The Occupational Safety and Health Act of 1970 (Public Law 91-596) states that "the Secretary of Health and Human Services shall... produce criteria... enabling the Secretary of Labor to meet his responsibility for the formulation of safety and health standards" [29 USC* 669(a)(2)]. An occupational safety and health standard is defined as one that is reasonably necessary or appropriate to provide safe or healthful employment or places of employment [29 USC 652]. In promulgating standards dealing with harmful physical agents under both the Occupational Safety and Health Act of 1970 (Public Law 91-596) and the Federal Mine Safety and Health Act of 1977 (Public Law 95-164), the Secretary of Labor shall set the standard which most adequately assures, to the extent feasible, that no employee will suffer material impairment of health or functional capacity even if such employee has regular exposure to the hazard for the period of his working life. In addition to the attainment of the highest degree of health and safety protection for the employee, other considerations shall be the latest available scientific data in the field, the feasibility of the standard, and experience gained under this and other health and safety laws [29 USC 655(b)(5); 30 USC 811(a)(6)(A)]. The National Institute for Occupational Safety and Health (NIOSH) is authorized under 29 USC 671 and 30 USC 811(a)(6)(B) to develop new and improved occupational safety and health standards and to perform all functions of the Secretary of Health and Human Services.
In 1972, NIOSH published Criteria for a Recommended Standard: Occupational Exposure to Noise, which provided the basis for a recommended standard to reduce the risk of developing permanent hearing loss as a result of occupational noise exposure [NIOSH 1972]. NIOSH has now evaluated the latest scientific information and is revising some of its previous recommendations.
The NIOSH recommended exposure limit (REL) of 85 dBA for occupational noise exposure was reevaluated using contemporary risk assessment techniques and incorporating the 4000-Hz audiometric frequency in the definition of hearing impairment. The new risk assessment reaffirms support for the 85-dBA REL. The excess risk of developing occupational noise-induced hearing loss (NIHL) for a 40-year lifetime exposure at the 85-dBA REL is 8%, which is considerably lower than the 25% excess risk at the 90-dBA permissible exposure limit currently enforced by the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA).
NIOSH previously recommended an exchange rate of 5 dB for the calculation of time-weighted average exposures to noise, but it is now recommending a 3-dB exchange rate, which is more firmly supported by scientific evidence. The 5-dB exchange rate is still used by OSHA and MSHA, but the 3-dB exchange rate has been increasingly supported by national and international consensus.
NIOSH recommends an improved criterion for significant threshold shift, which is an increase of 15 dB in hearing threshold at 500, 1000, 2000, 3000, 4000, or 6000 Hz that is repeated for the same ear and frequency in back-to-back audiometric tests. The new criterion has the advantages of a high identification rate and a low false-positive rate. In comparison, the criterion recommended in the 1972 criteria document has a high false-positive rate, and the OSHA criterion, called the Standard Threshold Shift, has a relatively low identification rate.
Differing from the 1972 criteria document, NIOSH no longer recommends age correction on individual audiograms. This practice is not scientifically valid, and would delay intervention to prevent further hearing losses in those workers whose hearing threshold levels have increased due to occupational noise exposure. OSHA currently allows age correction only as an option.
The Noise Reduction Rating (NRR) is a single-number, laboratory-derived rating required by the Environmental Protection Agency to be shown on the label of each hearing protector sold in the U.S. In calculating the noise exposure to the wearer of a hearing protector at work, OSHA has implemented the practice of derating the NRR by one-half for all types of hearing protectors. In 1972, NIOSH recommended the use of the full NRR value, but now it recommends derating the NRR by 25%, 50% and 70% for earmuffs, formable earplugs and all other earplugs, respectively. This variable derating scheme, as opposed to OSHA's straight derating scheme, takes into consideration the performances of different types of hearing protectors.
This document also provides recommendations for the management of hearing loss prevention programs for workers whose noise exposures equal or exceed 82 dBA (i.e., 1/2 of the REL). The recommendations include program evaluation, which was not articulated in the 1972 criteria document and is not included in the OSHA and MSHA standards.
Adherence to the revised recommended standard will minimize the risk of
developing occupational NIHL.
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This document was prepared by the staff of the Education and
Information Division, Paul Schulte, Ph.D., Acting Director, and the
Division of Biomedical and Behavioral Science, Janet C. Haartz, Ph.D.,
Director. The document manager was Henry S. Chan, M.P.H., CIH. The
contributions of consultants and other NIOSH personnel are gratefully
|Julia D. Royster, Ph.D., OCC-A||Alice H. Suter, Ph.D., OCC-A|
|John R. Franks, Ph.D., OCC-A||Stephen J. Gilbert, M.S.|
|Barry L. Lempert, B.S.||Carol J. Merry, Ph.D.|
|Mary M. Prince, Ph.D.||Randall J. Smith, M.A.|
|Leslie T. Stayner, Ph.D.||Mark R. Stephenson,Ph.D.,OCC-A|
|Christa L. Themann, M.A., OCC-A|
[more acknowledgement later]
|GLOSSARY AND ABBREVIATIONS
|1 RECOMMENDATIONS FOR A STANDARD||1|
|Section 1.1 Recommended Exposure Limit||1|
|Section 1.2 Hearing Loss Prevention Program||2|
|Section 1.3 Exposure Assessment||2|
|Section 1.4 Engineer Controls and Work Practices||3|
|Section 1.5 Hearing Protectors||3|
|Section 1.6 Medical Surveillance||4|
|Section 1.7 Posting||7|
|Section 1.8 Hazard Communication and Training||8|
|Section 1.9 Program Evaluation Criteria||9|
|Section 1.10 Recordkeeping||10|
|Section 1.11 ANSI Standards||12|
|Appendix Determination of dBA Reduction R for Ear Protectors||15|
|2.1 Recognition of Noise as a Health Hazard||16|
|2.2 Noise-Induced Hearing Loss||16|
|2.3 Physical Properties of Sound||18|
|2.4 Number of Noise-Exposed Workers in the United States||20|
|2.5 Legislative History||24|
|3 BASIS FOR THE EXPOSURE STANDARD||27|
|3.1 Quantitative Risk Assessment||27|
|3.2 Ceiling Limit||33|
|3.3 Exchange Rate||34|
|3.4 Impulsive Noise||41|
|4 INSTRUMENTATION FOR NOISE MEASUREMENT||47|
|4.1 Sound Level Meter||47|
|4.2 Noise Dosimeter||49|
|5 HEARING LOSS PREVENTION PROGRAMS||51|
|5.1 Personnel Requirements||53|
|5.2 Initial and Annual Audits (Component 1)||54|
|5.3 Exposure Assessment (Component 2)||55|
|5.4 Engineering and Administrative Controls (Component 3)||57|
|5.5 Audiometric Evaluation and Monitoring (Component 4)||60|
|5.6 Use of Hearing Protectors (Component 5)||74|
|5.7 Education and Motivation (Component 6)||74|
|5.8 Recordkeeping (Component 7)||80|
|5.9 Evaluation of Program Effectiveness (Component 8)||83|
|5.10 Age Correction||87|
|6 HEARING PROTECTORS||89|
|7 RESEARCH NEEDS||98|
|7.1 Noise Control||98|
|7.2 Impulsive Noise||98|
|7.3 Nonauditory Effects||99|
|7.4 Auditory Effects of Ototoxic Chemical Exposures||99|
|7.5 Exposure Monitoring||99|
|7.6 Hearing Protectors||100|
|7.7 Training and Motivation||101|
|7.8 Program Evaluation||101|
|1-1||Exposure Level (L) and Duration (T)||13|
|1-2||Noise Dose (D) and Time-Weighted Average (TWA)||14|
|2-1||Estimated numbers of workers exposed to noise levels at 85 dBA or greater by economic sector, based on data collected in the National Occupational Exposure Survey (NOES) and on data collected by the Bureau of Census||21|
|3-1||Estimated excess risk of incurring hearing impairment as a function of noise exposure over a 40-year working lifetime||29|
|3-2||Excess risk estimates for hearing impairment at age 60,based on 40-year lifetime occupational exposures to 80, 85 and 90 dBA - Comparison of the 1971-ISO, 1972-NIOSH, 1973-EPA, 1990-ISO and 1996-NIOSH models against the 0.5-1-2-kHz, the 1-2-3-kHz and the 1-2-3-4-kHz definitions of hearing impairment||32|
|4-1||Relative Response of Sound Level Meter Weighting Networks||48|
|5-1||Number of classifiable first tags (those occurring in 66 comparisons of tests 2 through 7 back to test 1 across of 15 databases (N=2903), and numbers and percents of first tags classified as true positive, for each of the 6 shift criteria|
|5-2||Advantages and Disadvantages of Each Criterion for Significant Threshold Shift||68|
|6-1||Summary of Real-World NRRs Achieved by 84% of the Wearers (NRR84) of Hearing Protectors (HPs) in 20 Independent Studies||95|
A graph or table obtained from an audiometric test showing hearing
level as a function of frequency.
The audiogram obtained from an audiometric examination administered
prior to employment or within the first 30 days of employment that is
preceded by a period of at least 14 hours of quiet. The baseline audiogram
is the audiogram against which subsequent audiograms will be compared for
the calculation of significant threshold shift.
Noise levels that vary minimally as a function of time.
Ratio of the peak sound pressure to the RMS (root mean square) sound
Daily noise dose (D)
A descriptor for noise exposure, in percent, expressed by the following
D = [C1/T1 + C2/T2 +
.... + Cn/Tn] x 100
Cn = total time of exposure at a specified noise level
Tn = total time of exposure permitted at that noise level
A dimensionless unit used in physics, which is equal to 10 times the
logarithm to the base 10 of the ratio of two values:
dB = 10*log(value1/value2)
[For the application of this general equation in acoustics, see sound
intensity level and sound pressure level in this glossary.]
Decibels, A-weighted (dBA)
Unit representing the sound level measured with the A-weighting network
on a sound level meter. [Refer to Table 4-1 for the characteristics of the
Decibels, C-weighted (dBC)
Unit representing the sound level measured with the C-weighting network
on a sound level meter. [Refer to Table 4-1 for the characteristics of the
To use a fraction of a hearing protector's noise reduction rating (NRR)
to calculate the noise exposure of a worker wearing that hearing
protector. [See noise reduction rating in this glossary.]
A hypothesis stating that equal amounts of sound energy will produce
equal amounts of hearing impairment, regardless of how the sound energy is
distributed in time.
Equivalent continuous noise level (Leq)
Varying, intermittent or impulsive noise exposure that is equal in
energy to a continuous noise level for a certain duration.
Percentage with hearing impairment in an occupational-noise-exposed
population after subtracting the percentage who would normally incur such
impairment from other causes in a population not exposed to occupational
An increment of decibels that requires the halving of exposure time, or
a decrement of decibels that requires the doubling of exposure time. For
examples, a 3-dB exchange rate requires that noise exposure time be halved
for each 3-dB increase in noise level; likewise, a 5-dB exchange rate
requires that exposure time be halved for each 5-dB increase.
The hearing threshold level above which a hearing loss is considered to
The number of times that a function occurs per unit time. [For sound,
the unit is cycles per second, or Hertz (Hz).]
Hearing threshold level (HTL)
The amount, in decibels, by which the threshold of audibility for an
ear differs from a standard audiometric threshold.
A descriptor for noise exposure, in decibels, representing the total
sound energy incident on the ear over a specified period of time (e.g.,
A reverberant type of impulsive noise.
A nonreveberant type of impulsive noise.
"Impulsive noise" is characterized by a sharp rise and rapid
decay in sound levels and is less than 1 second in duration. For the
purpose of this document, it includes impact or impulse noise.
Noise levels that are interrupted by intervals of relatively low sound
Any undesired or unwanted sound, usually of high intensity.
Noise Reduction Rating (NRR)
The NRR, which indicates a hearing protector's noise reduction
capabilities, is a single-number rating which is required by law to be
shown on the label of each hearing protector sold in the United States.
Permanent threshold shift (PTS)
An irreversible increase in hearing threshold level.
Difference in decibels between the peak level of an impulsive signal
and the RMS level of a continuous noise.
Root-mean-square (RMS) sound pressure
The square root of the sum of the square values of sound pressure over
a specified period of time.
Significant threshold shift
A shift in hearing threshold, outside the range of audiometric testing
variability (+5 dB), that warrants follow-up action to prevent
further hearing loss. NIOSH recommends that a change in hearing threshold
of 15 dB for the worse at any frequency (0.5, 1, 2, 3, 4, or 6 kHz) that
is repeated for the same ear and frequency to meet the criteria for
significant threshold shift.
An auditory sensation evoked by oscillations in pressure in a medium
with elasticity and viscosity (e.g., air).
Sound intensity (I)
The average rate at which sound energy is transmitted through a unit
area normal to the direction of sound propagation.
Sound intensity level (SIL)
The sound intensity level, in decibels, of a sound is 10 times the
logarithm to the base 10 of the ratio of the sound intensity to the
reference sound intensity.
SIL = 10*log(I/I0)
where I = sound intensity
I0 = reference sound intensity
For sound transmitted in air, the reference sound intensity is 10-12
watts per square meter.
Sound pressure (P)
The total instantaneous pressure at a point in the presence of a sound
wave minus the static pressure at that point.
Sound pressure level (SPL)
The sound pressure level, in decibels, of a sound is 10 times the
logarithm to the base 10 of the ratio of the sound pressure squared to the
reference sound pressure squared.
SPL = 10*log(P2/P02)
SPL = 10*log(P/P0)2
SPL = 20*log(P/P0)
where P = sound pressure
P0 = reference sound pressure
For sound transmitted in air, the reference sound pressure is 20
Temporary threshold shift (TTS)
A temporary increase in hearing threshold level, after exposure to
noise, that reverts to the pre-exposure hearing threshold level.
Noise levels that fluctuate considerably as a function of time.
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Section 1.1 RECOMMENDED EXPOSURE LIMIT
The recommended exposure limit (REL) for occupational noise exposure
encompasses the provisions in Sections 1.1.1 through 1.1.4.
1.1.1 Exposure Levels and Durations
Occupational noise exposure shall be controlled so that workers are not exposed in excess of the combination of exposure level (L) and duration (T) as calculated by the following formula (or as shown in Table 1-1).
|T (minutes) =||480|
1.1.2 Time-Weighted Average
In accordance with Section 1.1.1, the REL is a time-weighted average
(TWA) of 85 dBA based on an 8-hour workshift.
1.1.3 Daily Noise Dose
When the daily noise exposure consists of periods of different noise levels, the daily dose (D) shall not exceed 100, as calculated according to the following formula:
D = [C1/T1 + C2/T2 +
.... + Cn/Tn] x 100
Cn = total time of exposure at a specified noise level
Tn = total time of exposure permitted at that noise level
The daily dose can be converted into an 8-hour TWA level according to
the following formula (or as shown in Table 1-2):
TWA = 10.0 x Log(D/100) + 85
1.1.4 Ceiling Limit
Occupational exposure to continuous, varying and/or intermittent noise
shall not exceed 115 dBA.
Section 1.2 HEARING LOSS PREVENTION PROGRAM
The employer shall institute an effective hearing loss prevention
program (HLPP) described in Sections 1.3 through 1.11 when any worker's
8-hour TWA exposure equals or exceeds 82 dBA, except where noted.
Section 1.3 EXPOSURE ASSESSMENT
The employer shall conduct exposure assessment when any worker's 8-hour
TWA exposure equals or exceeds 82 dBA.
1.3.1 Initial Monitoring
An initial monitoring shall be conducted to determine the noise
exposure levels representative of all workers whose 8-hour TWA noise
exposures may equal or exceed 82 dBA.
1.3.2 Periodic Monitoring
Exposure monitoring shall be repeated at least bi-annually when any
worker's 8-hour TWA exposure equals or exceeds 82 dBA, or when equipment
or process changes may impact on the exposure levels.
Instruments used to measure workers' noise exposures shall be
calibrated to ensure measurement accuracy. In addition, they shall conform
to the American National Standard Specification for Sound Level Meters,
ANSI S1.4-1983 and S1.4A-1985, Type 2 [ANSI 1983; ANSI 1985] or, with the
exception of the operating range, to the American Standard Specification
for Personal Noise Dosimeters, ANSI S1.25-1991 [ANSI 1991a].
For the purpose of determining TWA exposures, all continuous, varying,
intermittent and impulsive sound levels from 80 dBA to 140 dBA shall be
integrated into the noise measurements.
Section 1.4 ENGINEERING AND ADMINISTRATIVE CONTROLS
AND WORK PRACTICES
Wherever feasible, engineering controls, administrative controls,
and/or work practices shall be used to ensure that workers are not exposed
to noise above the REL of 85 dBA, 8-hour TWA.
Section 1.5 HEARING PROTECTORS
Workers shall be required to wear hearing protectors when their noise
exposures exceed 85 dBA, regardless of duration. Hearing protectors shall
be provided by the employer.
Hearing protectors shall provide such attenuation as to reduce the
worker's effective noise exposure (i.e., the noise exposure when hearing
protectors are worn) to 82 dBA, 8-hour TWA, or below. Methods for
calculating the effective noise exposure are provided in the Appendix,
The employer shall provide training in the fitting and wearing of
Section 1.6 MEDICAL SURVEILLANCE
The employer shall provide audiometry for all workers whose exposures
equal or exceed 82 dBA, 8-hour TWA.
