Occupational Noise Exposure
Revised Criteria 1998
June 1998
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DHHS (NIOSH) Publication No. 98-126
In the Occupational Safety and Health Act of 1970 (Public Law 91-596), Congress declared that its purpose was to assure, so far as possible, safe and healthful working conditions for every working man and woman and to preserve our human resources. In this Act, the National Institute for Occupational Safety and Health (NIOSH) is charged with recommending occupational safety and health standards and describing exposure concentrations that are safe for various periods of employment—including but not limited to concentrations at which no worker will suffer diminished health, functional capacity, or life expectancy as a result of his or her work experience. By means of criteria documents, NIOSH communicates these recommended standards to regulatory agencies (including the Occupational Safety and Health Administration [OSHA]) and to others in the occupational safety and health community.
Criteria documents provide the scientific basis for new occupational safety and health standards. These documents generally contain a critical review of the scientific and technical information available on the prevalence of hazards, the existence of safety and health risks, and the adequacy of control methods. In addition to transmitting these documents to the Department of Labor, NIOSH also distributes them to health professionals in academic institutions, industry, organized labor, public interest groups, and other government agencies.
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 has revised some of its previous recommendations. The 1998 recommendations go beyond attempting to conserve hearing by focusing on preventing occupational noise-induced hearing loss (NIHL).
The NIOSH recommended exposure limit (REL) for occupational noise exposure (85 decibels, A-weighted, as an 8-hour time-weighted average [85 dBA as an 8-hr TWA]) was reevaluated using contemporary risk assessment techniques and incorporating the 4000-hertz (Hz) audiometric frequency in the definition of hearing impairment. The new risk assessment reaffirms support for the 85-dBA REL. With a 40-year lifetime exposure at the 85-dBA REL, the excess risk of developing occupational NIHL is 8%—considerably lower than the 25% excess risk at the 90-dBA permissible exposure limit (PEL) 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 (TWA) exposures to noise. However, NIOSH now recommends 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: an increase of 15 dB in the hearing threshold level (HTL) at 500, 1000, 2000, 3000, 4000, or 6000 Hz in either ear, as determined by two consecutive audiometric tests. The new criterion has the advantages of a high identification rate and a low false-positive rate. In comparison, the criterion NIOSH recommended in 1972 has a high false-positive rate, and the OSHA criterion (called the standard threshold shift) has a relatively low identification rate.
In contrast with the 1972 criterion, the new NIOSH criterion no longer recommends age correction on individual audiograms. This practice is not scientifically valid and would delay intervention to prevent further hearing losses in workers whose HTLs have increased because of occupational noise exposure. OSHA currently allows age correction only as an option.
The noise reduction rating (NRR) is a single-number, laboratory-derived rating that the U.S.Environmental Protection Agency (EPA) requires to be shown on the label of each hearing protector sold in the United States. In calculating the noise exposure to the wearer of a hearing protector at work, OSHA derates the NRR by one-half for all types of hearing protectors. In 1972, NIOSH recommended the use of the full NRR value; however, in this document, NIOSH recommends derating by subtracting from the NRR 25%, 50%, and 70% for earmuffs, formable earplugs, and all other earplugs, respectively. This variable derating scheme, as opposed to OSHAs straight derating scheme, considers the performances of different types of hearing protectors.
This document also provides recommendations for the management of hearing loss prevention programs (HLPPs) for workers whose noise exposures equal or exceed 85 dBA. 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 noise standard will minimize the risk of developing occupational NIHL.
Linda Rosenstock, M.D., M.P.H.
Director, National Institute for
Occupational Safety and Health
Centers for Disease Control and Prevention
This criteria document reevaluates and reaffirms the recommended exposure limit (REL) for occupational noise exposure established by the National Institute for Occupational Safety and Health (NIOSH) in 1972. The REL is 85 decibels, A-weighted, as an 8-hr time-weighted average (85 dBA as an 8-hr TWA). Exposures at or above this level are hazardous.
By incorporating the 4000-Hz audiometric frequency into the definition of hearing impairment in the risk assessment, NIOSH has found an 8% excess risk of developing occupational noise-induced hearing loss (NIHL) during a 40-year lifetime exposure at the 85-dBA REL. NIOSH has also found that scientific evidence supports the use of a 3-dB exchange rate for the calculation of TWA exposures to noise.
The recommendations in this document go beyond attempts to conserve hearing by focusing on prevention of occupational NIHL. For workers whose noise exposures equal or exceed 85 dBA, NIOSH recommends a hearing loss prevention program (HLPP) that includes exposure assessment, engineering and administrative controls, proper use of hearing protectors, audiometric evaluation, education and motivation, recordkeeping, and program audits and evaluations.
Audiometric evaluation is an important component of an HLPP. To provide early identification of workers with increasing hearing loss, NIOSH has revised the criterion for significant threshold shift to an increase of 15 dB in the hearing threshold level (HTL) at 500, 1000, 2000, 3000, 4000, or 6000 Hz in either ear, as determined by two consecutive tests. To permit timely intervention and prevent further hearing losses in workers whose HTLs have increased because of occupational noise exposure, NIOSH no longer recommends age correction on individual audiograms.