Audiometric tests shall be performed by an audiologist, or by an
occupational hearing conservationist certified by the Council for
Accreditation in Occupational Hearing Conservation (CAOHC) working under
the supervision of an audiologist or physician. If the audiometric test is
performed by an occupational hearing conservationist, the CAOHC
certification number shall be recorded on each worker's audiogram.
Audiometric tests shall be pure tone, air conduction, hearing threshold
examinations, with test frequencies including 500, 1000, 2000, 3000, 4000,
6000 and 8000 Hz, and shall be taken separately for the right and left
Audiometric tests shall be conducted with audiometers that meet the
specifications of, and are maintained and used in accordance with the
American National Standard Specifications for Audiometers, ANSI S3.6-1995
[ANSI 1995]. Audiometers shall be given an annual comprehensive
calibration, a bimonthly acoustic calibration check, and a daily
functional check whenever the audiometers are used. The date of the last
annual calibration shall be recorded on each worker's audiogram.
Audiometric tests shall be conducted in a room where ambient noise
levels conform to all requirements of the American National Standard
Institute Maximum Permissible Ambient Noise Levels for Audiometric Test
Rooms, ANSI S3.1-1991 [ANSI 1991b], when measured by instruments
conforming to the American National Standard Specification for Sound Level
Meters, ANSI S1.4-1983 and S1.4A-1985, Type 1 [ANSI 1983; ANSI 1985] and
the American National Standard Specification for Octave-Band and
Fractional-Octave-Band Analog and Digital Filters, ANSI S1.11-1986 [ANSI
1986]. For permanent, on-site testing facilities, ambient noise levels
shall be checked at least annually. For mobile testing facilities, ambient
noise levels shall be tested daily, or each time the facility is moved,
whichever is more often. Ambient noise levels shall be recorded on the
worker's audiogram or made otherwise accessible to the professional
reviewer of the audiograms.
1.6.2 Baseline Audiogram
For each worker in a HLPP, a baseline audiogram shall be obtained prior
to employment or within 30 days of enrollment in the HLPP.
Because the baseline audiogram is intended to be the best estimate of
the worker's hearing before any exposure to potentially harmful noise, the
worker shall not be exposed to workplace noise for a minimum of 14 hours
before the baseline audiometric test. The required quiet period shall not
be substituted by the use of hearing protectors.
1.6.3 Monitoring Audiogram, Retest Audiogram, and
Significant Threshold Shift
On an annual basis, each worker's hearing thresholds shall be monitored
by an audiometric test, which shall be conducted during the worker's
normal workshift. For the purpose of this section, the audiogram from this
test is called the monitoring audiogram.
At the completion of this test, the worker's monitoring audiogram shall
be examined immediately to determine whether it indicates any threshold
shift (higher threshold) in either ear that equals or exceeds 15 dB at
500, 1000, 2000, 3000, 4000 or 6000 Hz as evidenced by a comparison of
that audiogram with the worker's baseline audiogram.
If the monitoring audiogram indicates a hearing threshold shift, a
retest shall be conducted immediately, following re-instruction of the
worker and refitting of the earphones.
If the retest audiogram shows a shift of 15 dB or more at the same
frequency in the same ear as in the monitoring audiogram, the worker shall
be considered to have met the significant threshold shift criterion for
the purpose of this section and shall be given a confirmation audiometric
test within 30 days. This confirmation test shall be conducted under the
same conditions as in a baseline audiometric test.
1.6.4 Confirmation Audiogram and Fellow-Up Action
If the worker's confirmation audiogram shows a shift of 15 dB or more
at the same frequency in the same ear as in the previous retest audiogram,
the worker's audiograms and other appropriate records shall be reviewed by
an audiologist or a physician.
If this review confirms that the significant threshold shift is
persistent, the significant threshold shift shall be recorded in the
worker's medical record, and the confirmation audiogram shall be used for
the calculation of any subsequent significant threshold shift in future
A worker whose significant threshold shift is of any etiology other
than noise, as determined in this review, shall be referred to the
worker's physician. If the probable etiology is of occupational noise
exposure, the employer shall take appropriate action to protect the worker
from additional hearing loss due to occupational noise exposure. Examples
of appropriate action include, but is not limited to, re-instruction and
refitting of hearing protectors, additional training of the worker on
hearing loss prevention, and reassignment of the worker to a quieter work
1.6.5 Exit Audiogram
An exit audiogram shall be obtained for a worker who is leaving employment or is permanently rotated out of an occupational noise exposure at or above 82 dBA, 8-hour TWA. The audiometric test shall be conducted following a minimum of 14 hours of quiet.
Section 1.7 POSTING
A warning sign shall be located at the entrance to or the periphery of
areas where noise exposures exceed 85 dBA, regardless of duration. All
warning signs shall be printed both in English and the predominant
language of workers who do not read English, if applicable. Workers unable
to read the warning signs shall be informed verbally regarding the
instructions printed on signs in hazardous work areas of the facility. The
warning sign shall consist of the following:
The type size of the word "warning" shall be no less than 36
point, and the rest of the sign shall be no less than 24 point.
1.8.1 Hazard Communication
All workers who are exposed to noise at or above 82 dBA, 8-hour TWA,
shall be informed of the potential consequences of noise exposure and the
methods of preventing noise-induced hearing loss.
The employer shall institute a continuing education program conducted
by persons qualified by experience or training in occupational hearing
loss prevention. The program shall ensure that all workers exposed to
noise understand how noise damages the ear and causes hearing loss and how
to prevent hearing loss. The program shall have methods of delivery from
formal presentation with oral and written materials to informal, on the
spot presentations. Allowances shall be made to allow for one-on-one
training to be presented at the time of the annual audiogram so that the
worker may judge his or her own effectiveness in preventing occupational
hearing loss. The scope of educational and motivational materials shall
include, at a minimum, what the employer is doing to prevent noise-induced
hearing loss and what the worker can do to protect his or her own hearing.
The employer shall maintain a record of provision of educational and
motivational sessions and shall have documentation of the materials used
in these sessions.
The effectiveness of the HLPP shall be evaluated at the level of the individual worker and at the programmatic level.
The evaluation at the worker level shall take place at the time of the
annual audiometry. If the worker's audiogram meets the condition for
significant threshold shift, all possible steps shall be taken to make
sure that the change in hearing sensitivity is not caused by something
other than exposure to workplace noise. If the significant threshold shift
is attributed to workplace noise, then steps shall be taken to prevent
further significant threshold shift for that worker.
The evaluation at the programmatic level shall take place no less than
annually. If the only data available come from a summary of workers
experiencing significant threshold shift, then a significant threshold
shift incidence rate based on the total number of workers in the HLPP of
5% or less should be considered evidence of an effective HLPP. If other
data, such as the hearing threshold levels (HTLs) of workers not exposed
to noise, are available, then a successful program should have a similar
incidence rate of significant threshold shift for exposed and non-exposed
The employer shall establish and maintain recordkeeping in accordance
with the requirements in Sections 1.10.1 through 1.10.5.
1.10.1 Exposure Assessment
The employer shall establish and maintain an accurate record of all
exposure measurements required in Section 1.3. These records shall include
no less than the name of the worker being monitored; social security
number; duties performed and job locations; dates and times of
measurements; type, make, model and size of hearing protectors used (if
any); the measured exposure levels; and the identification of the person
taking the measurements. Copies of a worker's exposure history resulting
from this requirement shall also be included in the worker's medical file
along with the worker's audiograms.
1.10.2 Medical Surveillance
The employer shall establish and maintain an accurate record for each
worker subject to the medical surveillance specified in Section 1.6. These
records shall include no less than the name of the worker being tested;
social security number; duties performed and job locations; the worker's
medical, employment and noise-exposure history; dates, times and types of
tests (i.e., baseline, annual, retest, confirmation); hours since last
noise exposure before each test; hearing threshold levels at the required
audiometric frequencies; tester's identification and assessment of test
reliability; the etiology of any significant threshold shift; and the
identification of the reviewer.
1.10.3 Record Retention
In accordance with the requirements of 29 CFR 1910.20(d), Preservation
of Records, the employer shall retain the records described in Sections
1.3 and 1.6 of this document for at least the following periods:
In addition, records of audiometer calibrations and the ambient noise
measurements in the audiometric testing room shall be kept for 5 years.
1.10.4 Availability of Records
In accordance with 29 CFR 1910.20, Access to Employee Exposure and
Medical Records, the employer shall, upon request, allow examination and
copying of exposure monitoring records by the subject worker, the former
worker, or anyone having the specific written consent of the subject or
Any medical records required by this recommended standard shall be
provided upon request for examination and copying, to the subject worker,
the former worker, or anyone having the specific written consent of the
subject or former worker.
1.10.5 Transfer of Records
The employer shall comply with the requirements for the transfer of
records as set forth in 29 CFR 1910.20(h), Transfer of Records.
All ANSI standards referenced in this standard shall be superseded with
the latest versions as they become available.
|L (dBA)||T||L (dBA)||T|
*Any exposure time above the ceiling limit of 115 dBA shall be included
in the calculation of the 8-hour time-weighted-average exposure.
|Dose %||8-hr TWA||Dose %||8-hr TWA||Dose %||8-hr TWA|
*TWA = 10 x Log(D/100) + 85
Appendix - Determination of
Effective Noise Level (dBA') for Ear Protectors
This appendix is a summary of Appendix B, Methods for Estimating the
Adequacy of Hearing Protector Attenuation, in the Occupational Noise
Standard [29 CFR 1910.95] with two exceptions: (1) The 8-hour TWA noise
exposure is based on the 3-dB exchange rate. (2) The manufacturer's Noise
Reduction Rating (NRR) of a hearing protector is derated. The "NRR"
in the equations shown below shall be a derated NRR as follows:
|earmuffs||75% of manufacturer's labelled NRR|
|formable earplugs||50% of manufacturer's labelled NRR all|
|other earplugs||30% of manufacturer's labelled NRR|
Measure noise exposure levels in dBC or dBA with a sound level meter or
1. When the noise exposure level in dBC is known, the effective noise
level dBA' is:
dBA' = dBC - NRR
2. When the noise exposure level in dBA is known, the effective noise
level DBA' is:
dBA' = dBA - (NRR - 7)
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The sounds of industry, growing in volume over the years, have heralded
not only technical and economic progress, but also the threat of an ever
increasing incidence of hearing loss and other noise-related disturbances
to exposed workers. Noise, essentially any unwanted or undesirable sound,
is not a new hazard. Indeed, noise-induced hearing loss was observed
centuries ago. Ramazzini in "De Morbis Artificium Diatriba" in
1700 described how those hammering copper "have their ears so injured
by that perpetual din.....that workers of this class became hard of
hearing and, if they grow old at this work, completely deaf." Before
the Industrial Revolution, however, comparatively few people were exposed
to high levels of workplace noise. It was the advent of steam power in
connection with the Industrial Revolution that first brought general
attention to noise as an occupational hazard. Workers who fabricated steam
boilers were found to develop hearing loss in such numbers that such a
malady was dubbed "boilermakers disease." Increasing
mechanization in all industries and most trades has since proliferated the
The ear is the organ structure of the body especially adapted and most
responsive to the pressure changes in sound production. Anatomically, it
is divided into three subdivisions--the outer, middle, and inner ear. The
principal functions of the outer and middle ear are to collect and
transmit sound pressure to the inner ear where the hair cell receptors for
hearing are located. The latter are arranged in several rows along the
entire length of the basilar membrane, one of the two partitions which
spiral around the bony axis of the cochlea. These hair cells together with
their supporting cells comprise the organ of Corti, the auditory sense
organ. In the cochlea, mechanical energy from the sound pressure is
transformed into electrical energy that is carried by the auditory nerve
to the brain. Excessive noise exposure produces a sensorineural hearing
loss involving injury to the hair cells.
The most recognized effect of exposure is noise-induced hearing loss
(NIHL). Initially, the exposure to high levels of noise causes temporary
threshold shift (TTS), which refers to the difference in a person's
hearing level measured before and after exposure to noise. This shift is
temporary because the individual normally recovers his or her pre-exposure
hearing level several hours after cessation of exposure. Repeated
exposures over several years can result in a permanent threshold shift
(PTS), which is an irreversible, sensorineural hearing loss.
There are other causes of hearing loss besides occupational noise.
Hearing loss due to aging occurs naturally and is called presbycusis.
Hearing losses caused by non-occupational or recreation noise sources
(e.g., loud music, guns, lawnmowers, etc.) effect the ear the same as
occupational noise, but are collectively called sociocusis. Conductive
hearing losses, as opposed to sensorineural hearing losses caused by
noise, are usually traceable to diseases of the outer and middle ear.
Concomitant exposures to noise and certain physical or chemical agents
(e.g., vibration, organic solvents, carbon monoxide, ototoxic drugs, and
certain metals) appear to have synergistic effects on hearing [Jauhianien
et al. 1972; Hamernik and Henderson 1976; Brown et al. 1978; Gannon et al.
1979; Brown et al. 1980; Hamernik et al. 1980; Hamernik et al. 1981; Pryor
et al. 1983; Rebert et al. 1983; Humes 1984; Boettcher et al. 1987; Young
et al. 1987; Byrne et al. 1988; Fechter et al. 1988; Johnson et al. 1988;
Morata et al. 1993].
The most obvious effect of noise exposure is the loss of hearing.
However, there are other effects that do not involve the auditory system.
These nonauditory effects include psychological stress and disruption of
job performance [Cohen 1973; EPA 1973; Taylor 1984; Ohrstrom et al. 1988;
Suter 1989], and may include hypertension [Parvizpoor 1976; Jonsson and
Hansson 1977; Takala et al. 1977; Lees and Roberts 1979; Malchaire and
Mullier 1979; Manninen and Aro 1979; Singh et al. 1982; Belli et al. 1984;
Delin 1984; Talbott et al. 1985; Verbeek et al. 1987; Wu et al. 1987;
Talbott et al. 1990]. Noise may also be a contributory factor in
industrial accidents [Cohen 1976; Schmidt et al. 1980; Wilkins and Acton
The perception of sound begins when vibration or turbulence causes
pressure changes in the air (or in some other medium). These pressure
changes produce longitudinal waves that propagate away from the vibrating
or turbulent source to the receiver (i.e., human ear) in the form of
alternating compression and rarefaction of molecules. The receiver in turn
translates these waves into what is known as sound. The effects of sound
on the receiver depend on three physical parameters of sound: amplitude,
frequency, and duration.
Sound pressure level (SPL), expressed in decibels (dB), is a measure of
the amplitude of the pressure change that produces sound. This amplitude
is perceived by the listener as loudness. In sound measuring instruments,
weighting networks, which will be described in Chapter 4, are used to
modify the sound pressure level. The A-weighted network is commonly used
in measuring noise to evaluate its effect on people. Exposure limits are
expressed in decibels, A-weighted network (dBA). When used without a
weighted network suffix, the expression "dB" usually implies "dB
SPL" (i.e., no network).
Each wave of compression and rarefaction of molecules described above
represents a complete cycle. The frequency, in Hertz (Hz), represents the
number of cycles occurring in one second, and determines the pitch
perceived by the listener. Humans with normal hearing can hear a frequency
range of about 20 to 20,000 Hz. An ordinary sound generally contains a
mixture of frequencies.