Foreword
Abstract
Abbreviations
Glossary
Acknowledgments
1 Recommendations for a Noise Standard
3 Basis for the Exposure Standard
4 Instrumentation for Noise Measurement
5 Hearing Loss Prevention Programs (HLPPs)
5.5.1 Audiometry
5.5.2 Audiometers
5.6 Use of Hearing Protectors (Component 5)
5.7 Education and Motivation (Component 6)
5.8 Recordkeeping (Component 7)
5.9 Evaluation of Program Effectiveness (Component 8)
5.10 Age Correction
| AAO-HNS | American Academy of Otolaryngology-Head and Neck Surgery |
| AIHA | American Industrial Hygiene Association |
|
ANSI |
American National Standards Institute |
| AOMA | American Occupational Medical Association |
| ASHA | American Speech-Language-Hearing Association |
| CAOHC | Council for Accreditation in Occupational Hearing Conservation |
| CFR | Code of Federal Regulations |
| CHABA | Committee on Hearing, Bioacoustics, and Biomechanics |
| CI | confidence interval |
| dB | decibel(s) |
| dB SPL | decibel(s), sound pressure level |
| dBA | decibel(s), A-weighted |
| EPA | U.S. Environmental Protection Agency |
| Fed. Reg. | Federal Register |
| HLPP | hearing loss prevention program |
| hr | hour(s) |
| HTL | hearing threshold level |
|
Hz |
hertz |
| ISO | International Standards Organization |
| kHz | kilohertz |
| LAeq 8 hr | equivalent continuous sound for 8 hr |
| min | minute(s) |
| ms | millisecond(s) |
| MSHA | Mine Safety and Health Administration |
| NHANES | National Health and Nutrition Examination Survey |
| NHCA | National Hearing Conservation Association |
| NIHL | noise-induced hearing loss |
| NIOSH | National Institute for Occupational Safety and Health |
|
NOES |
National Occupational Exposure Survey |
| NOHSM | National Occupational Health Survey of Mining |
| NRR | noise reduction rating |
| ONHS | Occupational Noise and Hearing Survey |
| OSHA | Occupational Safety and Health Administration |
| PEL | permissible exposure limit |
| REAT | real ear attenuation at threshold |
| REL | recommended exposure limit |
| s | second(s) |
| SIC | standard industrial classification |
| SPL | sound pressure level |
| STS | standard threshold shift |
| T-BEAM | task-based exposure assessment model |
| TTS2 | temporary threshold shift 2 min after a period of noise exposure |
|
TWA |
time-weighted average |
Where possible, the definition is quoted from the appropriate American National Standards Institute (ANSI) standard, ANSI S1.1-1994 [ANSI 1994] or ANSI S3.20-1995 [ANSI 1995], under the term(s) used in that standard.
Audiogram: Graph of hearing threshold level as a function of frequency (ANSI S3.20-1995: audiogram).
Baseline audiogram: The audiogram obtained from an audiometric examination administered before employment or within the first 30 days of employment that is preceded by a period of at least 12 hr of quiet. The baseline audiogram is the audiogram against which subsequent audiograms will be compared for the calculation of significant threshold shift.
Continuous noise: Noise with negligibly small fluctuations of level within the period of observation (ANSI S3.20-1995: stationary noise; steady noise).
Crest factor: Ten times the logarithm to the base ten of the square of the wideband peak amplitude of a signal to the time-mean-square amplitude over a stated time period. Unit, dB (ANSI S3.20-1995: crest factor).
Decibel (dB): Unit of level when the base of the logarithm is the 10th root of 10 and the quantities concerned are proportional to power (ANSI S1.1-1994: decibel).
Decibel, 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 weighting networks.)
Decibel, 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 weighting networks.)
Derate: To use a fraction of a hearing protectors noise reduction rating (NRR) to calculate the noise exposure of a worker wearing that hearing protector. (See NRR below.)
Dose: The amount of actual exposure relative to the amount of allowable exposure, and for which 100% and above represents exposures that are hazardous. The noise dose is calculated according to the following formula:
D = [C1/T1 + C2/T2 + ... + Cn/Tn] H 100
Where
Effective noise level: The estimated A-weighted noise level at the ear when wearing hearing protectors. Effective noise level is computed by (1) subtracting derated NRRs from C-weighted noise exposure levels, or (2) subtracting derated NRRs minus 7 dB from A-weighted noise exposure levels. Unit, dB. (See Appendix.)
Equal-energy hypothesis: 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 sound level: Ten times the logarithm to the base ten of the ratio of time-mean-square instantaneous A-weighted sound pressure, during a stated time interval T, to the square of the standard reference sound pressure. Unit, dB; respective abbreviations, TAV and TEQ; respective letter symbols, LAT and LAeqT (ANSI S1.1-1994: time-average sound level; time-interval equivalent continuous sound level; time-interval equivalent continuous A-weighted sound pressure level; equivalent continuous sound level).
Excess risk: Percentage with material impairment of hearing 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 noise.
Exchange rate: An increment of decibels that requires the halving of exposure time, or a decrement of decibels that requires the doubling of exposure time. For example, 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.
Fence: The hearing threshold level above which a material impairment of hearing is considered to have occurred.
Frequency: For a function periodic in time, the reciprocal of the period. Unit, hertz (Hz) (ANSI S1.1-1994: frequency).
Hearing threshold level (HTL): For a specified signal, amount in decibels by which the hearing threshold for a listener, for one or both ears, exceeds a specified reference equivalent threshold level. Unit, dB (ANSI S1.1-1994: hearing level; hearing threshold level).
Immission level: A descriptor for noise exposure, in decibels, representing the total sound energy incident on the ear over a specified period of time (e.g., months, years).
Impact: Single collision of one mass in motion with a second mass that may be in motion or at rest (ANSI S1.1-1994: impact).
Impulse: Product of a force and the time during which the force is applied; more specifically, impulse is the time integral of force from an initial time to a final time, the force being time-dependent and equal to zero before the initial time and after the final time (ANSI S1.1-1994: impulse).