Duration refers to the pattern of sound levels over time. Industrial
noise can be arbitrarily classified as continuous, varying, intermittent
and impulsive. Lacking quantitative definitions, continuous, varying and
intermittent noises together denote a continuum of relatively constant
sound levels to noticeably different sound levels occurring within an
exposure period (usually an 8-hour workday). For simplicity, when compared
with impulsive noise, the term "continuous noise" would normally
include continuous, varying and intermittent noises. Impulsive noise is
distinguished from continuous noise by a steep rise in sound pressure
level to a high peak, followed by a rapid decay, and can be either
reverberant (impact noise) or nonreverberant (impulse noise). In many
workplaces, the exposures are often a mixture of continuous and impulsive
From 1981 to 1983, NIOSH conducted the National Occupational Exposure
Survey (NOES). The NOES was designed to provide data descriptive of the
occupational safety and health conditions in the United States. The
surveyors visited and gathered information at various workplaces
throughout the country. For the purposes of the NOES, workers were
considered to be noise-exposed if the noise levels were 85 dBA or greater,
regardless of the exposure duration. On the basis of the information
collected, the percentages of noise-exposed workers in different economic
sectors were estimated. When these percentages are applied to the more
recent Bureau of Census  data, the estimated number of noise-exposed
workers in the United States is approximately 30 million. Table 2-1
provides a breakdown of the estimated numbers of noise-exposed workers by
|Economic Sector (2-Digit SIC Code)1||NOES Estimated Number of Noise-Exposed Workers2||NOES Number of Production Workers in 19832||NOES Percent of Noise-Exposed Workers2||Number of Production Workers in 19923||Estimated Number of Noise-Exposed Workers in 19924|
|Agricultural services (07)||2,826,096||3,364,000||84.01%||3,233,000||2,716,043|
|Oil and gas extraction (13)||365,000||400,000||91.25%||354,000||821,276|
|General building contractors (15)||654,400||749,000||87.37%||940,000||821,276|
|Construction, other than building (16)||513,270||632,000||81.21%||694,000||563,622|
|Special trade contractors (17)||1,448,800||1,646,000||88.02%||2,401,000||2,113,347|
|Food and kindred products (20)||344,964||1,118,000||30.86%||1,214,000||374,585|
|Lumber and wood (24)||210,305||548,000||38.38%||563,000||216,062|
|Furniture and fixtures (25)||126,988||355,000||35.77%||367,000||131,281|
|Printing and publishing (27)||160,293||710,000||22.58%||841,000||189,868|
|Petroleum and coal (29)||31,998||118,000||27.12%||101,000||27,388|
|Rubber and plastics (30)||138,208||556,000||24.86%||678,000||168,534|
|Stone, clay, and glass (32)||103,596||436,000||23.76%||401,000||95,280|
|Primary metals (33)||275,791||625,000||44.13%||531,000||234,312|
|Fabricated metals (34)||367,826||997,000||36.89%||979,000||361,185|
|Machinery, except electrical (35)||256,277||1,202,000||21.32%||1,164,000||248,175|
|Electrical machinery (36)||108,425||1,229,000||8.82%||1,056,000||93,163|
|Transportation equipment (37)||249,605||1,103,000||22.63%||1,148,000||259,788|
|Instruments and related products (38)||50,515||390,000||12.95%||5,000,000||647,628|
|Miscellaneous manufacturing industries (39)||47,462||268,000||17.71%||264,000||46,754|
|Local and suburban transit (41)||148,320||236,000||62.85%||331,000||208,025|
|Freight transportation and warehouses (42)||40,709||1,056,000||3.86%||1,618,000||62,374|
|Water transportation (44)||99,170||211,000||47.00%||1,410,000||662,700|
|Transportation by air (45)||339,750||453,000||75.00%||732,000||549,000|
|Transportation services (47)||164,340||198,000||83.00%||274,000||227,420|
|Wholesale trade (50, 51)||130,146||551,427||23.60%||5,983,000||1,412,088|
|Retail trade (53-58)||3,167,296||15,018,000||21.09%||19,138,000||4,036,204|
|Financial services (60-67)||17,277||5,160,000||0.33%||6,672,000||22,275|
|Services (70-79, except 75)||3,856,800||17,890,000||21.56%||28,903,000||6,231,028|
|Automotive services (75)||138,806||275,243||50.43%||754,000||380,245|
|Health services (80)||182,420||5,366,000||3.40%||878,000||29,848|
1Standard Industry Classification (SIC) (Source: Anonymous )
2Numbers not available from NOES are extracted from unpublished data from the following sources: NIOSH Farm Family Health Hazard Survey (agricultural services), Mine Safety and Health Administration (mining), and U.S. Army Center for Health Promotion and Preventive Medicine (government civilian work force).
3Source: Bureau of Census 
% of Noise-Exposed Worker) x (Number of Production Workers in 1992)
Efforts to effectively regulate occupational noise in the United States
began about 1955. The military was first to establish such regulations for
members of the armed forces [U.S. Air Force 1956]. Under the Walsh-Healey
Public Contracts Act of 1936, as amended, safety and health standards had
been issued that contained references to excessive noise, but they
prescribed neither limits nor acknowledged the occupational hearing-loss
problem. A later regulation under this act [41 CFR 50-204.10] promulgated
in 1969, defined noise limits for occupational exposure for purposes of
hearing conservation. These limits were applicable to only those firms
having supply contracts with the government in excess of $10,000; similar
limits were made applicable to work under Federal service contracts of
$2,500 or more under the Service Contract Act. The noise rule in the
Walsh-Healey Act regulations was adopted under the Coal Mine Health and
Safety Act of 1969 (Public Law 91―173) and thereby became applicable
to underground and surface coal mine operations.
In 1970, the Occupational Safety and Health Act (Public Law 95-164) was
enacted, which established the Occupational Safety and Health
Administration (OSHA) within the Department of Labor as the law
enforcement agency responsible for protecting the safety and health of a
large segment of the American work force. The Act also stipulated that the
Secretary of Health, Education, and Welfare (now the Secretary of Health
and Human Services) would, on the basis of available data, develop
criteria for harmful physical agents that describe exposure levels safe
for various periods of employment. In compliance with this provision,
NIOSH published Criteria for a Recommended Standard.... Occupational
Exposure to Noise in 1972 [NIOSH 1972]. The document provided the
basis for a recommended standard to reduce the risk of developing
permanent hearing loss as a result of occupational noise exposure. The
criteria document presented a recommended exposure limit (REL) of 85 dBA,
8-hour time-weighted average (TWA); and methods for measuring noise,
calculating noise exposure, and providing a hearing conservation program.
However, the criteria document acknowledged that NIOSH was not able to
determine the technological feasibility of the REL and that approximately
15% of the population exposed to occupational noise at the 85-dBA level
for a working lifetime would still be at risk of developing noise-induced
hearing loss (NIHL).
Initially, OSHA adopted the Walsh-Healey exposure limit of 90 dBA,
8-hour TWA with a 5-dB exchange rate, as its permissible exposure limit
(PEL) [29 CFR 1910.95] for general industry. In 1974, responding to the
NIOSH criteria document, OSHA proposed a revised noise standard [39 Fed.
Reg. 37,773 (1974a)], but left the PEL unchanged. The proposed standard
was not promulgated, but it articulated the requirement for a hearing
conservation program. In 1981, and again in 1983, OSHA amended its noise
standard to include specific provisions of a hearing conservation program
for occupational exposures at 85 dBA or above [46 Fed. Reg. 4,078 (1981);
48 Fed. Reg. 9,738 (1983)]. The OSHA noise standard as amended does not
cover all industries. Specifically, the Hearing Conservation Amendments do
not cover noise-exposed workers in transportation, oil/gas well drilling
and servicing, agriculture, construction, and mining. For examples, the
construction industry is covered by another OSHA noise standard [29 CFR
1926.52], and the mining industry is regulated by four separate standards
[30 CFR 56; 30 CFR 57; 30 CFR 70; 30 CFR 71] which are enforced by the
Mine Safety and Health Administration (MSHA). These standards vary in
specific requirements regarding exposure monitoring and hearing
conservation, but all maintain an 8-hour TWA exposure limit of 90 dBA.
Although they are required to comply with OSHA regulations by Executive
Order 12196, the U.S. Air Force  and the U.S. Army  have
chosen a more stringent exposure limit of 85 dBA, 8-hour TWA with a 3-dB
exchange rate. Thus, the American working population is given various
degrees of protection from occupational noise.
Thus far, the exposure limits discussed above apply only to continuous
noise. The generally-accepted exposure limit for impulsive noise, 140 dB
peak SPL, the level not to be exceeded at any time, is dealt with
differently among the regulatory standards. It is either enforceable or
non-enforceable, indicated by the word "shall" or "should,"
respectively. In the MSHA standards [30 CFR 56; 30 CFR 57; 30 CFR 70; 30
CFR 71], this exposure limit is enforceable, but in the OSHA standards [29
CFR 1910.95; 29 CFR 1926.52], it is non-enforceable. The NIOSH REL of 85
dBA, in principle, includes impulsive noise, but for practical purposes it
is an exposure limit for continuous noise only.
This document evaluates and presents the criteria for recommending the
exposure limit, the exchange rate, control methods and occupational
hearing loss prevention programs. Where the information is incomplete to
support definitive recommendations, research needs are suggested for
future criteria development. Nonauditory effects of noise, and hearing
losses due to causes other than noise, are beyond the scope of this
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The selection of an exposure limit depends on defining two parameters:
1) the maximum hearing threshold level (HTL) that is acceptable, above
which there is material impairment and below which hearing is considered
to be within normal limits; and 2) the proportion of the noise-exposed
population to be protected from material hearing impairment. Also referred
to as the fence, the HTL that divides normal hearing from impaired hearing
is often calculated as the average of the HTLs for two, three, or four
contiguous audiometric frequencies. The proportion of the population to be
protected from material hearing impairment is related to the excess risk,
which is the difference in the risk of developing material hearing
impairment between two populations: one exposed to occupational noise and
the other whose members are from a similar population but not exposed to
occupational noise. The risk of developing material hearing impairment in
these populations is denoted as the expected proportion of individuals
developing material hearing impairment. Mathematical models are used to
estimate the risk of developing material hearing impairment among workers
in these populations.
The most common protection goal has been one aimed at the preservation
of hearing for speech discrimination. Using this protection goal, NIOSH
 defined hearing impairment as an average of HTLs at the audiometric
frequencies 1000, 2000 and 3000 Hz that exceeds 25 dB (the 1-2-3-kHz
definition), and the U.S. Public Health Service (PHS)  also
reaffirmed this definition as "significant hearing impairment."
With this definition, NIOSH  assessed the excess risk of hearing
impairment as a function of levels and durations (e.g., 40-year working
lifetime) of occupational noise exposure. For a 40-year lifetime exposure
to average daily noise levels of 80, 85 and 90 dBA in the workplace, the
excess risk estimates were 3%, 16% and 29%, respectively, and on the basis
of this risk assessment, NIOSH recommended an 8-hour TWA exposure limit of
85 dBA [NIOSH 1972].
In order to compare the NIOSH excess risk estimates with those
developed by other organizations, the NIOSH data were also analyzed using
the same 25-dB fence but averaging the HTLs at 500, 1000 and 2000 Hz (the
0.5-1-2-kHz definition) [NIOSH 1972]. Table 3-1 presents the excess risk
estimates developed by NIOSH , EPA , and the International
Organization for Standardization (ISO)  for hearing impairment
caused by occupational noise exposure. OSHA used these estimates as the
basis for requiring hearing conservation programs for occupational noise
exposures at or above 85 dBA, 8-hour TWA [46 Fed. Reg. 4,078 (1981)].
|Reporting Organization||Noise Exposure (dBA)||Excess Risk (%)***|
*For purposes of comparison in this table, hearing impairment is defined as an average of the hearing threshold levels at 500, 1000, and 2000 Hz that exceeds 25 dB.
**Adapted from 39 Fed. Reg. 43,802 [1974b].
***Percentage with hearing impairment in an occupational-noise-exposed
population after subtracting the percentage who would normally incur such
impairment from other causes in an unexposed population.
The data used for the NIOSH risk assessment were collected by NIOSH in
13 noise and hearing surveys (collectively known as the Occupational Noise
and Hearing Survey, or ONHS) from 1968 to 1971. The industries that were
involved in the surveys included steel making, paper bag processing,
aluminum processing, quarrying, printing, tunnel traffic controlling,
woodworking and trucking. Questionnaires and audiometric examinations were
given to noise-exposed and non-noise-exposed employees who had consented
to participate in the surveys. More than 4,000 audiograms were collected,
but excluded from the sample were audiograms of employees not exposed to a
specified continuous noise level over their working lifetime and those
with abnormal hearing levels as a result of their medical history and a
variety of otological problems. Thus, 1,172 audiograms were used, which
represented 792 noise-exposed and 380 non-noise-exposed workers (controls)
[NIOSH 1972; Lempert and Henderson 1973].
Because of the prolific use of hearing protectors in the U.S. workplace
since the early 1980's, which would confound the determination of
dose-response relationships, new data that can be used for risk assessment
of occupational NIHL in the U.S. are not known to exist. Prince et al.
 derived a new set of excess risk estimates using the ONHS data with
a model (referred to as the "1996-NIOSH" model) which differed
from the model used in 1972 [NIOSH 1972] (referred to as the "1972-NIOSH"
model). A noteworthy difference between the two models is that Prince et
al.  considered more flexible shapes for the effects of noise in the
1996-NIOSH model, whereas the 1972-NIOSH model was based on a linear
assumption for the effects of noise. Prince et al.  found that
linear models similar to the 1972-NIOSH model did not fit the data as
well. In addition to using the 0.5-1-2-kHz and the 1-2-3-kHz definitions
of hearing impairment to assess the risk of occupational NIHL, Prince et
al.  also used the definition of hearing handicap1
proposed by the American Speech-Language-Hearing Association (ASHA) Task
Force on the Definition of Hearing Handicap, which is an average of the
HTLs at 1000, 2000, 3000 and 4000 Hz that exceeds 25-dB (the 1-2-3-4-kHz
definition) [ASHA 1981].
In 1971, ISO issued the first edition of ISO 1999, Assessment of
Occupational Noise Exposure for Hearing Conservation Purposes [ISO
1971] (referred to as the 1971-ISO model). This edition included estimates
of the risk of hearing impairment from occupational noise exposures. In
1990, the ISO issued a second edition of ISO 1999, Acoustics -
Determination of Occupational Noise Exposure and Estimation of
Noise-Induced Hearing Impairment [ISO 1990] (referred to as the
1990-ISO model). Both ISO models are based on broadband, steady noise
exposures for 8-hour workshifts during a working lifetime of up to 40
years. For comparison among the risk analyses by different organizations
over the years, Prince et al.  also calculated the excess risk
estimates using the 1990-ISO model and the information in Annex A of ISO
1999 [ISO 1990]. The comparison is summarized in Table 3-2.
|Average Daily Exposure Level||Excess Risk Estimates (%) for Hearing Impairment|
|0.5-1-2 kHz||1-2-3 kHz||1-2-3-4 kHz|
The excess risk estimates derived from the 1971-ISO, 1972-NIOSH,
1973-EPA and 1996-NIOSH2
models are reasonably similar. However, except for the 1-2-3-4-kHz
definition, the excess risk estimates derived from the 1990-ISO model are
considerably lower than those derived from the other models. These
disparities may be due to differences in the ISO 1999  statistical
methodology or in the underlying data used. However, regardless of which
set of excess risk estimates is used, an excess risk of hearing impairment
remains at and below 85 dBA.
As mentioned earlier in this section, the protection goal incorporated
in the definitions of hearing impairment has been to preserve hearing for
speech discrimination. The 4000-Hz audiometric frequency has been
recognized as not only being sensitive to noise but also extremely
important for hearing and understanding speech in unfavorable or noisy
listening conditions [Kuzniarz 1973; Aniansson 1974; Suter 1978]. Because
listening conditions are not always ideal in everyday life, and on the
basis of the ASHA  Task Force's proposal, NIOSH has modified its
definition of hearing impairment to include the 4000-Hz audiometric
frequency for use in assessing the risk of occupational NIHL. Hence, with
this modification, NIOSH defines hearing impairment as an average of the
HTLs at 1000, 2000, 3000 and 4000 Hz that exceeds 25 dB. Based on this
definition of hearing impairment, Prince et al.  have estimated that
the excess risk is 8% for a 40-year lifetime occupational exposure to an
average daily noise level of 85 dBA. This excess risk estimate is lower
than the 16% derived from the 1972-NIOSH model with the 1-2-3-kHz
definition for the same exposure parameters; thus, NIOSH continues to
recommend the exposure limit of 85 dBA, 8-hour TWA.
3.2 CEILING LIMIT
NIOSH  also recommended a ceiling limit of 115 dBA. Exposures to
noise levels greater than 115 dBA would not be permitted regardless of the
duration of the exposure. This ceiling limit is based on the assumption
that above a critical intensity level the ear's response to energy no
longer has a relation to the duration of the exposure, but is only related
to the intensity of the exposure. Recent research with animals indicates
that the critical level is between 115 and 120 dBA [Price and Kalb 1991;
Henderson et al. 1991; Danielson et al. 1991]. Below this critical level,
the amount of PTS is related to the intensity and duration of exposure;
but above this critical level, the relationship does not hold. For a noise
standard to be protective, there should be a noise ceiling level above
which no unprotected exposure is permitted. Given the recent data, 115 dBA
is a reasonable ceiling limit beyond which no unprotected exposure should
The following discussion is summarized from a NIOSH contract report
[Suter 1992a], of which the conclusion is supported by NIOSH.
Health effect outcomes are dependent on exposure level and duration.
For some time, scientists have attempted to identify the relationship
between noise level and duration that will best predict hearing
impairment. Currently, this relationship is called the "exchange
rate," although other terms have been used to describe it, including
the "doubling rate," "trading ratio," and "time-intensity
tradeoff." The most commonly used exchange rates incorporate either 3
dB or 5 dB per doubling or halving of exposure duration.
The 3-dB exchange rate, which is used by the EPA, Great Britain, and
many European countries, is also known as the equal-energy rule or
hypothesis. First proposed by Eldred et al. , it was later supported
and expanded by Burns and Robinson . This hypothesis maintains that
equal amounts of sound energy will produce equal amounts of hearing
impairment, regardless of how the sound energy is distributed in time.
Theoretically, this principle could apply to exposures ranging from a few
minutes to many years. Ward and Turner , however, suggest
restricting its use to the sound energy accumulated in one day only. They
make a distinction between an interpretation of the "total energy"
theory that would allow a whole lifetime's exposure to be condensed into a
few hours, and a restricted "equal-A-weighted-daily-energy"
interpretation of the theory. Burns  also cautions against the
misuse of the equal-energy hypothesis, noting that it was based on data
gathered from individuals who experienced daily 8-hour occupational
exposures for periods of months to years, and thus, extrapolation to very
different conditions would be inappropriate.
On an energy basis, the 3-dB exchange rate provides for the calculation
of a mathematically-valid TWA exposure to noise. This is true when the
general definition of "decibel" is examined. A decibel (dB) is a
dimensionless unit used in physics, which is equal to 10 times the
logarithm to the base 10 of the ratio of two values:
dB = 10*log(value1/value2)
The "value" in the above equation can be one of several types
of measures, including sound intensity (a measure of energy). Based on the
above equation, the relationship between sound intensity and sound
intensity level is defined by the following equation:
Sound Intensity Level (dB) = 10*log(I/Io)
I = sound intensity
Io = reference sound intensity
By this mathematical relationship, every doubling of energy is
represented by an increase of 3 dB in the following calculations:
Let X = the exchange rate
10*log(I/Io) + X = 10*log(2I/Io)
X = 10*log(2I/Io) - 10*log(I/Io)
|= 10*log 2I/Io|
= 3.01 dB
This same relationship does not hold true for the 5-dB exchange rate.
To derive X = 5 dB, the sound intensity would have to be more than doubled
in this equation. Thus, the 5-dB exchange rate does not provide for the
doubling or halving of energy per 5-dB increment.
The 5-dB exchange rate is sometimes called the OSHA rule, and it is
less protective than the equal-energy hypothesis. It attempts to account
for the interruptions in noise exposures that commonly occur during the
work day [40 Fed. Reg. 12,336 (1975)], presuming that some recovery from
TTS occurs during these intermittencies, and the hearing loss is not as
great as it would be if the noise were continuous. The rule itself makes
no distinction between continuous and non-continuous noise, and it will
permit comparatively long exposures to continuous noise at higher sound
levels than would be allowed by the 3-dB rule. Based on the limited data
that existed in the early 1970's, NIOSH  recommended the 5-dB
exchange rate, but after reviewing the more recent scientific evidence,
NIOSH now recommends the 3-dB exchange rate.
The evolution of the 5-dB exchange rate began in 1965 when the National
Academy of Sciences-National Research Council, Committee on Hearing,
Bioacoustics, and Biomechanics (CHABA) issued criteria for assessing
allowable exposures to continuous, fluctuating, and intermittent noise
[Kryter et al. 1966]. The CHABA criteria was an attempt to predict the
hazard from nearly every conceivable noise exposure pattern, based on TTS
experimentation. In the development of its criteria, CHABA used the
1. TTS2 (temporary threshold shift measured 2 minutes after
a period of noise exposure) is a consistent measure of the effects of a
single day's exposure to noise.
2. All noise exposures that produce a given TTS2 will be
equally hazardous (the "equal temporary effect" theory).
3. PTS produced after many years of habitual noise exposures, 8 hours
per day, is about the same as the TTS2 produced in normal ears
by an 8-hour exposure to the same noise.
The CHABA postulates were not validated, and also because TTS proved
not to be a good predictor of permanent hearing damage, criteria based on
TTS patterns could not be relied upon for predicting the long-term adverse
effects of noise exposure. TTS2 is not a consistent measure of
the effects of a single day's exposure to noise, and the PTS after many
years may be quite different from the TTS2 produced at the end
of an 8-hour day. Research has failed to show a significant correlation
between TTS and PTS [Burns and Robinson 1970; Ward 1980], and the
relationships among TTS, PTS, and cochlear damage are equally
unpredictable [Ward 1970; Ward and Turner 1982; Hetu 1982; Clark and Bohne
1978; Clark and Bohne 1986].
CHABA's assumption of the equal temporary effect theory is also
questionable in that some of the CHABA-permitted intermittent exposures
can produce delayed recovery patterns even though the magnitude of the TTS
was within "acceptable" limits and chronic incomplete recovery
will hasten the advent of PTS. The CHABA criteria also assume regularly
spaced noise bursts, interspersed with periods that are sufficiently quiet
to permit the necessary amount of recovery from TTS. Both of these
assumptions fail to characterize noise exposures in the manufacturing
Botsford  published a simplified set of criteria based on the
CHABA criteria, having observed that the CHABA method had proved too
complicated for general use. The Botsford  method assumes that
interruptions will be of "equal length and spacing so that a number
of identical exposure cycles are distributed uniformly throughout the day."
These interruptions would occur during coffee breaks, trips to the
washroom, lunch, and periods when machines are temporarily shut down.
During the same period, there was another parallel, but related,
development that led to the 5-dB exchange rate. Simplifying the criteria
developed by Glorig et al.  and adopted by the International
Organization for Standardization (ISO) , the Intersociety Committee
 published its criteria that consisted of a table showing
permissible exposure levels (starting at 90 dBA) as a function of duration
and the number of occurrences per day. The exchange rates varied
considerably depending on noise level and frequency of occurrence. For
continuous noise with durations less than 8 hours, the Committee
recommended maximum exposure levels based on a 5-dB exchange rate.
In 1968, the Department of Labor proposed a noise standard under the
authority of the Walsh-Healey Public Contracts Act [33 Fed. Reg. 14,258
(1968)]. The proposal contained a permissible exposure limit of 85 dBA for
continuous noise. Exposure to non-continuous noise was to be assessed over
a weekly period according to a large table of exposure indices. The
exchange rate varied according to level and duration; a rate of 2 to 3 dB
was used for long-duration noises of moderate level, and 6 to 7 dB for
short-duration, high-level bursts. This standard was promulgated early in
1969 [34 Fed. Reg. 790 (1969a)], but was withdrawn after a short period.
Later in that same year the Walsh-Healey noise standard that is in effect
today was issued [34 Fed. Reg. 7,948 (1969b)]. In this version, any
special criteria for non-continuous noise had disappeared and the 5-dB
exchange rate became official. Thus, the 5-dB exchange rate appears to
have been the outgrowth of the many simplifying processes that preceded
Although the exact history of the 3-dB rule is not certain, the study
of Burns and Robinson  adds to the credibility of the 3-dB rule,
which has been increasingly supported by national and international
consensus [EPA 1973; EPA 1974; 39 Fed. Reg. 43,802 (1974b); IS0 1971; von
Gierke at al. 1981; IS0 1990; U.S. Air Force 1993; U.S. Army 1994; ACGIH
1995]. The only field study that has been repeatedly cited as supporting
the 5-dB rule is the study of miners by Sataloff et al. .
Data from animal experiments support the use of the 3-dB exchange rate
for single exposures of various levels within an 8-hour day [Ward and
Nelson 1971; Ward and Turner 1982; Ward et al. 1983], but there is
increasing evidence that intermittency can be beneficial, especially in
the laboratory [Bohne and Pearse 1982; Ward and Turner 1982; Ward et al.
1982; Bohne et al. 1985; Bohne et al. 1987; Clark et al. 1987]. However,
these benefits are likely to be smaller or even nonexistent in the
industrial environment, where sound levels during intermittent periods are
considerably higher and where interruptions are not evenly spaced.
Data from a number of field studies correspond well to the equal-energy
hypothesis, as Passchier-Vermeer [1971 and 1973] and Shaw  have
demonstrated. In Passchier-Vermeer's portrayal of the data
[Passchier-Vermeer 1973], the Passchier-Vermeer  and the Burns and
Robinson  prediction models for hearing losses as a function of
continuous-noise exposure level fit the data on hearing losses from
intermittent or varying noise exposures quite well. The fact that
comparisons using the newer ISO standard [ISO 1990] corroborate
Passchier-Vermeer's findings lend additional support to the equal-energy
Some field data from certain occupations, such as forestry and mining,
show less hearing loss than expected when compared with continuous noise
data [Sataloff et al. 1969; Holmgren et al. 1971; Johansson 1973; and INRS
1978], although these findings have not been supported by the two NIOSH
[1976 and 1982] studies of intermittently exposed workers or the analyses
conducted by Passchier-Vermeer  and Shaw . If such a trend
exists, it is further supported by the evidence with experimental animals
that laboratory intermittencies produce a savings over continuous noise
However, the ameliorative effect of intermittency does not support the
use of the 5-dB exchange rate. For example, although Ward  noted
that some industrial studies have shown lower PTS from intermittent noise
exposure than would be predicted by the 3-dB rule, he did not favor
selection of the 5-dB exchange rate as a compromise to compensate for the
effects of intermittency because it would allow single exposures at
excessively high levels. In his opinion, "this compromise was futile
and perhaps even dangerous." [Ward 1970]
One response to the evidence from the animal studies and certain field
studies would be to select the 3-dB exchange rate, but to allow an
adjustment (increase) to the maximum permissible exposure limit for
certain intermittent noise exposures, as suggested by EPA  and
Johansson et al. . This would be in contrast to a 5-dB exchange
rate, for which there is little scientific justification. Ideally, if an
adjustment is needed, the amount of such an adjustment should be
determined by the temporal pattern of the noise and the levels of quiet
between noise bursts. At this time, however, there is little quantitative
information about these parameters in industrial environments. Therefore,
the need for an adjustment should await clarification by further research.
While the 3-dB rule may be somewhat conservative in truly intermittent
conditions, the 5-dB rule will be under-protective in most others. Whether
or not an adjustment is used for certain intermittent exposures, the 3-dB
exchange rate is the method most firmly supported by the scientific
evidence for assessing hearing impairment as a function of noise level and
Impulsive noise is a short burst of acoustic energy characterized by a
rapid rise to a peak sound level followed by a rapid decay. With peak
levels ranging from less than 100 dB to well above 140 dB, burst durations
may vary from several microseconds to several hundred milliseconds. As a
general rule, the burst duration of an impulsive noise is less than 1
second. Coles et al.  defined two basic types of impulsive noise: a
nonreverberant A-type, also known as impulse noise (e.g., gunfire) and a
reverberant B-type, also known as impact noise (e.g., a hammer striking a
The OSHA occupational noise standard [29 CFR 1910.95] states: "Exposure
to impulsive or impact noise should not exceed 140 dB peak sound pressure."
Thus, in this context, the 140-dB limit is advisory rather than mandatory.
This number was first proposed by Kryter et al. , and later
acknowledged by Ward  as "little more than a guess." NIOSH
 did not address the hazard of impulsive noise although it stated
that the provisions of the recommended standard in the criteria document
were "intended to apply for all noise." To date, there is not
yet consensus as to what criteria best describe the relationship between
NIHL and exposure to impulsive noise, either by itself or in the presence
of continuous noise.
In many industrial operations, impulsive noise occurs in a background
of continuous noise. There are at least two approaches to evaluating
exposures to impulsive and continuous noises. One approach, based on the
equal-energy hypothesis, is to integrate impulsive noise with continuous
noise in determining the TWA exposure level. Another approach favors the
evaluation of impulsive noise exposure separate from that of continuous
3.4.1 Impulsive Noise Conforming to the
In 1968, CHABA published damage risk criteria for impulsive noise based
on the equal-energy hypothesis [CHABA 1968]. Over the years, there has
been individual and organizational support for treating impulsive noise on
an equal-energy basis [Coles et al. 1973; EPA 1974; Coles 1980; ISO 1990].
Burns and Robinson  proposed the concept of "immission,"
which is based on the equal-energy hypothesis, to describe the total
energy from a worker's exposure to continuous noise over a period of time
(i.e., months or years). Atherley and Martin  modified this concept
to include impulsive noise in the calculation of the equivalent continuous
noise level (Leq), which is the A-weighted TWA level of
continuous noise equivalent to the same amount of energy contributed by
all noises, including impulsive noise, in an 8-hour workday.
Studying 76 men who were exposed to impact noise in two drop-forging
factories, Atherley and Martin  calculated each man's noise exposure
(immission level) during his employment period, and plotted it against his
age-corrected HTLs over 6 audiometric frequencies. They found that the
observed HTLs of the population came close to the predicted HTLs according
to Robinson , and concluded that the equal-energy hypothesis was
applicable to impact noise. Similarly, Atherley  examined the HTLs
of 50 men exposed to impact noise produced by pneumatic chisels used on
metal castings, and found good agreement between observed and predicted
Guberan et al.  compared the HTLs of 70 workers exposed to impact
noise in drop-forging workshops with the predicted HTLs according to
Robinson  at the 3, 4 and 6 kHz audiometric frequencies. Again, the
observed HTLs were in close agreement with the predicted.
A study of 716 hammer and press operators in seven drop forges by
Taylor et al.  indicates that hearing losses resulting from impact
and continuous noises in the drop-forging industry are as great or greater
than those resulting from equivalent continuous noise. Using noise
dosimetry, Taylor et al.  found that the hammer operators were
exposed to average Leq of 108 dBA, and the press operators, to
99 dBA. The investigators also conducted audiometry for the operators. The
median HTLs of hammer operators of all age groups approximated those
predicted by the Robinson  immission model. The median HTLs of
younger press operators (15 to 34 years of age) also corresponded closely
with the predicted values, but those of older press operators (34 to 54
years of age) were significantly higher than predicted. The results
indicate that, up to certain limits, the equal-energy hypothesis can be
applied to combined exposure to impact and continuous noises.
3.4.2 Impulsive Noise Not Conforming to the
Despite its simplicity, the equal-energy hypothesis as applied to
exposure to combined impulsive and continuous noises is not universally
accepted. In her evaluation of the effects of continuous and varying
noises on hearing, Passchier-Vermeer  found that the HTLs of workers
in steel construction works did not conform to the equal-energy
hypothesis; that is, the hearing losses in these workers, who were exposed
to noise levels with impulsive components, were higher than predicted.
Later studies reported by Ceypek et al. , Hamernik and Henderson
 and Nilsson et al.  also indicated that continuous and
impulsive noises have a synergistic, rather than additive, effect on
Comparing the studies of Passchier-Vermeer  and of Burns and
Robinson , Henderson and Hamernik  suggested that the steeper
slope of Passchier-Vermeer's exposure-response curve at the 4000-Hz
audiometric frequency might have been due to noise exposures which
contained impulsive components, a characteristic not present in the Burns
and Robinson data. Citing the similarity of Passchier-Vermeer's data to
those collected by Taylor and Pelmear  and Kuzniarz et al.  on
workers exposed to impulsive noise environments containing impulses,
Henderson and Hamernik  indicated that exposure to continuous and
impulsive noises in combination may be more hazardous than exposure to
continuous noise alone.
Voight et al.  studied noise exposure patterns in the building
construction industry and related the Leq to audiometric
records of over 81,000 construction workers in Sweden. They found
differences in hearing loss among groups exposed to noise of the same Leq
but with different temporal characteristics. Groups exposed to impulsive
noise had more hearing loss than those exposed to continuous noise of the
Sulkowski and Lipowczan  conducted noise measurement and
audiometric testing in a drop-forge factory. The HTLs of 424 workers in
the factory were compared to the predicted values according to the Burns
and Robinson equation . The observed and the predicted values
differed in that there was a smaller observed hearing loss in the lower
audiometric frequencies but a greater observed hearing loss at the higher
audiometric frequencies. In their study of hearing loss in weavers, who
were exposed to continuous noise, and drop-forge hammermen, who were
exposed to impact noise of equivalent energy to the continuous noise
level, Sulkowski et al.  found that the latter group of workers had
substantially worse hearing than did the former group.
Thiery and Meyer-Bisch  conducted a cross-sectional
epidemiological study at an automobile manufacturing plant. The automotive
workers were exposed to continuous and impulsive noises at Leq
levels ranging from 87 to 90 dBA. When their HTLs were compared to those
of workers exposed to continuous noise at Leq of 95 dBA for
the same exposure time, the automotive workers showed greater hearing
losses at the 6000-Hz audiometric frequency than the reference population
after nine years of exposure.
Starck et al.  compared the HTLs at the 4000-Hz audiometric
frequency of forest workers using chain saws and shipyard workers using
hammers and chippers. The former group was exposed to continuous noise,
and the latter group was exposed to impact noise. Starck et al. 
also used the immission model developed by Burns and Robinson  to
predict the HTLs for both groups. They found that Burns and Robinson's
model was accurate at 4000 Hz for the forest workers, but that it
substantially underestimated the HTLs at 4000 Hz for the shipyard workers.
The studies described above provide evidence to indicate that the
effects of combined exposure to impulse and continuous noises are
synergistic rather than additive as the equal-energy hypothesis would
support. One measure for protecting a worker from such synergistic effects
would be to require that a correction factor be added to a measured TWA
noise exposure level when impulsive components are present in the noise.
The magnitude of such a correction has not been quantified. The matter
becomes more complicated when other parameters of impulsive noise are
considered. It appears that noise energy is not the only factor that
affects hearing. The amplitude, duration, rise time, number of impulses,
repetition rate and crest factor appear to be involved [Henderson and
Hamernik 1976; Starck and Pekkarinen 1987; Pekkarinen 1989]. The criteria
for exposure to impulsive noise based on the inter-relationships of these
parameters await the results of further research.
3.4.3 Combined Exposure to Continuous and
Whether the effects of combined exposure to continuous and impulsive
noises are additive or synergistic, exposure to these noises does cause
hearing loss. In many industrial environments, impulsive noise is often
present with continuous noise. It is also a matter of practicality to
measure all noise levels when different types of noises are present.
Therefore, the criteria for limiting exposure to continuous noise should
be extended to include impulsive noise.
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While a variety of instruments for measuring noise is available today,
the choice of a particular instrument depends on the measurement need.
This discussion is limited to the two most commonly used instruments for
measuring noise exposures, the sound level meter and the noise dosimeter.
More detailed discussions on instrumentation can be found in available
reference sources [NIOSH 1973; Earshen 1986; Johnson et al. 1991].