Impulsive noise: Impulsive noise is characterized by a sharp rise and rapid decay in sound levels and is less than 1 sec in duration. For the purposes of this document, it refers to impact or impulse noise.
Intermittent noise: Noise levels that are interrupted by intervals of relatively low sound levels.
Noise: (1) Undesired sound. By extension, noise is any unwarranted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device. (2) Erratic, intermittent, or statistically random oscillation (ANSI S1.1-1994: noise).
Noise reduction rating (NRR): The NRR, which indicates a hearing protectors noise reduction capabilities, is a single-number rating that is required by law to be shown on the label of each hearing protector sold in the United States. Unit, dB.
Permanent threshold shift (PTS): Permanent increase in the threshold of audibility for an ear. Unit, dB (ANSI S3.20-1995: permanent threshold shift; permanent hearing loss; PTS).
Pulse range: Difference in decibels between the peak level of an impulsive signal and the root-mean-square level of a continuous noise.
Significant threshold shift: A shift in hearing threshold, outside the range of audiometric testing variability (5 dB), that warrants followup action to prevent further hearing loss. NIOSH defines significant threshold shift as an increase in the HTL of 15 dB or more at any frequency (500, 1000, 2000, 3000, 4000, or 6000 Hz) in either ear that is confirmed for the same ear and frequency by a second test within 30 days of the first test.
Sound: (1) Oscillation in pressure, stress, particle displacement, particle velocity, etc. in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillations. (2) Auditory sensation evoked by the oscillation described above (ANSI S1.1-1994: sound).
Sound intensity: Average rate of sound energy transmitted in a specified direction at a point through a unit area normal to this direction at the point considered. Unit, watt per square meter (W/m2); symbol, I (ANSI S1.1-1994: sound intensity; sound-energy flux density; sound-power density).
Sound intensity level: Ten times the logarithm to the base ten of the ratio of the intensity of a given sound in a stated direction to the reference sound intensity of 1 picoWatt per square meter (pW/m2). Unit, dB; symbol, L (ANSI S1.1-1994: sound intensity level).
Sound pressure: Root-mean-square instantaneous sound pressure at a point during a given time interval. Unit, Pascal (Pa) (ANSI S1.1-1994: sound pressure; effective sound pressure).
Sound pressure level: (1) Ten times the logarithm to the base ten of the ratio of the time-mean-square pressure of a sound, in a stated frequency band, to the square of the reference sound pressure in gases of 20 micropascals (µPa). Unit, dB; symbol, Lp. (2) For sound in media other than gases, unless otherwise specified, reference sound pressure in 1 µPa (ANSI S1.1-1994: sound pressure level).
Temporary threshold shift: Temporary increase in the threshold of audibility for an ear caused by exposure to high-intensity acoustic stimuli. Such a shift may be caused by other means such as use of aspirin or other drugs. Unit, dB. (ANSI S3.20-1995: temporary threshold shift; temporary hearing loss).
Time-weighted average (TWA): The averaging of different exposure levels during an exposure period. For noise, given an 85-dBA exposure limit and a 3-dB exchange rate, the TWA is calculated according to the following formula:
where D = dose.
Varying noise: Noise, with or without audible tones, for which the level varies substantially during the period of observation (ANSI S3.20-1995: nonstationary noise; nonsteady noise; time-varying noise).
This document was prepared by the staff of the National Institute for Occupational Safety and Health (NIOSH). Principal responsibility for this document rested with the Education and Information Division, Paul A. Schulte, Ph.D., Director, and the Division of Biomedical and Behavioral Science, Derek E. Dunn, Ph.D., Director. Henry S. Chan was the document manager. John R. Franks, Ph.D., Carol J. Merry, Ph.D., Mark R. Stephenson, Ph.D., and Christa L. Themann contributed the principal input on the technical aspects of noise measurements, noise health effects, and the requisite components of a hearing loss prevention program. Mary M. Prince, Ph.D., Randall J. Smith, Leslie T. Stayner, Ph.D., and Stephen J. Gilbert provided risk assessment and statistical calculations. Barry Lempert recovered and reformatted the Occupational Noise and Hearing Survey (ONHS) data. David H. Pedersen, Ph.D. and Randy O. Young provided data from the National Occupational Exposure Survey (NOES). Dennis W. Groce and Janet M. Hale provided data from the National Occupational Health Survey of Mining (NOHSM). Ralph D. Zumwalde and Marie Haring Sweeney, Ph.D., provided policy review. Robert J. Tuchman, Anne C. Hamilton, Jane Weber, and Susan Feldmann edited the document. Susan Kaelin and Vanessa Becks provided editorial assistance and desktop publishing. Judy C. Curless, Sharon L. Cheesman, and Michelle Brunswick provided word processing and production support.
NIOSH gratefully acknowledges the contributions of Alice H. Suter, Ph.D. (Alice Suter and Associates, Ashland, Oregon) and Julia D. Royster, Ph.D. (Environmental Noise Consultants, Inc., Raleigh, North Carolina), who served as consultants in the areas of the 3-decibel exchange rate and criteria for significant threshold shift, respectively.
NIOSH thanks the following consultants for their participation in the public meeting held on June20-21, 1997, in Cincinnati, Ohio:
| Henning von Gierke, Dr. Eng. | Daniel L. Johnson, Ph.D. |
| Yellow Springs, OH | Interactive Acoustics |
| Provo, UT | |
| Scott Schneider | Anne R. Shields, Ph.D. |
| Center to Protect Workers Rights | Aberdeen Proving Ground, MD |
| Washington, DC | |
| Thomas Simpson, Ph.D. | Alice H. Suter, Ph.D. |
| Wayne State University | Alice Suter and Associates |
| Detroit, MI | Ashland, OR |
Edwin Toothman
Noise/Hearing Construction
Bethlehem, PA
We also thank James E. Lankford, Ph.D. (Northern Illinois University, DeKalb, Illinois) and Charles W. Nixon, Ph.D. (Wright Patterson Air Force Base, Dayton, Ohio) for reviewing the draft.