The sound level meter is the basic measuring instrument for noise
exposures. It consists of a microphone, a frequency selective amplifier,
and an indicator. When it is calibrated, it measures sound pressure level
4.1.1 Frequency Weighting Networks
The definition of sound pressure level makes no reference to sound
frequency. In actuality, the ear does not show equal response to all
frequencies. Most sound level meters have A and C weighting networks or
scales, and some include the B scale also. The frequency characteristics
of these scales are shown in Table 4-1. These scales, which modify sound
pressure level detected by the sound level meter to approximate the ear's
response, are empirically derived. The A scale is commonly used in
measuring noise to evaluate its effect on people and has been incorporated
in many occupational noise standards.
|Octave-Center Frequency, Hertz (Hz)||Weighted Response, decibels (dB)|
*Adapted from ANSI .
4.1.2 Meter Response
The meter response can be set to either SLOW or FAST, corresponding to
the integration periods 1 and 0.125 second, respectively. The indicator
reflects the average sound pressure level measured by the meter during the
period selected. In general, continuous sounds can be measured easily
using either SLOW or FAST response, but fluctuating sounds should be
measured using SLOW response to reduce indicator fluctuations. The
indicator fluctuates less with the longer 1-second integration time of
SLOW response relative to the 0.125-second integration time of FAST
Measuring noise exposure with a sound level meter is relatively simple
when the noise levels are continuous and non-varying and where the worker
remains essentially stationary during the workshift. When the noise levels
are varying or intermittent, or contain impulsive components, and where
the worker moves around frequently during the workshift, the use of a
noise dosimeter to measure a worker's noise exposure is preferred.
The noise dosimeter may be thought of as a sound level meter with an
additional storage and computational function. It measures and stores the
sound levels during an exposure period and computes the readout as the %
dose or TWA. Many dosimeters available today can measure the noise dose or
time-weighted average using various exchange rates (e.g., 3-, 4-, and
5-dB), 8-hour criterion levels (e.g., 80, 84, 85 and 90 dBA) and sound
measurement ranges (e.g., 80 to 130 dBA). The wide selections exist to
accommodate different noise standards. It should be noted that the choice
of FAST or SLOW meter response on the dosimeter does not affect the
computed noise dose or time-weighted average when the 3-dB exchange rate
is used. However, this does not apply when other exchange rates are used.
OSHA requires that, for the purposes of the Hearing Conservation
Amendment, all sound levels from 80 to 130 dBA must be included in the
noise measurements [29 CFR 1910.95 (d)(2)(i)]. This range was specified on
the basis of instrument capabilities available at that time [ANSI 1978],
and OSHA had intended to increase the upper limit of the range to 140 or
150 dB as improved dosimeters became readily available [46 Fed. Reg. 4,135
To measure all sound levels from 80 to 140 dBA, a noise dosimeter
should have an operating range of at least 60 dB and a pulse range of the
same magnitude. In contrast, the ANSI S1.25-1991 standard specifies that
dosimeters should have an operating range of at least 50 dB and a pulse
range of at least 53 dB [ANSI 1991a]. Today, noise dosimeters that have
operating and pulse ranges in excess of 65 dB are quite common. Therefore,
NIOSH considers that measuring all sound levels from 80 to 140 dBA with a
noise dosimeter is technically feasible.
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Whenever potentially hazardous noise exists in the workplace, measures
should be taken to reduce those levels as much as possible, to protect
workers who are exposed to the noise, and to monitor the effectiveness of
these intervention processes. Employers have an obligation to protect
their workers from this debilitating occupational hazard [46 Fed. Reg.
4,078 (1981); 48 Fed. Reg. 9,738 (1983)]. Additionally, research has shown
that there are numerous other benefits of implementing effective hearing
loss prevention programs (HLPPs)(also known as hearing conservation
programs) in the workplace. For example, Cohen  found reduced
employee absenteeism following the establishment of a hearing conservation
program. Similarly, Schmidt et al.  reported a reduction in
workplace injuries following the introduction of a hearing conservation
program. Alternatively, other reports have documented detrimental
non-auditory effects of noise such as decreased productivity in high noise
environments [Noweir 1984; Suter 1992b]. It has also been suggested that
the employer who effectively protects his workers' hearing may also reap
the economic benefits of lower worker's compensation rates because of
fewer claims for noise-induced hearing loss.
NIOSH recommends that HLPPs be implemented for all workers whose
unprotected (i.e., without the use of hearing protectors) 8-hour TWA
exposures equal or exceed 82 dBA (i.e., 1/2 of the REL)3
and that the programs include at least the following components [NIOSH
1. Initial and annual audits of procedures
2. Assessment of noise exposures
3. Engineering or administrative control of noise exposures
4. Audiometric evaluation and monitoring of workers' hearing
5. Use of hearing protectors for exposures greater than 85 dBA,
regardless of exposure duration
6. Education and motivation of workers
8. Program evaluation for effectiveness
Implementation of a HLPP must hinge on the fact that occupational
noise-induced hearing loss is 100% preventable. The key to developing and
implementing an effective program lies in a commitment by both management
and workers to truly prevent hearing loss [Helmkamp et al. 1984].
Achievement of this end is facilitated by integrating the HLPP into the
company's overall health and safety program [Berger 1981; NIOSH 1995].
This gives the prevention of hearing loss the same weight as the
prevention of other work-related illnesses and injuries, thus indicating
to workers and management that occupational hearing loss is something to
be taken seriously. Other factors which facilitate an effective HLPP
include encouraging workers to carry over their good hearing conservation
practices to off-the-job situations; using simple, clearly-defined
procedures; making compliance with the HLPP a condition of employment; and
incorporating safety requirements into written company policy. Today,
there is no legitimate reason for any worker to incur an occupational
hearing loss [NIOSH 1995].
Responsibility for developing and implementing a HLPP usually resides
with a team of professionals. The American Occupational Medical
Association (AOMA)  identifies the team approach to hearing
conservation as necessary for its success. The number of team members and
their professional disciplines may vary with the kind of company and the
number of noise-exposed workers; however, members frequently include
audiologists, physicians, occupational health nurses, occupational hearing
conservationists, engineers, industrial hygienists, safety professionals,
management representatives, and employee/union safety representatives.
Regardless of whether program responsibility resides in a team or a
single individual, one person should act as champion for the program and
must hold overall responsibility for its implementation [NIOSH 1995;
Royster and Royster 1990]. This individual will be referred to in this
document as the program implementor. The program implementor should ensure
that all aspects of the program are fully and properly administered, and
should enlist the support of management and workers to actively prevent
hearing loss. Royster and Royster  recommend that the primary
qualification of the program implementor should be a genuine interest in
preserving workers' hearing. AOMA  recommends that the program
implementor be a physician. NIOSH  maintains that the professional
discipline of the program implementor is not as important as his/her
ability to act as the "champion" of the HLPP by focusing
management and worker attention on hearing conservation issues.
Additionally, the program implementor's stature in the organization should
allow him/her to make decisions, correct deficiencies, enforce compliance,
and supervise other team members with regard to the program.
In addition to the program implementor, there should be an individual
specifically responsible for the audiometric aspects of the HLPP; this
individual will be referred to in this document as the audiometric
manager. The professional qualifications of this individual are critical.
The audiometric manager should be an audiologist or a physician
specializing in otological or occupational medicine. The overall program
implementor and the audiometric manager may be the same person, provided
that the individual meets the qualifications for both positions. If the
overall program implementor and the audiometric manager are not the same
person, the audiometric manager should report to the overall manager,
regardless of the professional credentials of either party.
Ideally, an initial audit should be conducted before a HLPP is
implemented or any changes are made to an existing program. This audit
will serve as a basis for comparison against which the effectiveness of an
improved program can be assessed. The audit should begin by examining
administrative issues such as corporate responses to health and safety
regulations, official policies promoting good safety and health practices,
assurance of adequate resources to conduct the program, and the status
within the company of the program implementor. Current engineering and
administrative controls should be evaluated, and the systems for
monitoring noise exposures and conducting audiometry should be critically
examined. Employee and management training should be noted, with past
successes and failures analyzed so that improvements can be made. In
particular, if engineering and administrative controls are insufficient,
it should be noted whether effective training is provided in the
selection, fitting, and daily use of hearing protectors. Recordkeeping
procedures should receive a meticulous inspection, since how the records
concerning audiometry, noise exposure, and other aspects of the overall
program are maintained can greatly influence the success or failure of a
program. NIOSH recommends that a HLPP audit should be conducted annually
as a part of an overall program evaluation so that the strengths of the
program may be clearly identified, and any weakness promptly addressed
Section 6(b)(7) of the Occupational Safety and Health Act of 1970 [29
USC 651 et seq.] directs that, where appropriate, occupational health
standards shall provide for monitoring or measuring employee exposure at
such locations and intervals in such a manner as may be necessary for the
protection of employees. Accurate characterization of the noise hazard
present in the workplace and the subsequent identification of affected
workers are both extremely important. These two elements form the basis
for all subsequent actions within the HLPP [NIOSH 1995]. Monitoring
procedures should be specifically defined to ensure consistency.
Instrumentation, calibration, measurement parameters, and methods for
linking results to worker records should be clearly delineated. Exposure
assessment should be done during typical production cycles; however, if
noise levels vary significantly during different phases of production,
then exposures should be assessed separately for each phase [Royster and
Royster 1990; NIOSH 1995].
Exposure assessment should be conducted by an industrial hygienist, an
audiologist, or other professional with appropriate training [NIOSH 1995].
Workers should be permitted and encouraged to observe and participate in
monitoring activities, insofar as such observation or participation does
not interfere with the monitoring procedure. Their participation will help
ensure valid results, as the workers frequently have the experience to
identify the prevailing noise sources, to indicate periods when noise
exposure may differ, and to recognize whether given noise levels are
typical or atypical. They can explain how different operating modes affect
equipment sound levels, and describe worker tasks and positions. Having
the cooperation of workers is also critical in order to ensure that
workers do not advertently or inadvertently interfere with obtaining valid
measurements. The initial exposure monitoring can serve as an introduction
to the HLPP by raising the awareness of workers and management regarding
noise as a hazard. The monitoring survey, if conducted cooperatively, can
help establish a rapport that will assist in obtaining the cooperation of
both employees and essential management in later phases of the program
[Royster and Royster 1990; NIOSH 1995].
The frequency with which noise exposure assessments are updated depends
upon a number of variables. These might include the intensity of the
noise, potential changes in exposure due to changes in equipment or
production, the rate of significant threshold shift noted among workers,
other changes noted in additional measures of program effectiveness, the
requirements of various governmental regulations, directives from the
Bureau of Workers' Compensation, union contract stipulations, and specific
company policies [Royster et al. 1986].
In general, NIOSH  recommends that exposure monitoring be
repeated at least bi-annually. Monitoring should be repeated sooner if
there is a change in production, process, equipment, or personnel which
might affect exposure levels [Royster et al. 1986; Royster and Royster
1990; NIOSH 1995].
Workers should be notified of the noise exposure level determined for
their particular job, and the relative risk that such an exposure poses to
their hearing. This information should also be cross-referenced to
individual worker records. Notification should include a description of
the specific hazardous noise sources in the worker's area, the purpose and
proper use of any noise control devices, and requirements for hearing
protectors, if appropriate. This notification can be incorporated into the
worker training program [Royster and Royster 1990; NIOSH 1995].
Noise contour maps should be posted and readily available for the
entire facility, so that workers may be aware of the noise levels in other
areas. Also, warning signs should be posted on the periphery of noise
areas [Royster and Royster 1990; NIOSH 1995]. The warning signs should
include a requirement that hearing protectors be worn in the area, and a
supply of several types of hearing protectors should be readily
accessible. Signs should be printed in English and in the predominant
language of the workers who do not read English.
For occupational hearing loss prevention, NIOSH defines engineering
control as "any modification or replacement of equipment, or related
physical change at the noise source or along the transmission path (with
the exception of hearing protectors) that reduces the noise level at the
employee's ear" [NIOSH 1995]. Typical mechanisms for engineering
noise controls include reducing noise at the source (installing a
muffler), altering the noise path (building an acoustic enclosure or
barrier), reducing reverberation (covering walls with sound-absorbing
materials), and reducing equipment vibration (installing vibration
mounts). Engineering controls should be the first order of protection from
excessive noise exposure [Suter 1986; AOMA 1987]. When the noise can be
reduced or eliminated through engineering controls, so also is the danger
to hearing. Any reduction in noise level - even if only a few
decibels - serves to make the noise hazard more manageable, reduces the
risk of hearing loss, improves communication, and lowers annoyance and
related extra-auditory problems associated with high noise levels [NIOSH
1995]. Furthermore, when the noise can be reduced to acceptable levels
through engineering controls, employers may forego some of the additional
difficulties and expenses related to providing hearing protectors,
education and motivation programs, and program evaluation [Royster and
To reduce noise in an existing facility, it is generally necessary to
retrofit engineering controls. Development of these controls should
involve engineers, safety and/or industrial hygiene personnel, and the
workers who operate, service, and/or maintain the equipment. Development
of special noise control measures must be predicated upon a thorough
assessment of the noise source and individual worker exposure.
Consideration should be given to the relative contribution of each noise
source to the overall sound levels. Various noise control options should
be evaluated on the basis of their effectiveness, cost, technical
feasibility, and implications for the equipment's use, service, and
maintenance. Other potential complications of new noise control measures
(such as effects on lighting, heat production, ventilation, and
ergonomics) should be considered [NIOSH 1995]. Engineering controls must
always take into consideration proper maintenance of equipment.
Additionally, the function and purpose of any planned or existing
engineering controls should be fully discussed with the workers, so that
they support the controls and do not inadvertently interfere with them
Management should also consider noise reduction in planning for new or
remodeled facilities. Engineering controls can be most effective when they
are incorporated into the design and purchase of equipment from the start.
Additionally, the cost of incorporating engineering controls during the
design phase is generally much lower than retrofitting them at a later
date. By substituting more sound-absorbent materials, modifying equipment
structure or mechanical process, and/or isolating sources within the
equipment, substantial reductions can be made in the ultimate noise level
A "buy-quiet" policy for new equipment acquisitions should be
adopted by management [Royster and Royster 1990; Brogan and Anderson 1994;
NIOSH 1995]. Haag [1988b] describes a four-part process which management
can implement in order to have an effective buy-quiet policy. The process
includes selection of specific products or operations to be targeted for
noise reduction through new purchases, setting criteria for new equipment
noise levels, requesting noise level specifications from manufacturers,
and including this noise level data in bid evaluation. Again, input from
workers should be incorporated in the buying process.
When engineering controls are inadequate, supplemental administrative controls may be utilized to help limit exposures. Administrative controls are defined as changes in the work schedule or operations which reduce worker noise exposures. For example, sometimes workers can be scheduled so that their time in a noisy environment is minimized. When extremely noisy operations are unavoidable, they might be scheduled during the shift with the fewest number of protected employees. So administrative controls are best used for workers for whom hearing protectors alone may not be sufficient because workers still are exposed to hazardous noise (protected level greater than 85 dBA). Finally, a quiet, clean, and conveniently located lunch and break area should be provided to facilitate employees periodically gaining relief from workplace noise.
Audiometric evaluation of workers' hearing is crucial to the success of
a HLPP, since it is the only way to actually determine whether
occupational hearing loss is being prevented. Because occupational hearing
loss occurs gradually, affected employees often will not notice a change
in hearing ability until a relatively large change in their hearing
sensitivity has occurred. The annual comparison of audiometric tests can
trigger prompt hearing loss program interventions, initiating protective
measures and motivating employees to prevent further hearing loss.
Audiometry should be conducted by an audiologist or by an occupational
hearing conservationist certified by the Council for Accreditation in
Occupational Hearing Conservation (CAOHC). All testing should be
supervised by an audiologist, an otologist, or an occupational physician.
Occupational hearing conservationists should follow the training
guidelines proposed by the National Hearing Conservation Association
(NHCA) . Use of microprocessor-based or self-recording audiometers
should not waive the qualification requirements for the tester.
For audiometric testing to be beneficial, management must allocate
sufficient time and resources to allow for timely and accurate testing.
The testing must be conducted carefully to ensure the integrity of the
audiometric data. Effective communication and coordination are critical
among company personnel, health service providers, and employees.
Audiometry should minimally consist of pure-tone air-conduction
threshold testing of each ear at 500, 1000, 2000, 3000, 4000, 6000, and
8000 Hz. While this entire frequency range is not utilized in the
assessment of OSHA=s
Astandard threshold shift@,
all of these frequencies are important in deciding the probable etiology
of a hearing loss. Sufficient time should be taken to conduct the test
accurately. Testing too quickly sacrifices accuracy and gives the worker
the impression that audiometry and the HLPP are unimportant [NIOSH 1995].
Audiograms are displayed and stored as tables or charts of hearing
thresholds measured in each ear at specified test frequencies. In
OSHA-mandated hearing conservation programs, thresholds must be measured
for pure tone signals at the test frequencies of 500, 1000, 2000, 3000,
4000, and 6000 Hz [29 CFR 1910.95(h)(1)]. At each frequency, the threshold
recorded for an ear is the lowest signal output level of the audiometer at
which the individual responds in a specified percentage of trials (such as
50%) or in 2 of 3 trials. Thresholds are measured in decibels hearing
threshold level (dB HTL) with 0 dB HTL representing average hearing
ability for young people without any otological pathology. Larger
threshold values indicate poorer-than-average hearing, while smaller
threshold values (negative thresholds such as -5 or -10 dB HTL) indicate
An individual's audiometric threshold at a given test frequency is not
an invariant quantity. Measurement variability is associated with the
state of the subject (including the subject's prior audiometric
experience, attention, motivation, the influence of upper respiratory
problems, drugs, and other factors) and with the testing equipment and
methodology [Morrill 1986]. The higher the measurement variability, the
more difficult it is to distinguish actual changes in hearing threshold.
Noise exposure increases hearing thresholds, resulting in threshold
shifts toward higher values (poorer hearing). Occasionally, exposure to
extremely intense noise may cause an immediate permanent hearing loss
known as acoustic trauma. Most often, exposure to less intense noise
causes the gradual development of hearing damage over months and years.
During each overexposure to noise the ear develops a temporary reduction
in sensitivity called temporary threshold shift (TTS). This TTS recovers
over a period of hours or days if the ear is allowed to rest in a quieter
environment. However, if the exposure is high enough or if exposures are
repeated, the TTS may not recover completely, and a permanent threshold
shift (PTS) begins to develop.
It is because TTS serves as a precursor of PTS, that NIOSH recommends
conducting monitoring audiometry on noise-exposed workers at the end of or
late in their daily work shifts. Discovering and taking action to prevent
further TTS will result in more thorough worker protection from permanent
hearing damage. If the annual monitoring audiometry is performed at the
beginning of work shifts or before the work day begins, temporary
threshold shifts which might have been present from the previous day=s
noise exposure will have resolved - any threshold shifts observed will
represent permanent shifts in hearing. This type of audiometric monitoring
will serve only to document the development of permanent hearing loss, not
to prevent it.
Some reports have indicated that industrial audiometry is too variable
to be useful in detecting initial threshold shifts [Hetu 1979; Atherley
and Johnston 1981]. Certainly, if testing procedures are too inconsistent,
then TTS or PTS may not be distinguishable from measurement variability.
The challenge is to select a criterion for significant threshold shift
which is stringent enough to detect incipient hearing loss, yet not so
stringent as to identify large numbers of workers whose thresholds are
simply showing normal variability.
This challenge is compounded by the fact that the incipient PTS may
manifest itself with the same order of magnitude as typical audiometric
measurement variability - about 10 dB change in hearing thresholds.
However, daily TTS could be larger in magnitude than the developing PTS,
so testing workers near the end of their work shifts (when TTS may be
present) increases the probability of identifying workers who are not
adequately protected from noise.
In 1972, a significant threshold shift criterion was initially
recommended by NIOSH [NIOSH 1972]. In 1992, the performance of this
criterion [NIOSH 1972] against 5 other "significant threshold shift"
criteria, was examined by Dr. Julia Royster under contract with NIOSH
[Royster 1992]. The following threshold shift criteria were evaluated:
1. OSHA STS (Standard Threshold Shift): a change of 10 dB or
more in the average of hearing thresholds at 2000, 3000 and 4000 Hz in
2. AAO-HNS SHIFT: a change of 10 dB or more in the average of hearing
thresholds at 500, 1000 and 2000 Hz, or at 3000, 4000 and 6000 Hz in
3. 1972 NIOSH SHIFT: a change of 10 dB or more at 500, 1000, 2000 or
3000 Hz; or 15 dB or more at 4000 or 6000 Hz in either ear
4. 15-dB SHIFT: a change of 15 dB or more at any test frequency from
500 through 6000 Hz in either ear
5. 15-dB TWICE: a shift of 15 dB or more at any test frequency from 500
through 6000 Hz in either ear, which is present in one annual audiogram
and is persistent at the same frequency in the same ear on the next
6. 10-dB AVG. 3-4 kHz: a shift of 10 dB or more in the average of
thresholds at 3000 and 4000 Hz in either ear
The study methodology, database characteristics, and results are
described in detail in the Royster  report. This study compared each
of the above six criteria for threshold shifts by applying each criterion
to 15 different industrial hearing conservation databases which were
contributed to ANSI S12 Working Group 12 (S12/WG12).
Within each database, analyses were restricted to the first eight
audiograms for male workers who had at least eight tests. The numbers of
workers included from each database ranged from 39 to 1056. Data were
analyzed for a total of 2903 workers.
A significant threshold shift for a worker, according to the three
non-averaging, any frequency shift criteria (1972 NIOSH shift, 15-dB
shift, and 15-dB twice shift), was considered a true positive if the shift
was confirmed by the succeeding audiogram, but only if the worker's next
test showed persistence of the shift for at least one of the same
frequencies in the same ear. For example, if a worker's test 3 showed a
1972 NIOSH SHIFT at 2000, 4000 and 6000 Hz in the left ear, then the shift
would be confirmed as a true positive if test 4 results showed the shift
to be persistent in the left ear at one or more of these same frequencies.
For the three criteria calculated on frequency averages (i.e., OSHA STS,
AAO-HNS SHIFT, and 10-dB AVG. 3-4 kHz), a shift was considered a true
positive if the worker's next audiogram showed a change by that same
criterion whether or not the confirming shift occurred in the same ear
and/or the same frequency range (applicable to AAO-HNS). In other words,
the original shift could be counted as confirmed not only by a persistent
shift in the same ear at the same frequency average, but also by a new
shift in the other ear at any frequency average.
The data for classifying true positives from all 15 databases are
presented in Table 5-1. The 15-dB TWICE criterion yields 70.9% true
positive tags, while the other criteria all yield between 44.4% and 46.1%
true positive tags.
|Criterion||Number of Classifiable Tags||Number of Positive Tags||Percent of Positive Tags|
|1972 NIOSH SHIFT||2268||1045||46.1|
|10-dB AVG. 3-4 kHz||1175||524||44.6|
*Adapted from Royster .
None of the shift criteria evaluated are best in every respect. The
relative merits of each criterion are tabulated in Table 5-2. An
acceptable criterion should be able to promptly identify a worker with any
measurable threshold shift at the most noise-sensitive audiometric
frequencies, and should tag a reasonably high number of true positives.
Relative to the "any one frequency" criteria, those criteria
that average thresholds at two or more audiometric frequencies (i.e., OSHA
STS, AAO-HNS SHIFT, and 10-dB AVG. 3-4 kHz) yield lower numbers of tags
with lower percentages of true positives [Royster 1992].
The 15-dB TWICE criterion requires that a threshold shift persist on
two tests before the worker is identified or "tagged" for
meeting the criterion of significant threshold shift, resulting in a high
percentage of true positives. However, this criterion could be misused if
the second test was not administered until a year later.
The 1972 NIOSH SHIFT, which shares with 15-dB TWICE the advantage of
not requiring any frequency-averaging, uses such a small amount of shift
at 500-3000 Hz (only 10 dB) that it tags many audiograms that reflect
normal testing variability. Thus, the 1972 NIOSH SHIFT tags so many
workers that it loses its usefulness as a problem identifier. This
disadvantage can be partially overcome by increasing the amount of
required shift to 15 dB (the 15-dB SHIFT); however, too many workers are
still tagged by a one-time 15-dB SHIFT to allow any meaningful follow-up.
|OSHA STS||15dB TWICE||10-dB AVG 3-4 kHz||AAO-HNS SHIFT||15-dB SHIFT||1972 NIOSH SHIFT|
|- tags a moderate percentage of workers||X||X||X|
|- gives highest percentage true positive tags||X|
|- tags workers earliest||X|
|- shows largest differences between control databases and non-control databases||X||X|
|- no calculation of frequency-averages required||X||X||X|
|- averages noise-frequencies separately or examines each frequency separately||X||X||X||X||X|
|- tags the lowest percentage of workers||X|
|- tags such a high percentage of workers that follow-up impractical||X||X|
|- tags workers early in fewer cases||X|
|- requires calculations of frequency averages||X||X||X|
|- averages low frequencies that are unlikely to be affected by noise exposure||X|
|- averages together frequencies which vary in susceptibility to noise||X|
|- uses a shift magnitude within the range of normal audiometric variability||X|
*Adapted from Royster .
The ideal significant threshold shift criterion should tag workers with
temporary threshold shifts before they develop into permanent hearing
impairment. On the basis of the data analysis presented by Royster ,
NIOSH now recommends a modified 15-dB TWICE criterion requiring that a
test be followed by an immediate retest if a 15 dB change in threshold is
noted at any frequency. The value of two back-to-back tests was observed
by Rink , who reported that performing an immediate retest reduced
the proportion of workers meeting the OSHA Standard Threshold Shift
criterion by more than 70%. Thus, if an annual audiogram indicates a 15-dB
shift or more in either ear at any one of the test frequencies (500, 1000,
2000, 3000, 4000 or 6000 Hz), the worker should be reinstructed, the
earphones should be refitted, and a retest should be administered. If the
retest shows the same results (i.e., 15-dB shift or more in the same ear
and at the same frequency), the 15-dB TWICE criterion for a significant
threshold shift has been met and the worker should be rescheduled for a
confirmation test within 30 days.
With this recommendation in mind, and to provide maximum protection for
employees and maximum documentation for employers, NIOSH advocates that
audiograms be performed on the following occasions:
1. Pre-employment (baseline).
2. Prior to initial assignment into a hearing hazard work area.
3. Annually as long as the employee is assigned to a noisy job with an
exposure level equal to or greater than 85 dBA (monitoring audiometry).
Annual testing may lead to a number of retests if a significant threshold
shift occurs. Additionally, it may be a good practice to provide
audiometry twice a year to workers exposed to more than 100 dBA, because
the most susceptible ten percent of a population who are exposed to daily
average noise levels of 100 dBA with inadequate hearing protectors could
be expected to develop significant hearing loss well before the end of one
year [NIOSH 1995].
4. At the time of reassignment out of a hearing hazard job.
5. At the termination of employment.
Baseline audiogram: The baseline audiogram should be obtained within 30 days of enrollment in the HLPP [NIOSH 1972]. It should be preceded by a minimum of 14 hours of unprotected quiet. Use of hearing protectors should not be considered as a substitute for an actual 14 hour quiet period. Use of a mobile testing service should not waive these requirements. It is unacceptable to wait up to a year, as permitted by OSHA [29 CFR 1910.95], for a mobile service to conduct a baseline audiogram because permanent hearing loss can occur within relatively short periods of time (months or even days in susceptible individuals), especially when high levels of noise are involved [ISO 1990]. If a mobile service cannot meet these time constraints, other arrangements should be made to obtain the baseline audiograms before or promptly after employment.
Monitoring audiograms: Monitoring audiometry should be
conducted no less than annually. Unlike baseline audiometry, these annual
tests should be scheduled at the end of, or well into, the work shift so
that temporary changes in hearing due to insufficient noise controls
and/or inadequate use of hearing protector will be noted. The results
should be compared immediately to the baseline audiogram to check for any
change in hearing sensitivity. The collection of audiograms for batch
comparison to baseline audiograms at a later date in another location is
an unacceptable practice, because it does not afford the opportunity to
conduct retests or to discuss the findings with employees in a timely
Retest audiograms: Audiometry should be repeated immediately
after any monitoring audiogram which indicates a threshold shift of 15 dB
or more at 500, 1000, 2000, 3000, 4000 or 6000 Hz in either ear. The
worker must be re-instructed and the headphones refitted before conducting
Confirmation audiograms: Audiometry should be conducted again
within 30 days of any retest audiogram which continues to show a
significant threshold shift. A minimum of 14 hours of quiet should precede
the confirmation audiogram in order to determine whether the shift is
temporary (TTS) or indicative of a permanent change in hearing sensitivity
(PTS). The use of hearing protectors as a substitute for an actual quiet
environment is not acceptable. Confirmation audiograms indicating
persistent threshold shifts should trigger written notification to the
worker and a referral to the audiometric manager for review and
determination of probable etiology. In this review, all possible
etiologies in addition to occupational noise should be explored, including
age-related hearing loss, familial hearing loss, medical history,
non-occupational noise exposure, etc. [Franks et al. 1989; Stepkin 1993].
Workers showing a threshold shift determined by the audiometric manager to
have any etiology other than noise should be counseled by the manager and
referred to their physician for evaluation and treatment. Workers should
also be referred if they meet any of the otologic or medical criteria
recommended by the American Academy of Otolaryngology-Head and Neck
Surgery [AAO-HNS 1983]. Appropriate action should be triggered for workers
showing a threshold shift determined by the audiometric manager to have a
probable etiology of occupational noise exposure. Actions should minimally
include re-instruction and refitting of hearing protectors, additional
training in worker responsibilities for effective hearing loss prevention,
and/or reassignment to quieter work areas. The audiometric manager should
be responsible for making whatever recommendations he/she feels are
necessary, and for seeing that they are carried out.
Exit audiogram: Audiometry should be conducted when a worker
leaves employment or is permanently rotated out of an occupational noise
exposure at or above 82 dBA, 8-hour TWA. This exit audiogram, like the
baseline, should be performed after a minimum of 14 hours of quiet. The
use of hearing protectors as a substitute for quiet is not acceptable.
It is also suggested that hearing testing be offered as a health
benefit to workers who are not exposed to hazardous noise levels. In
addition to providing a valuable internal control group for comparison to
the noise-exposed workers, this policy elevates the perceived importance
of the HLPP for management and workers [NIOSH 1995].
Audiometers should minimally conform to the specifications of the
appropriate ANSI standard [ANSI S3.6-1995] for Type 4 audiometers, with
the additional stipulation that they have the capacity for testing at 8000
Hz. Type 5 audiometers, which only test to 70 dB HTL, are unacceptable for
threshold testing within an occupational HLPP.
Audiometers must be kept in calibration for the audiograms to have any
value. An audiometer must receive a functional check (sometimes called a
biologic check) each day the instrument is used [Morrill 1986; NIOSH
1995]. This type of calibration check involves obtaining an audiogram from
an individual with known, stable thresholds and verifying that there have
been no changes in HTL that exceed 10 dB. A bioacoustic simulator check
may be substituted for this procedure. Additionally, the audiometer
attenuator and frequency selection dials should be cycled through while
carefully listening for any extraneous noise or distortion which might
interfere with testing. The earphone cords should be manipulated to check
for any unwanted static or noise. A check for "crosstalk"
(presence of the test signal in the non-test earphone) should be made,
using an attenuator setting of 60 dB HTL [Morrill 1986].
An acoustic calibration check should be performed whenever the
functional check indicates a threshold difference exceeding 10 dB in
either earphone at any frequency. An acoustic calibration includes checks
of output levels, attenuator linearity, and frequency. If the sound
pressure levels differ by more than the allowable variances specified by
ANSI 3.6-1995 (or its successor), or if the attenuator linearity differs
by more than 1 dB, or if frequency drift exceeds 3%, an exhaustive
calibration is necessary [Morrill 1986].
An exhaustive calibration check should be conducted annually or
whenever an acoustic calibration indicates the need for such. An
exhaustive calibration includes adjusting the audiometer so that it is in
compliance with all specifications of ANSI S3.6-1995 (or its successor)
and must be done by an audiometer service technician. It is best to have
exhaustive calibrations performed on-site. If the audiometer must be
shipped out for this service, then an acoustic calibration should be
conducted upon its return to ensure that calibration changes did not occur
during shipping [Morrill 1986].
The audiometric test area should conform to the ambient noise
requirements of ANSI S3.1-1991 [ANSI 1991b]. For permanent, on-site test
areas, ambient noise levels should be checked at least annually. For
mobile test areas, ambient noise levels should be checked daily or at each
new site, whichever is more frequent. Ambient noise levels should be
checked with a calibrated sound level meter placed in the test environment
at the approximate position the worker's head will occupy during the test
procedure. Some bio-acoustic simulators have the capability of measuring
ambient noise levels; this is acceptable provided that the unit is placed
near the area of the worker's head. All audiometric test equipment, as
well as lights, heaters, air conditioners, etc., should be set as they
would be during actual testing. The ambient noise levels should also be
measured during audiometric testing, and they should be recorded in a log
through which they can be traced for each audiogram obtained.
NIOSH  defines a hearing protector as "anything that can be
worn to reduce the level of sound entering the ear." Hearing
protectors are discussed more fully in Chapter 6, but a few brief points
should be made here. Hearing protectors are subject to many problems, and
should be considered the last resort against hazardous noise. Berger
 identified seven reasons why hearing protectors can fail to provide
adequate protection in real-world situations: discomfort, poor
utilization, poor fit, incompatibility with other safety equipment,
dislodging, deterioration, and abuse. Additionally, hearing protectors
generally provide greatest protection at high frequencies and
significantly less protection from low frequency noise [Berger 1986].
Nevertheless, hearing protectors can work as a short term solution
to prevent noise-induced hearing loss if their use is carefully planned,
evaluated, and supervised [Berger 1986; Royster and Royster 1990; NIOSH
1995; Franks and Berger 1997].
On November 21, 1983, OSHA promulgated an occupational safety and
health standard entitled "Hazard Communication" [29 CFR
1910.1200]. Under the provisions of this standard, employers in the
manufacturing sector must establish a comprehensive hazard communication
program that includes, at a minimum, container labeling, material safety
data sheets, and a worker training program. The hazard communication
program is to be written and made available to workers and their
designated representatives. Although the Hazard Communication standard
does not specifically address occupational noise exposure, the intent of
the standard to inform workers of health hazards should apply. Annual
training should be provided to employees exposed to noise levels of 82 dBA
TWA or above. Workers must be informed of the possible consequences of
noise exposure and of the various control methods available to protect
their hearing. When a HLPP is implemented, workers should be informed of
the provisions of the program and of the benefits of their full
participation in the program.