The National Institute for Occupational Safety and Health (NIOSH) recommends the following standard for promulgation by regulatory agencies such as the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA) to protect workers from hearing losses resulting from occupational noise exposure. If this recommended standard is promulgated by a regulatory agency, the mandatory and nonmandatory provisions of the standard are indicated by the words shall and should, respectively.
The NIOSH recommended exposure limit (REL) for occupational noise exposure encompasses the provisions in Sections 1.1.1 through 1.1.4. The REL is 85 decibels, A-weighted, as an 8-hr time-weighted average (85 dBA as an 8-hr TWA). Exposures at and above this level are considered hazardous.
Occupational noise exposure shall be controlled so that worker exposures are less than the combination of exposure level (L) and duration (T), as calculated by the following formula (or as shown in Table 1-1).
where 3 = the exchange rate.
In accordance with Section 1.1.1, the REL for an 8-hr work shift is a TWA of 85 dBA using a 3-decibel (dB) exchange rate.
When the daily noise exposure consists of periods of different noise levels, the daily dose (D) shall not equal or exceed 100, as calculated according to the following formula:
D = [C1/T1 + C2/T2 + ... + Cn/Tn] H 100
where
Cn = total time of exposure at a specified noise level, and
Tn = exposure duration for which noise at this level becomes hazardous.
The daily dose can be converted into an 8-hr TWA according to the following formula (or as shown in Table 1-2):
Exposure to continuous, varying, intermittent, or impulsive noise shall not exceed 140 dBA.
The employer shall institute an effective hearing loss prevention program (HLPP) described in Sections 1.3 through 1.11 when any worker's 8-hr TWA exposure equals or exceeds 85 dBA.
The employer shall conduct a noise exposure assessment when any worker's 8-hr TWA exposure equals or exceeds 85 dBA. Exposure measurements shall conform to the American National Standard Measurement of Occupational Noise Exposure, ANSI S12.19-1996 [ANSI 1996a]. Noise exposure is to be measured without regard for the wearing of hearing protectors.
When a new HLPP is initiated, an initial monitoring of the worksite or of noisy work tasks shall be conducted to determine the noise exposure levels representative of all workers whose 8-hr TWA noise exposures may equal or exceed 85 dBA. For workers remaining in essentially stationary, continuous noise levels, either a sound level meter or a dosimeter may be used. However, for workers who move around frequently or who perform different tasks with intermittent or varying noise levels, a task-based exposure monitoring strategy may provide a more accurate assessment of the extent of exposures.
If any worker's 8-hr TWA exposure to noise equals or exceeds 85 dBA, monitoring shall be repeated at least every 2 years. Monitoring shall be repeated within 3 months of the occurrence when there is a change in equipment, production processes or maintenance routines. It may also be prudent to assess noise exposures when work practices have changed and/or if workers are developing significant threshold shifts (see Section 1.6.4).
Instruments used to measure workers' noise exposures shall be calibrated to ensure measurement accuracy and, at a minimum, 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, 1985] or, with the exception of the operating range, to the American National Standard Specification for Personal Noise Dosimeters, ANSI S1.25-1991 [ANSI 1991a]. If a sound level meter is used, the meter response shall be set at SLOW.
In determining TWA exposures, all continuous, varying, intermittent, and impulsive sound levels from 80 to 140 dBA shall be integrated into the noise measurements.
To the extent feasible, engineering controls, administrative controls, and work practices shall be used to ensure that workers are not exposed to noise at or above 85 dBA as an 8-hr TWA. The use of administrative controls shall not result in exposing more workers to noise.
Workers shall be required to wear hearing protectors when engaged in work that exposes them to noise that equals or exceeds 85 dBA as an 8-hr TWA.* The employer shall provide hearing protectors at no cost to the workers.
Hearing protectors shall attenuate noise sufficiently to keep the worker's "real-world" exposure (i.e., the noise exposure at the worker's ear when hearing protectors are worn) below 85 dBA as an 8-hr TWA. Workers whose 8-hr TWA exposures exceed 100 dBA should wear double hearing protection (i.e., they should wear earplugs and earmuffs simultaneously).†
*This recommendation should not be construed to imply that workers need not wear hearing protection unless their 8-hr TWAs equal or exceed 85 dBA. For example, it would be prudent for a worker in and out of noise or habitually exposed to loud noise (e.g., 91 dBA for 1 hr and 59 min) to wear hearing protection while in noise—even though his or her dose was less than 100%.
†The intent of this section is not to advocate hearing protectors as the primary means of control; however, when engineering controls, administrative controls, and work practices cannot keep workers= exposures below 85 dBA as an 8-hr TWA, the use of hearing protectors shall be required. For most TWA exposures exceeding 105 dBA, hearing protectors will be necessary to supplement engineering and administrative controls.
To compensate for known differences between laboratory-derived attenuation values and the protection obtained by a worker in the real world, the labeled noise reduction ratings (NRRs) shall be derated as follows: (1) earmuffs—subtract 25% from the manufacturers' labeled NRR; (2) slow-recovery formable earplugs—subtract 50%; and (3) all other earplugs—subtract 70% from the manufacturers' labeled NRR. These derating values shall be used until such time as manufacturers test and label their products in accordance with a subject-fit method such as method B of ANSI S12.6-1997, American National Standard Methods for Measuring the Real-Ear Attenuation of Hearing Protectors [ANSI 1997]. Chapter 6 (p. 62) describes methods for using the NRR.