The success of a HLPP depends in large measure on effective worker
education regarding all aspects of the program. In his review of the
hearing conservation literature, Berger  suggests that there are
several keys to a successful program: support from management, enforcement
of safety policies, education and motivation of the workers, and
comfortable and effective hearing protectors. All of these issues depend
to some degree on a well-constructed, thorough program of educating and
training everyone who is involved in the HLPP.
Obviously, the primary focus of the training component of the HLPP is
on the workers. Workers need to be informed about the reasons for and the
requirements of the HLPP at the time that they are enrolled in it. The
education process should be ongoing, and highlighted by periodic programs
focusing on one or more particular aspects of the program. Furthermore, to
be optimally effective, education should be tailored to the specific
exposure and prevention needs of each worker or group of workers, rather
than generic. Education and training will be easily dismissed unless it
can be related to each worker's day-to-day functions [Berger 1981]. Worker
education should cover all relevant aspects of the hearing conservation
program. At a minimum, the following topics should be included [AOMA 1987;
Royster and Royster 1990; NIOSH 1995]:
Despite the emphasis on employee training, management also needs to be
educated about the need for and elements of the HLPP. Strong management
support is critical to an effective HLPP [AOMA 1987]. This support must be
more than just implicit approval of company hearing loss prevention
policies; it must be an outward, active show of approval and compliance
with the established policies. It must be clearly evident to lower
management, foremen, and the workers. Management needs to know the basics
of the legal and professional requirements for effective hearing loss
prevention, as well as the administrative requirements for compliance and
the liability consequences of non-compliance. Motivation of upper
management may be heightened by emphasizing the possible financial
benefits of an effective HLPP on worker's compensation costs, improved
productivity, and worker retention [Royster and Royster 1990].
In addition to the workers and managers, members of the hearing loss
prevention team need to be educated regarding company policy for the
program and their role within it. They must receive appropriate training
to enable them to successfully fulfill their duties. This is especially
important for those who will be responsible for fitting hearing protectors
and training workers in their proper use [Royster and Royster 1990]. If
there is a hierarchy of responsibility within the program's team, each
member should know his/her place in it. Consultants, including physicians
or audiologists who conduct follow-up examinations, should also be
well-informed about the company's hearing loss prevention policies. This
will help prevent recommendations or decisions which might conflict with
established company policy [Royster and Royster 1986].
Choice of educational and motivational strategies is critical to the
success of the training phase of the HLPP. The techniques used as well as
the content selected for presentation must be tailored to the particular
needs of the audience [Royster and Royster 1990].
For all groups involved, an effective training program requires that
there be both episodic and ongoing educational opportunities. The most
useful opportunity for episodic training of the workers occurs at the time
of each worker's annual monitoring audiogram. During this time, the worker
is most interested in his/her hearing status, and recommendations will
have the most relevance. Time should be taken immediately after testing to
explain the results of the hearing test, its relationship to the worker's
baseline audiogram, and its implications for the adequacy of the worker's
hearing protector usage. Stable hearing should be praised in order to
reinforce the worker's proper use of noise controls and hearing
protectors, and hearing shifts should result in a sincere warning about
the need for more consistent use of appropriate hearing
protectors. The worker must be given the opportunity to ask questions
about his/her role in the HLPP, and should be encouraged to discuss
hearing protector difficulties, etc. [Royster and Royster 1986].
Other opportunities for episodic training also exist. Information about
specific company policies, results of bi-annual noise exposure monitoring,
overviews of the effect of noise on hearing, and related topics should be
periodically addressed in special training sessions or regularly planned
safety meetings. These training sessions should not be limited to showing
a film, but should be personally presented by an educator who is
knowledgeable about hearing conservation and has an interesting
presentation style. Group size should be small enough to permit
interaction with the speaker and among the workers. Content should be
varied and continually updated [Royster and Royster 1986; NIOSH 1995].
In addition to these episodic training sessions, there should be an
ongoing educational process. HLPP personnel, especially the program
implementor, should take the time to go out to the workers' job sites and
see how they are doing. They should talk to workers about the program when
they meet them in the halls, at lunch, etc. Posters, bulletin boards,
informational pamphlets, etc. can be used as a constant reminder of the
importance the company places on hearing conservation. Contests or awards
for effective hearing conservation practices can be used to promote safety
behavior [Royster and Royster 1986; Royster and Royster 1990]; however,
incentive programs should be planned and implemented with full worker
participation or they may be perceived by the workers as manipulative
attempts by management to control workers' behavior [Merry 1995].
Recordkeeping involves creating and maintaining documents on each
aspect of the HLPP. This documentation is more than just an exercise in
paperwork or computer data entry. It provides the only compelling evidence
that the HLPP components were properly, consistently, and thoroughly
conducted. Program records are often needed many years after they are
collected; and if it cannot be established that they are valid, then the
records are useless. Clearly, documentation needs to be viewed as one of
the most critical aspects of a HLPP [Gasaway 1985].
HLPP records are medical records and should be treated with the same
degree of integrity and confidentiality. The recordkeeping system should
be compatible with the company's general health and safety record system.
The company should keep copies of all records, even if a contractor is
used to collect the data [NIOSH 1995]. Additionally, each worker's noise
exposure records, audiometric records, hearing protection records, and
training participation records should be cross-referenced so that
information regarding one program component can be readily linked with
information regarding all other program components for that worker. Such
cross-referencing is critical to building a total hearing history and
establishing the probable etiology of any hearing loss should a claim ever
be filed [Gasaway 1985; NIOSH 1995].
5.8.1 Noise Exposure Records
Noise exposure records should include the worker's name, identification
number, job code, job description, department, and similar related
information such as the current noise exposure level, the date of the last
exposure assessment, the monitoring method used, and the name of the
person who did the monitoring [NIOSH 1995]. The employee's record should
also include the previous noise exposure history. It is useful to include
both calculated exposure levels and the raw data from which the
calculations were made [Royster et al. 1986].
Noise exposure records should be kept for a minimum of 30 years. This
is the length of time OSHA requires employers to keep other industrial
hygiene records [29 CFR 1910.20]. However, it may be prudent to keep noise
exposure records even longer. Royster et al.  recommend that
exposure records be kept for the length of employment plus thirty years.
Employers might also consult the Bureau of Workers' Compensation for their
state. Most states have a statute of limitations for filing a claim for
occupational hearing loss; however, some states do not [ASHA 1992].
Prudence dictates a check with state regulations to be certain that
records are kept until it is determined that there will be no further use
for them [Royster et al. 1986].
5.8.2 Audiometric Records
Audiometric records should include the worker's name, identification
number, gender, date of birth, and a self-reported worker history. The
history should include medical information which may have an impact on
hearing status, history of past occupational or military noise exposure,
and types of non-occupational noise exposure [Helmkamp 1984; NIOSH 1995].
Occupational exposure to potentially ototoxic chemicals should also be
recorded [Rybak 1992]. Morrill recommends a brief "high risk"
history, which can be readily taken by a technician; this history can then
be used as a framework for a more detailed history, as necessary, if the
worker is ever referred to an audiologist or physician for further
evaluation [Morrill 1986]. The more detailed the history, of course, the
more accurately the audiometric manager will be able to determine the
actual etiology of any threshold shifts.
For each audiometric examination, the test date, time, and hours since
the worker's last noise exposure should be recorded. Audiometric
thresholds at all required frequencies should be recorded. The
audiometer's make, model, and serial number should be noted; as well as
the dates of the last exhaustive calibration, the last acoustic
calibration, the last functional check, and the last check of room ambient
noise levels. Additionally, the identity of the tester, and the tester's
subjective assessment of test reliability should be recorded [NIOSH 1995].
Any time that a significant threshold shift is documented, the etiology
determined by the audiometric manager should be recorded. Also, all
follow-up actions that were taken should be documented [Gasaway 1985].
Audiometric test results and etiologies of any confirmed shifts should
be kept for the length of employment plus thirty years, which is the OSHA
requirement for worker health records [29 CFR 1910.20]. Other related
records - calibration records, ambient noise level checks, etc. - should
be minimally kept for five years. However, remembering that audiometric
records are only as valid as documentation indicates, it may be prudent to
keep all supporting records for as long as the thresholds themselves are
maintained [Gasaway 1985].
5.8.3 Hearing Protection Records
Hearing protection records should include the types of hearing
protectors used, including make, model, and size as relevant. Records
should also be kept to document training the workers have received in the
proper fitting and use of protectors, and any records that may have been
made documenting the consistency of compliance with requirements for
wearing hearing protectors [NIOSH 1995]. Hearing protection records should
be kept for a minimum of 30 years.
5.8.4 Education Records
Education records should include date and type of training provided,
who conducted the training, and attendance (if training was a group
program) [NIOSH 1995]. Education and training records should also be kept
for a minimum of thirty years.
5.8.5 Other Records
Other necessary records might include documentation of periodic audits,
exposure assessments, plans for engineering and administrative controls,
and results of overall program evaluations [NIOSH 1995]. These records,
and any other documentation relevant to the HLPP, should be kept a minimum
of thirty years.
The effectiveness of a HLPP should be evaluated in terms of the hearing
losses prevented for each worker and the overall rate of hearing loss in
the population of workers. This evaluation should occur on a continual
5.9.1 Individual Effectiveness
The effectiveness of the HLPP in preserving workers' hearing is best
evaluated through audiometric monitoring of each noise exposed worker. All
workers whose noise exposure meets or exceeds 82 dBA should receive
audiometric testing at no cost to the worker at the intervals noted
previously under audiometric evaluation. Comparison of a current audiogram
with the baseline audiogram will permit the audiometric manager to assess
the adequacy of the program elements for that particular worker. Thus,
each audiogram serves as a marker of the effectiveness of the hearing loss
prevention effort for that individual worker. Any apparent changes in
hearing signal a possible failure in the program.
5.9.2 Overall Program effectiveness
To assess the effectiveness of the HLPP from an overall programmatic
level, it is necessary to have an evaluation method that can monitor
trends in the population of workers enrolled in the program and thus
indicate program effectiveness before many individual shifts occur. This
evaluation has two parts. The first part evaluates the internal integrity
of the audiometric data. Currently, there is a draft ANSI standard which
details a method for such an evaluation - Draft ANSI S12.13-1991, "Evaluating
the Effectiveness of Hearing Conservation Programs" [ANSI 1991c].
This standard is based on an assumption that year-to-year variability in a
population's hearing thresholds reflects the adequacy of the audiometric
monitoring program. High variability in sequential thresholds is viewed as
indicative of inadequate control of audiometric test procedures,
audiometric calibration problems, or poor recordkeeping. Low variability
in sequential thresholds is viewed as indicative of a well-controlled
program producing results which may be relied upon for accuracy and
The second part of the program evaluation involves comparing the rate
of threshold shift among noise-exposed workers to that of persons not
exposed to occupational noise. Toward this end, Melnick  evaluated
the efficacy of several different methods. The first method was based on
the OSHA estimation that a noise exposed population that was in compliance
with the current noise regulations would still demonstrate a prevalence of
hearing loss (defined as thresholds exceeding 25 dB at the frequencies of
500, 1000, and 2000 Hz) up to 10% greater than a non-noise exposed
population by the time workers reached retirement (later OSHA calculations
have revised this estimate to be 10-15%). This method has the obvious
disadvantage of delaying evaluation of the HLPP until a number of workers
have reached retirement age, and by then, improvements to the HLPP will be
too late to prevent their loss of hearing.
Another method involves evaluating the effectiveness of the overall
program on the basis of the percentage of workers showing significant
threshold shifts. Ideally, the criterion percentage of significant
threshold shifts could be based on a control group (i.e.,
non-noise-exposed) within the same company. Others who have
investigated the possibility of percent significant threshold shift as an
evaluation criterion have reported that 3-6% [Morrill and Sterrett 1981],
or 5% significant threshold shifts [Franks et al. 1989; Simpson et al.
1994] are reasonable incidence rates which can be met by effective
programs. Significant threshold shift incidence rates exceeding these
percentages might then be considered evidence of a deficient program. A
disadvantage of this technique is that it does not account for the effects
of other variables (e.g., age, gender, race, previous noise exposure
history) that might differentially affect the significant threshold shift
incidence rates if the noise and non-noise populations differ
substantially along these variables. Another disadvantage is that this
technique does not differentiate possible causes of program deficiencies.
Problems could be as likely due to poor audiometry as to excessive noise
exposure [Melnick 1984; Simpson et al. 1994].
An alternative evaluation method, used by Pell  in evaluating the
effectiveness of the hearing conservation program at DuPont, involves a
longitudinal analysis of the rate of increased hearing loss (10th, 50th,
and 90th percentiles) as a function of age for three classes of worker
noise exposure: quiet (<85 dBA), low noise (85-94 dBA), and high noise
(95 dBA or greater). Pell judged his hearing conservation program
effective by demonstrating that the rate of increase of hearing loss with
respect to age did not significantly differ among the three noise
categories. This system, however, requires that all workers receive annual
audiometric evaluations, regardless of whether or not they are noise
exposed. Also, because some individuals are susceptible to hearing loss at
the action level of 85 dBA, it would be preferable to define the "quiet"
group as those exposed to less than 80 dBA.
The U.S. Army Center for Health Promotion and Preventive Medicine
(CHPPM) (formerly the U.S. Army Environmental Hygiene Agency) evaluates
its HLPPs by rating each element and sub-element of the program on a 5
point scale ranging from maximally compliant to non-compliant. Total
points are added across the sub-elements to achieve a score for that
program element; and then a total score is computed for the overall
program. There are well-defined criteria for scoring the sub-elements, but
the program evaluator is also given some flexibility in assigning ratings.
Such a system is helpful in that it defines strict criteria for every
aspect of the program which must be met in order to have a fully
successful program. However, some of the currently-used criteria are not
perfect, as CHPPM has found several highly rated HLPPs to have
unacceptably high incidences of significant threshold shifts [Byrne and
In general, NIOSH suggests that the success of a smaller HLPP should be
judged by the audiometric results of individual workers. An overall
program evaluation becomes critical when the number of workers grows so
large that one cannot simply look at each worker's results and get an
adequate picture of the program's efficacy. At the present time, there is
not one generally accepted method for the overall evaluation of HLPPs.
Furthermore, there is not one method which stands out as being superior to
the rest. Therefore, at this time, NIOSH recommends considering a
significant threshold shift incidence rate of 5% or less as evidence of an
effective HLPP [Morrill and Sterrett 1981; Franks et al. 1989; Simpson et
al. 1994]. The 5% criterion method is currently the simplest procedure
available, and has no more disadvantages than other potential evaluation
NIOSH does not recommend that age correction be applied to an
individual's audiogram for significant threshold shift calculations.
Although many people experience some decrease in hearing sensitivity with
age, age correction cannot be accurately applied to audiograms in
determining an individual's significant threshold shift because the data
on age-related hearing losses describe only the statistical distributions
in populations. Thus, the median hearing loss attributable to presbycusis
for a given age group will not be generalizable to the presbycusis
experienced by an individual in that age group. Furthermore, the
age-correction tables developed in the 1972 criteria document [NIOSH
1972], and subsequently included in the 1983 OSHA Hearing Conservation
Amendment to the Occupational Noise Standard [48 Fed. Reg. 9,738 (1983)],
were based on a cross-sectional study. Thus, the age corrections were
estimated by calculating trends as a function of the age of each member of
the sample. When data from a cross-sectional study are used, the inherent
assumption is that a subject who was 20-years-old in 1970 can be expected
to experience the same hearing loss due to age by 2000 as a 50 year-old
subject had experienced due to age in 1970. This assumption may not be
valid as the general health and societal noise exposures of each
generation are likely to be different.
The adjustment of audiometric thresholds for aging has become a common
practice in workers' compensation litigation. In this application, age
corrections result in a reduction in the amount of hearing loss which is
considered attributable to noise exposure, with a consequent reduction in
the amount of compensation paid to workers for their hearing losses.
However common and regardless of the extent to which "age correcting"
has been and is applied, it is technically inappropriate to apply
population statistics to an individual in this manner. Each age-correction
number is nothing more than a median value from a population distribution.
In age correcting an audiogram, the underlying assumption is that the
individual value is given the 50th percentile, when in fact a different
value such as the 10th percentile or the 90th percentile may be correct.
Thus, one cannot apply age correction formulas to determine with certainty
how much of an individual's hearing loss is due to age and how much of it
is due to noise exposure.
It is even less appropriate to "age correct" audiograms
obtained as part of an occupational HLPP. The purpose of the program is to
prevent hearing loss. If an audiogram is "age corrected",
regardless of the source of the correction values, the time it takes for a
significant threshold shift to be noted will be prolonged. Delaying the
tagging of a worker with a significant threshold shift is completely
contrary to the purpose of a HLPP.
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The hearing protector is a device which covers or fills the ears so
that the sound reaching the ear drum is attenuated. How much attenuation a
hearing protector provides is dependent upon the characteristics of the
hearing protector and how the worker wears it. Thus, the selected hearing
protector should be capable of reducing the noise exposure at the ear to
below 82 dBA, 8-hour TWA.