The employer shall train workers at least annually to select, fit, and use a variety of appropriate hearing protectors. By making a variety of devices available and training the workers in their use, the employer will substantially increase the likelihood that hearing protector use will be effective and worthwhile.
The employer shall provide audiometry for all workers whose exposures equal or exceed 85 dBA as an 8-hr TWA.
Audiometric tests shall be performed by a physician, an audiologist, or an occupational hearing conservationist certified by the Council for Accreditation in Occupational Hearing Conservation (CAOHC) or the equivalent, working under the supervision of an audiologist or physician. The appropriate professional notation (e.g., licensure, certification, or CAOHC certification number) shall be recorded on each worker's audiogram.
Audiometric testing shall consist of air-conduction, pure-tone, hearing threshold measures at no less than 500, 1000, 2000, 3000, 4000, and 6000 hertz (Hz). Right and left ears shall be individually tested. The 8000-Hz threshold should also be tested as an option and as a useful source of information about the etiology of a hearing loss.
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 S3.6-1996 [ANSI 1996b]. Audiometers shall receive a daily functional check, an acoustic calibration check whenever the functional check indicates a threshold difference exceeding 10 dB in either earphone at any frequency, and an exhaustive calibration check annually or whenever an acoustic calibration indicates the need—as outlined in Section 5.5.2. 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 Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms, ANSI S3.1-1991 [ANSI 1991b]. Instruments used to measure ambient noise shall conform to the American National Standard Specification for Sound Level Meters, ANSI S1.4-1983 and S1.4A-1985, Type 1 [ANSI 1983, 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 onsite 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 measurements shall be obtained under conditions representing the typical acoustical environment likely to be present when audiometric testing is performed. Ambient noise levels shall be recorded on each audiogram or made otherwise accessible to the professional reviewer of the audiograms.
A baseline audiogram shall be obtained before employment or within 30 days of employment for all workers who must be enrolled in the HLPP. Workers shall not be exposed to noise levels at or above 85 dBA for a minimum of 12 hr before receiving a baseline audiometric test. Hearing protectors shall not be used in lieu of the required quiet period.
All workers enrolled in the HLPP shall have their hearing threshold levels (HTLs) measured annually. These audiometric tests shall be conducted during the worker's normal work shift. This audiogram shall be referred to as the "monitoring audiogram." The monitoring audiogram shall be examined immediately to determine whether a worker has a change in hearing relative to his or her baseline audiogram.
When the monitoring audiogram detects a change in the HTL in either ear that equals or exceeds 15 dB at 500, 1000, 2000, 3000, 4000, or 6000 Hz, an optional retest may be conducted immediately to determine whether the significant threshold shift is persistent. In most cases, the retest will demonstrate that the worker does not have a persistent threshold shift, thereby eliminating the need for a confirmation audiogram and followup action. If a persistent threshold shift has occurred, the worker shall be informed that his or her hearing may have worsened and additional hearing tests will be necessary.
When a worker=s monitoring audiogram detects a threshold shift as outlined in Section 1.6.3, he or she shall receive a confirmation audiogram within 30 days. This confirmation test shall be conducted under the same conditions as those of a baseline audiometric test. If the confirmation audiogram shows the persistence of a threshold shift, the audiograms and other appropriate records shall be reviewed by an audiologist or physician.
If this review validates the threshold shift, the threshold shift is considered to be a significant threshold shift. This shift shall be recorded in the worker's medical record, and the confirmation audiogram shall serve as the new baseline and shall be used to calculate any subsequent significant threshold shift. Whenever possible, the worker should receive immediate feedback on the results of his or her hearing test; however, in no case shall the worker be required to wait more than 30 days.
When a significant threshold shift has been validated, the employer shall take appropriate action to protect the worker from additional hearing loss due to occupational noise exposure. Examples of appropriate action include explanation of the effects of hearing loss, reinstruction and refitting of hearing protectors, additional training of the worker in hearing loss prevention, and reassignment of the worker to a quieter work area.
When the reviewing audiologist or physician suspects a hearing change is due to a nonoccupational etiology, the worker shall receive appropriate counseling, which may include referral to his or her physician.
The employer should obtain an exit audiogram from a worker who is leaving employment or whose job no longer involves exposure to hazardous noise. The exit audiogram should be conducted under the same conditions as those of baseline audiometry.
A warning sign shall be clearly visible at the entrance to or the periphery of areas where noise exposures routinely equal or exceed 85 dBA as an 8-hr TWA. All warning signs shall be in English and, where applicable, in the predominant language of workers who do not read English. Workers unable to read the warning signs shall be informed verbally about the instructions printed on signs in hazardous work areas of the facility. The warning sign shall textually or graphically contain the following information:
All workers who are exposed to noise at or above 85 dBA as an 8-hr TWA shall be informed about the potential consequences of noise exposure and the methods of preventing noise-induced hearing loss (NIHL). When noise measurements are initially conducted and confirm the presence of hazardous noise, or when followup noise measurements identify additional noise hazards, workers shall be notified within 30 days. New workers shall be alerted about the presence of hazardous noise before they are exposed to it.
The employer shall institute a training program in occupational hearing loss prevention for all workers who are exposed to noise at or above 85 dBA as an 8-hr TWA; the employer shall ensure worker participation in such a program. The training program shall be repeated annually for each worker included in the HLPP. Information provided shall be updated to be consistent with changes in protective equipment and work processes.
The employer shall ensure that the training addresses, at a minimum, (1) the physical and psychological effects of noise and hearing loss; (2) hearing protector selection, fitting, use, and care; (3) audiometric testing; and (4) the roles and responsibilities of both employers and workers in preventing NIHL.
The format for the training program may vary from formal meetings to informal on-the-spot presentations. Allowances shall be made for one-on-one training, which would be particularly suitable for workers who have demonstrated a significant threshold shift. Whenever possible, the training should be timed to coincide with feedback on workers' hearing tests.
The employer shall maintain a record of educational and training programs for each worker for the duration of employment plus 1 year. On termination of employment, the employer should provide a copy of the training record to the worker. The employer may wish to keep the training record with the worker's exposure and medical records for longer durations (see Section 1.10).
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 a worker demonstrates a significant threshold shift that is presumed to be occupationally related, all possible steps shall be taken to ensure that the worker does not incur additional occupational hearing loss.
The evaluation at the programmatic level shall take place annually. The incidence rate of significant threshold shift for noise-exposed workers shall be compared with that for a population not exposed to occupational noise. Similar incidence rates from this comparison indicate an effective HLPP. Data for calculating an incidence rate for a population not exposed to occupational noise should be drawn from Annex C in the American National Standard Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment, ANSI S3.44-1996 [ANSI 1996c] unless more appropriate data are available.
The employer shall establish and maintain records in accordance with the requirements in Sections 1.10.1 through 1.10.5.
The employer shall establish and maintain an accurate record of all exposure measurements required in Section 1.3. These records shall include, at a minimum, the name of the worker being monitored; identification number; duties performed and job locations; dates and times of measurements; type (refer to Section 6), brand, 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.
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, at a minimum, the name of the worker being tested; identification number; duties performed and job locations; 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; HTLs 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.
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 maintained for 5 years.
‡Code of Federal Regulations. See CFR in references.
In accordance with 29 CFR 1910.20, Access to Employee Exposure and Medical Records, the employer shall, upon request, allow examination and provide copies of these records to a worker, a former worker, or anyone having appropriate authorization for record access.
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 standards (e.g., American National Standards Institute [ANSI] standards) referred to in this document shall be superseded by the latest available versions.
Noise, which is essentially any unwanted or undesirable sound, is not a new hazard. Indeed, NIHL has been observed for centuries. Before the industrial revolution, however, comparatively few people were exposed to high levels of workplace noise. The advent of steam power in connection with the industrial revolution first brought general attention to noise as an occupational hazard. Workers who fabricated steam boilers developed hearing loss in such numbers that the malady was dubbed "boilermakers disease." Increasing mechanization in all industries and most trades has since proliferated the noise problem.
NIHL is caused by exposure to sound levels or durations that damage the hair cells of the cochlea. Initially, the noise exposure may cause a temporary threshold shift-that is, a decrease in hearing sensitivity that typically returns to its former level within a few minutes to a few hours. Repeated exposures lead to a permanent threshold shift, which is an irreversible sensorineural hearing loss.
Hearing loss has causes other than occupational noise exposure. Hearing loss caused by exposure to nonoccupational noise is collectively called sociocusis. It includes recreational and environmental noises (e.g., loud music, guns, power tools, and household appliances) that affect the ear the same as occupational noise. Combined 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 loss [Hamernik and Henderson 1976; Brown et al. 1978; Gannon et al. 1979; Brown et al. 1980; Hamernik et al. 1980; 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; Franks and Morata 1996]. Some sensorineural hearing loss occurs naturally because of aging; this loss is called presbycusis. Conductive hearing losses, as opposed to sensorineural hearing losses, are usually traceable to diseases of the outer and middle ear.
Noise exposure is also associated with nonauditory effects such as psychological stress and disruption of job performance [Cohen 1973; EPA 1973; Taylor 1984; Öhrström et al. 1988; Suter 1989] and possibly 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 contributing factor in industrial accidents [Cohen 1976; Schmidt et al. 1980; Wilkins and Acton 1982; Moll van Charante and Mulder 1990]. Nevertheless, data are insufficient to endorse specific damage risk criteria for these nonauditory effects.
The effects of sound on a person depend on three physical characteristics of sound: amplitude, frequency, and duration. Sound pressure level (SPL), expressed in decibels, 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 (described in Chapter 4) are used to modify the SPL. Exposure limits are commonly measured in dBA. When used without a weighted network suffix, the expression should be dB SPL.
The frequency of a sound, expressed in Hz, represents the number of cycles occurring in 1 sec and determines the pitch perceived by the listener. Humans with normal hearing can hear a frequency range of about 20 Hz to 20 kilohertz (kHz). Exposures to frequency ranges that are considered infrasonic (below 20 Hz), upper sonic (10 to 20 kHz), and ultrasonic (above 20 kHz) are not addressed in this document.
Although no uniformly standard definitions exist, noise exposure durations can be broadly classified as continuous-type or impulsive. All nonimpulsive noises (i.e., continuous, varying, and intermittent) are collectively referred to as "continuous-type noise." Impact and impulse noises are collectively referred to as "impulsive noise." Impulsive noise is distinguished from continuous-type noise by a steep rise in the sound level to a high peak followed by a rapid decay. In many workplaces, the exposures are often a mixture of continuous-type and impulsive sounds.
In 1981, OSHA estimated that 7.9 million U.S. workers in the manufacturing sector were occupationally exposed to daily noise levels at or above 80 dBA [46 Fed. Reg.* 4078 (1981a)]. In the same year, the U.S. Environmental Protection Agency (EPA) estimated that more than 9 million U.S. workers were occupationally exposed to daily noise levels above 85 dBA, as follows:
*Federal Register. See Fed. Reg. in references.