Hearing protectors should not be the sole or primary means by which
worker noise exposures are reduced. Hearing protectors should be used only
when engineering controls and work practices are not feasible for reducing
noise exposures, or during the implementation of engineering controls.
Hearing protectors are a temporary solution to the problem of preventing
NIHL. The permanent solution is to remove the hazardous noise from the
workplace or to remove the worker from the hazardous noise.
In general, hearing protectors should be used by workers who are
exposed to noise exceeding 85 dBA, regardless of duration, and should be
an integral part of a HLPP. The hearing protector is a form of noise
control although it is the least preferable of the noise control methods.
It alters the sound path by attenuating the noise at the ear of the
worker. It should be recognized that the effectiveness of hearing
protectors may vary widely from worker to worker. As such, each worker
wearing hearing protectors needs to be monitored to see that the hearing
protectors are being worn correctly, are fitted properly and are
appropriate for the noise in which they are worn [Helmkamp et al. 1984;
Gasaway 1985; Berger 1986; Royster and Royster 1990; NIOSH 1995].
Hearing protectors should be selected for use in the HLPP with
considerations made for the noises in which they will be worn, the workers
who will be wearing them, the need for compatibility with other safety
equipment, and work environmental conditions such as temperature,
humidity, and atmospheric pressure [Gasaway 1985; Berger 1986].
Hearing protectors should have adequate attenuation to provide
protection from the noise to which workers will be exposed using the
methods recommended later in this chapter. Hearing protectors should
reduce the noise level at the ear to below 82 dBA, 8-hour TWA, thus
reducing the likelihood of occupational NIHL [Gasaway 1985; Berger 1986;
A variety of styles should be provided so that workers may select the hearing protector on the basis of comfort, ease of use, ease of handling, and impact on communication [NIOSH 1995; Royster and Royster 1990]. The best hearing protector is the one the worker will wear all of the time because it is comfortable, effective, and has minimal impact on communication. What is the best hearing protector for some workers may not be the best for others [Casali 1990]. The most common reasons why workers won't use hearing protectors correctly are discomfort and interference with speech communication [Berger 1980; Helmkamp 1986].
Hearing protectors should not be relied upon when the noise exposure
level is such that noise under the protector cannot be estimated to be
less than 82 dBA, 8-hour TWA. If the condition exists that use of hearing
protectors cannot reduce noise adequately, then workers should not be sent
into that type of environment. The use of double protection may also be
considered [Gasaway 1985; Berger 1986].
Each worker should receive individual training in the selection,
fitting, use, repair and replacement of the hearing protector [Gasaway
1985; Royster and Royster 1990; NIOSH 1995]. Workers should not be allowed
to provide their own hearing protectors. Devices such as hearing aid
earmolds, swim molds, and personal stereo earphones should never be
considered as being hearing protective.
One consideration for selecting a hearing protector would be its noise
reduction capabilities, which are expressed in terms of a noise reduction
rating (NRR). The NRR is a single-number rating which is required by law
to be shown on the label of each hearing protector sold in the United
States [40 CFR 211].
The NRR is intended to be used to calculate the exposure under the
hearing protector by subtracting it from the environmental noise exposure
level. OSHA has prescribed six methods4
with which the NRR can be used [29 CFR 1910.95, Appendix B]. These methods
vary according to the instrumentation used to determine the noise exposure
levels, and can be summarized in two basic formulas:
When dBA level is known:
When dBC level is known:
NRRs and other hearing protector ratings are based on data from listeners in psychophysical experimental conditions. As such, there are many variables which may impact upon the ratings, making the predicted noise reduction different from what the worker would actually experience.
Use of statistical descriptors to select protectors for one employee: In an effort to account for the variance in attenuation for a given protector from person to person, the decision has been made to use the mean attenuation with a correction for variance (some constant times the standard deviation of the mean). In the case of the NRR, the mean attenuation adjusted by two standard deviations is used. However, there is no practical way of knowing how much attenuation a worker may experience with the protector or how his or her experience will vary from day to day and from fitting to fitting.
In the late 1970's and early 1980's, two NIOSH field studies found that
insert-type hearing protectors in the field provided less than one-half
the attenuation measured in the laboratory [Edwards et al. 1978; Lempert
and Edwards 1983]. Since the 1970's, additional studies of "real-world"
attenuation with hearing protectors have been conducted [Regan 1975;
Padilla 1976; Abel et al. 1978; Fleming 1980; Crawford and Nozza 1981;
Chung et al. 1983; Edwards et al. 1983; Hachey and Roberts 1983; Goff and
Blank 1984; Behar 1985; Mendez et al. 1986; Smoorenburg et al. 1986;
Edwards and Green 1987; Pekkarinen 1987; Pfeiffer et al. 1989; Hempstock
and Hill 1990; Berger and Kieper 1991; Casali and Parks 1991]. In general,
these studies involved testing the hearing thresholds of occluded and
unoccluded ears of subjects who wore the hearing protectors for the test
in the same manner as on the job. The tests were performed as an attempt
to simulate the actual conditions in which hearing protectors are normally
used in the workplaces. In Table 6-1, the NRRs
derived from these real-world attenuation data were compared with the
manufacturers' labelled or laboratory NRRs. The laboratory NRRs
consistently overestimated the real-world NRRs by 140% to 2000%. In
general, the data show that earmuffs provide the highest real-world
attenuation values, followed by foam earplugs, and all other insert-type
devices provide the least attenuation. From these results, it can also be
concluded that ideally workers should be fit tested for hearing protectors
individually. Currently, NIOSH is developing feasible methods for this
type of fit testing.
OSHA  has instructed its compliance officers to derate the NRR by 50% in enforcing the engineering control provision of the OSHA noise standard. However, because of the wide variation of real-world NRRs among different types of hearing protectors, and as an interim measure, NIOSH recommends that the labelled NRRs be derated by 25%, 50% and 70% for earmuffs, formable earplugs and all other earplugs, respectively. These percentages are estimates based on Table 6-1.
|Foam||E-A-R/Decidamp||Crawford and Nozza (1981)||58||29||19|
|Foam||E-A-R/Decidamp||Hachey and Roberts (1983)||31||29||9|
|Foam||E-A-R/Decidamp||Lempert and Edwards (1983)||56||29||12|
|Foam||E-A-R/Decidamp||Edwards and Green (1987)||28||29||19|
|Foam||E-A-R/Decidamp||Edwards and Green (1987)||28||29||14|
|Foam||E-A-R/Decidamp||Lempert and Edwards (1983)||56||29||5|
|Foam||E-A-R/Decidamp||Abel et. al. (1978)||55||29||9|
|Foam||E-A-R/Decidamp||Abel et al. (1978)||24||29||9|
|Foam||E-A-R/Decidamp||Pfeiffer et al. (1989)||69||29||10|
|Foam||E-A-R/Decidamp||Casali and Park (1991)||10||29||6|
|Foam||E-A-R/Decidamp||Casali and Park (1991)||10||29||23|
|Foam||E-A-R/Decidamp||Hempstead and Hill (1990)||72||29||13|
|Foam||E-A-R/Decidamp||Berger and Kieper (1985)||22||29||20||12.5||13.2|
|Premolded||Ultra-Fit||Casali and Park (1991)||10||21||4|
|Premolded||Ultra-Fit||Casali and Park (1991)||10||21||17|
|Premolded||Ultra-Fit||Royster et al. (1984)||19||21||5|
|Premolded||Ultra-Fit||Berger and Kieper (1985)||29||21||3||5.8||7.3|
|Premolded||V-51R||Royster et al. (1984)||12||23||3|
|Premolded||V-51R||Abel et al. (1978)||20||23||2|
|Premolded||V-51R||Edwards et al. (1978)||84||23||1|
|Premolded||Com-Fit||Abel et al. (1978)||18||26||7||4.9||4.5|
|Premolded||EP100||Crawford and Nozza (1981)||22||26||0|
|Premolded||EP100||Edwards et al. (1978)||28||26||-2|
|Premolded||EP100||Abel et al. (1978)||45||26||10|
|Premolded||EP100||Smoorenburg et al. (1986)||46||26||-2||2.1||1.5|
|Premolded||NA||Mendez et al. (1986)||30||NA||1||1.0||1.0|
|Fiberglass||Down||Lempert and Edwards (1983)||28||15||4|
|Fiberglass||Down||Edwards et al. (1978)||56||15||3||3.3||3.5|
|Fiberglass||POP||Lempert and Edwards (1983)||28||22||4|
|Fiberglass||POP||Pfeiffer et al. (1989)||51||22||7|
|Fiberglass||POP||Mendez et al. (1986)||30||22||10|
|Fiberglass||POP||Hempstock and Hill (1990)||39||22||8||7.7||7.8|
|Fiberglass||Soft||Hachey and Roberts(1983)||36||26||1|
|Fiberglass||Soft||Pfeiffer et al. (1989)||12||26||9|
|Fiberglass||Soft||Hempstock and Hill (1990)||32||26||4||3.4||4.7|
|Custom||Adcosil||Hachey and Roberts (1983)||44||24||4|
|Custom||NA||Crawford and Nozza (1981)||7||NA||7|
|Custom||Prtctear/vent||Lempert and Edwards (1983)||56||11||8|
|Custom||Peackeeper||Lempert and Edwards (1983)||56||15||4|
|Custom||NA||Abel et al. (1978)||48||NA||3|
|Semi-aural||Sound-Ban #10||Behar (1985)||32||17||10|
|Semi-aural||Sound-Ban #20||Casali and Park (1991)||10||19||6|
|Semi-aural||Sound-Ban #20||Casali and Park (1991)||10||19||12||9.6||9.3|
|Earmuffs||Bilsom UF-1||Hachey and Roberts (1983)||31||25||13|
|Earmuffs||Bilsom UF-1||Casali and Park (1991)||10||25||16|
|Earmuffs||Bilsom UF-1||Casali and Park (1991)||10||25||20|
|Earmuffs||MSA Mk IV||Abel et al. (1978)||47||23||11|
|Earmuffs||MSA Mk IV||Goff and Blank (1984)||15||23||4|
|Earmuffs||Optac 4000||Pfeiffer et al. (1989)||33||NA||14|
|Earmuffs||Peltor H9A||Pfeiffer et al. (1989)||34||22||14|
|Earmuffs||Rcal AGd III||Hempstock and Hill (1990)||42||NA||19|
|Earmuffs||Norseg||Mendez et al. (1986)||30||NA||8|
|Earmuffs||AO 1720||Goff and Blank (1984)||11||21||6|
|Earmuffs||Bilsom 2450||Pfeiffer et al. (1989)||11||NA||13|
|Earmuffs||Clark E805||Abel et al. (1978)||17||23||15|
|Earmuffs||Glendale 900||Goff and Blank (1984)||10||21||10|
|Earmuffs||Optac 4000s||Pfeiffer et al. (1989)||10||NA||14|
|Earmuffs||Safety 208||Abel et al. (1978)||15||22||12|
|Earmuffs||Safety 204||Behar (1985)||9||21||22|
|Earmuffs||Welsh 4530||Regan (1975)||5||25||20|
|Earmuffs||Safir E/ISF||Hempstock and Hill (1990)||20||NA||14|
|Earmuffs||Misc.||Chung et al. (1983)||64||24||18||13.8||13.8|
|Cap Muffs||Bilsom 2313||Hempstock and Hill (1990)||37||23||16|
|Cap Muffs||Hlbrg No Nse||Abel et al. (1978)||58||23||11|
|Cap Muffs||Peltor H7P3E||Behar (1985)||36||24||13|
|Cap Muffs||AO 1776K||Behar (1985)||26||21||14|
|Cap Muffs||Hlbrg 26007||Hempstock and Hill (1990)||20||NA||18|
|Cap Muffs||Misc.||Chung et al. (1983)||37||23||17||14.3||14.8|
|Plug+Muff||E-A-R + UF-1||Hachey and Roberts (1983)||25||25.0||25.0|
*Adapted from Berger et al. 
N = size of test population
NRR = labelled Noise Reduction Rating
Wght = weighted on the basis of test population size
The best hearing protection for any worker is removal of hazardous
noise from the workplace. Until that happens, the best hearing protector
for a worker is the one he or she will wear willingly and consistently.
The following factors are extremely important determinants of the workers'
acceptance of hearing protectors: comfort; adequate noise reduction; ease
of fit; communication enhancement; durability and repairability; and
proportion of time worn.
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The following research is needed to further reduce the risk of NIHL in the workplace.
Research is needed to reduce noise exposures through engineering controls in situations where the noise exposures are still being controlled primarily by hearing protectors. A hearing loss prevention program is complex and difficult to manage effectively, the need for which can be obviated by noise control procedures that reduce noise levels to less than 82 dBA. As important as applying new technologies such as active noise reduction are, it is equally important to apply traditional noise control engineering concepts to the building of new facilities and equipment. Research also is needed to improve retrofitting noise control to existing operations. A database of effective solutions should be created and made publicly accessible.
Research is needed to define the hazardous parameters of impulsive noise and their interrelationships. These parameters should include amplitude, duration, rise time, number of impulses, repetition rate and crest factor. In the absence of any other option, impulsive noise is integrated with continuous noise to determine hazard. While laboratory research with animals and some retrospective studies of workers exposed to impulsive noise in the presence or absence of other noises indicate that impulsive noise is more hazardous to hearing than continuous noise of the same spectrum and intensity, there are not sufficient data to support the development of damage risk criteria for impulsive noises.
Research is needed to define dose―response relationships between noise and nonauditory effects, such as hypertension and psychological stress. Studies of hypertension conducted to date on noise exposed workers have established a relationship between hypertension and NIHL, but have not established a relationship between noise exposure and hypertension. Workplace accidents need to be analyzed to determine whether noise interference with oral communication or audio alarms was a contributing factor. Technologies need to be developed that allow easy identification of warning signals and efficient communication in noisy environments but that also provide effective protection of hearing.
The ototoxic properties of industrial chemicals and the interaction between them and noise have been investigated only for a few substances. Research with laboratory animals to investigate the range of chemicals known to be ototoxic or neurotoxic is needed to appraise the risk of hearing loss from exposures to these chemicals, alone or in combination with noise. Research to support damage risk criteria for combined exposure is needed.
NIOSH was a pioneer in developing an exposure monitoring strategy for air contaminants based on the application of statistical methods [NIOSH 1977]. However, the appropriateness of the strategy for occupational noise exposure was not determined, and not much research has been done on this question since 1977. Limited studies have indicated that a different strategy for monitoring occupational noise exposure may be required [Behar and Plenar 1984; Henry 1992]. It is important that workers' exposures to noise be accurately monitored and that appropriate control measures be implemented when necessary. Several individuals and organizations have proposed different approaches to monitoring noise exposures [Behar and Plenar 1984; CSA 1986; Royster et al. 1986; AIHA 1991; Henry 1992; Simpson and Berninger 1992; Stephenson 1995]. NIOSH acknowledges the contributions of these individuals and organizations to this important subject, and encourages continued effort in the development of exposure monitoring strategies applicable to occupational noise exposure. An important component of HearSf 2000 that is being co―developed by NIOSH, the United Auto Workers―Ford National Joint Committee on Health and Safety, Hawkwa Group, and James, Anderson and Associates is noise monitoring with emphasis on noise exposure characterizations based on the principles of task―based exposure assessment. Research is needed to determine the most important factors for determining the hazard to hearing.
The attenuation of hearing protectors as they are worn in the occupational environment is usually quite different from that realized in the laboratory. The manufacturer's labeled NRRs, which are currently used by OSHA in determining compliance with the permissible exposure limit when engineering controls are being implemented or not feasible, usually do not reflect actual experiences. Thus, there is a continued need to research and develop a laboratory method that will best estimate the noise attenuation obtained with hearing protectors worn in the field. Research should also lead to the development of hearing protectors that provide increased comfort and improved communication. Just as important as laboratory methods to produce results with predictive value are the development of worksite test methods to determine the amount of protection each worker receives from his or her hearing protection. Additionally, as new technologies such as active level dependancy and active noise reduction are introduced into personal hearing protection, methods need to be developed to describe the effectiveness of these methods alone and when built into passive hearing protectors.
Research is needed in using behavioral survey tools as resources for developing training and educational programs that address worker's beliefs, attitudes and intentions regarding hearing loss prevention with the purpose of enhancing the effectiveness of training and education. To date, research in training and motivation, when conducted at all, has been focused on materials and their delivery with the worker considered as the passive receptacle. Research is needed to develop materials and programs that involve the worker in the process and give the worker ownership of the information.
Several methods for evaluating the effectiveness of a HLPP was
discussed in Chapter 5 of this document. There is currently not one
generally accepted method which stands out as being superior to the rest.
Further research and development of methods for evaluating the
effectiveness of HLPPs is needed, and the method deemed to have the best
balance between accuracy and ease of use should be adopted. All existing
methods rely upon the results of audiometric testing for evaluating
effectiveness of the hearing loss prevention program. While audiometric
data are crucial for managing the program for each worker, such that if no
workers develop occupational hearing loss the program is considered to be
100% effective, too much time must pass for a database of audiograms to be
built that can support queries about overall program effectiveness.
Methods that do not rely on audiograms, such as observed behaviors that
effect the success of a program or questionnaire―type survey
instruments that evaluate workers' beliefs and intents, need to be
considered as vehicles for immediate assessment of program effectiveness.
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A - E / F - J / K - O / P - R / S - V / W - Z
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