Major
groupNumber of
workers | Agriculture |
323,000 |
| Mining |
255,000 |
| Construction |
513,000 |
| Manufacturing and utilities |
5,124,000 |
| Transportation |
1,934,000 |
| Military |
976,000 |
| Total |
9,125,000 |
More than half of these workers were engaged in manufacturing and utilities [EPA 1981].
From 1981 to 1983, NIOSH conducted the National Occupational Exposure Survey (NOES), which was designed to provide data describing the occupational safety and health conditions in the United States [NIOSH 1988a,b, 1990]. The surveyors visited and gathered information at various workplaces throughout the United States. For the purposes of NOES, workers were considered noise-exposed if any noise (excluding impulsive noise) at or above 85 dBA occurred in their work environment at least once per week for 90% of the workweeks in a year [NIOSH 1988a]. Because not all industries were surveyed, NOES does not provide an all-inclusive estimate of the number of noise-exposed workers in the United States; however, it does provide reasonable estimates of the numbers of noise-exposed workers in the particular industries covered by NOES. These estimates are tabulated in Table 2-1, which shows that noise-exposed workers were employed in a wide range of industries, with the majority in manufacturing.
To collect occupational health data in mining industries not covered by NOES, NIOSH conducted the National Occupational Health Survey of Mining (NOHSM) from 1984 to 1989. Unlike NOES surveyors, the NOHSM surveyors did not measure the noise levels but used qualitative evaluation to determine noise exposures. As shown in Table 2B2, noise exposures occurred in all of the industries covered by NOHSM.
Efforts to 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; however, they prescribed neither limits nor acknowledged the occupational hearing loss problem. A later regulation under this act [41 CFR 50B204.10], promulgated in 1969, defined noise limits that were applicable only to those firms having supply contracts with the U.S. Government greater than $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 Federal Coal Mine Health and Safety Act of 1969 (Public Law 91-173) for underground and surface coal mine operations.
In 1970, the Occupational Safety and Health Act (Public Law 95-164) was enacted, which established OSHA within the U.S. Department of Labor as the enforcement agency responsible for protecting the safety and health of a large segment of the U.S. workforce. Concurrently, NIOSH was established under the Department of Health, Education, and Welfare (now the Department of Health and Human Services) to develop criteria for safe occupational exposures to workplace hazards. 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 noise-induced occupational hearing loss. The criteria document presented an REL of 85 dBA as an 8-hr TWA and methods for measuring noise, calculating noise exposure, and providing a hearing conservation program. However, the criteria document acknowledged that (1) NIOSH was not able to determine the technical feasibility of the REL, and (2) approximately 15% of the population exposed to occupational noise at the 85-dBA level for a working lifetime would develop occupational NIHL.
Initially, OSHA adopted the Walsh-Healey exposure limit of 90 dBA as an 8-hr 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. 37773 (1974a)] but left the PEL unchanged. The proposed standard was not promulgated; however, 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. 4078 (1981a); 48 Fed. Reg. 9738 (1983)]. The OSHA noise standard as amended does not cover all industries. For example, the Hearing Conservation Amendments do not cover noise-exposed workers in transportation, oil/gas well drilling and servicing, agriculture, construction, and mining. The construction industry is covered by another OSHA noise standard [29 CFR 1926.52]; the mining industry is regulated by four separate standards that are enforced by MSHA [30 CFR 56.5050; 30 CFR 57.5050; 30 CFR 70.500B70.508; 30 CFR 71.800B71.805]. These standards vary in specific requirements regarding exposure monitoring and hearing conservation; however, all maintain an exposure limit based on 90 dBA for an 8-hr duration. Although they are required to comply with OSHA regulations by Executive Order 12196, the U.S. Air Force [1993] and the U.S.Army [1994] have chosen a more stringent exposure limit of 85 dBA as an 8-hr TWA with a 3-dB exchange rate. Thus, the protection that a worker receives from occupational noise depends in part on the sector in which he or she is employed.
The exposure limits discussed above apply only to continuous-type noises. For impulsive noise, the generally accepted limit not to be exceeded for any time is a peak level of 140dBSPL. Among the regulatory standards, this peak level is either enforceable or nonenforceable, as indicated by the word "shall" or "should," respectively. For example, in the MSHA standards for metal and nonmetal mines [30 CFR 56.5050; 30 CFR 57.5050], this exposure limit is enforceable; in the OSHA standards [29 CFR 1910.95; 29 CFR 1926.52], it is nonenforceable.
The focus of this document is on the prevention of occupational hearing loss rather than on conservation. Prevention means to avoid creating hearing loss. Conservation means to sustain the hearing that is present, regardless of whether damage has already occurred. An emphasis on prevention evolves from beliefs that it should not be necessary to suffer an impairment, illness, or injury to earn a living and that it is possible to use methods to prevent occupational hearing loss. This document evaluates and presents recommended exposure limits, a 3-dB exchange rate, and other elements necessary for an effective HLPP. 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 document.
The selection of an exposure limit depends on the definitions of two parameters: (1) the maximum acceptable occupational hearing loss (i.e., the fence) and (2) the percentage of the occupational noise-exposed population for which the maximum acceptable occupational hearing loss will be tolerated. The fence is often defined as the average HTL for two, three, or four audiometric frequencies. It separates the maximum acceptable hearing loss from smaller degrees of hearing loss and normal hearing. Excess risk is the difference between the percentage that exceeds the fence in an occupational-noise-exposed population and the percentage that exceeds it in an unexposed population. Mathematical models are used to describe the relationship between excess risk and various factors such as average daily noise exposure, duration of exposure, and age group.
The most common protection goal is the preservation of hearing for speech discrimination. Using this protection goal, NIOSH [1972] employed the term "hearing impairment" to define its criteria for maximum acceptable hearing loss; and OSHA later used the slightly modified term "material hearing impairment" to define the same criteria [46 Fed. Reg. 4078 (1981a)]. In this context, a worker was considered to have a material hearing impairment when his or her average HTLs for both ears exceeded 25 dB at the audiometric frequencies of 1000, 2000, and 3000 Hz (denoted here as the "1-2-3-kHz definition").
NIOSH [1972] assessed the excess risk of material hearing impairment as a function of levels and durations (e.g., 40-year working lifetime) of occupational noise exposure. Thus, for a 40-year lifetime exposure in the workplace to average daily noise levels of 80, 85, or 90 dBA, the excess risk of material hearing impairment was estimated to be 3%, 16%, or 29%, respectively. On the basis of this risk assessment, NIOSH recommended an 8-hr TWA exposure limit of 85 dBA [NIOSH 1972].
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 [1972], EPA [1973], and the International Standards Organization (ISO) [1971] for material 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 85dBA (8-hr TWA) [46 Fed. Reg. 4078 (1981a)].
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 [ONHS]) from 1968 to 1971. The industries in the surveys included steelmaking, 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 workers who had consented to participate in the surveys. More than 4,000 audiograms were collected, but the sample excluded audiograms of (1) noise-exposed workers whose noise exposures could not be characterized relative to a specified continuous noise level over their working lifetime, and (2)noise-exposed workers with abnormal hearing levels as determined by their medical history. Thus, 1,172 audiograms were used. These represented 792 noise-exposed and 380 non-noise-exposed workers (controls) [NIOSH 1972; Lempert and Henderson 1973].
A review of relevant epidemiologic literature did not identify new data suitable for estimating the excess risk of occupational NIHL for U.S. workers. The prolific use of hearing protectors in the U.S. workplace since the early 1980s would confound determination of dose-response relationships for occupational NIHL among contemporary workers. Therefore, the current risk assessment is based on a reanalysis of data from the NIOSH ONHS [Prince et al. 1997].
Prince et al. [1997] (reprinted in the Appendix of this document) derived a new set of excess risk estimates using the ONHS data with a model referred to as the "1997-NIOSH model," which differed from the 1972-NIOSH model [NIOSH 1972]. A noteworthy difference between the two models is that Prince et al. [1997] considered the possibility of nonlinear effects of noise in the 1997-NIOSH model, whereas the 1972-NIOSH model was based solely on a linear assumption for the effects of noise. Table 3-2 provides an overview of the differences between the 1997- and the 1972-NIOSH models. Prince et al. [1997] found that nonlinear models fit the data well and that the linear models similar to the 1972-NIOSH model did not fit as well. In addition to using the 0.5-1-2-kHz and the 1-2-3-kHz definitions of material hearing impairment to assess the risk of occupational NIHL, Prince et al. [1997] used the definition of hearing handicap* proposed by the American Speech-Language-Hearing Association (ASHA) Task Force on the Definition of Hearing Handicap. Prince et al. [1997] found the ASHA Task Force definition† (average of HTLs at 1000, 2000, 3000, and 4000 Hz) [ASHA 1981] useful because it was geared toward excess risk of hearing loss rather than compensation. Phaneuf et al. [1985] also found that the audiometric average of 1000, 2000, 3000, and 4000 Hz provided "a superior prediction of hearing disability in terms of specificity, sensitivity, and overall accuracy." The ASHA Task Force definition is also referred to as the 1-2-3-4-kHz definition in this criteria document. Table 3-3 presents the excess risk estimates for this definition and associated 95% confidence intervals.
* ASHA makes a distinction between the terms impairment and handicap; however, for the purpose of the subsequent discussion in this criteria document, only the term material hearing impairment is used. The Prince et al. [1997] paper reports the use of a modified ASHA Task Force definition. This modification incorporates frequency-specific weights based on the articulation index for each frequency [ANSI 1969]. Negligible differences were found between excess risk estimates generated using the modified and the unmodified definitions. The excess risk estimates presented in this criteria document are based on the unmodified ASHA Task Force definition.
†Historical note, ASHA did not deliberate on the definition proposed by the ASHA Task Force.
The ISO has also developed procedures for estimating hearing loss due to noise exposure. In 1971, the 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"), which included risk estimates for material 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-hr work shifts during a working lifetime of up to 40 years.
The various models for estimating the excess risk of material hearing impairment are compared in Table 3-4. The excess risk estimates derived from the 1971-ISO, 1972-NIOSH, 1973-EPA, and 1997-NIOSH‡ 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 statistical methodology or in the underlying data used. Nevertheless, these five models confirm an excess risk of material hearing impairment at 85 dBA.
‡Prince et al. [1997] found that the excess risk estimates at exposure levels below 85 dBA were not well defined. Insufficient data for workers with average daily exposures below 85 dBA led to considerable variability in the estimation, depending on the statistical assumptions used in the modeling.
As mentioned earlier in this section, the protection goal incorporated in the definitions of material hearing impairment has been to preserve hearing for speech discrimation. The 4000-Hz audiometric frequency is recognized as being both sensitive to noise and important for hearing and understanding speech in unfavorable or noisy listening conditions [Kuzniarz 1973; Aniansson 1974; Suter 1978; Smoorenburg 1990]. In recognition of the fact that listening conditions are not always ideal in everyday life, and in concurrence with the ASHA [1981] Task Force proposal, NIOSH has modified its definition of material hearing impairment to include 4000-Hz when assessing the risk of occupational NIHL. Therefore, with this modification, NIOSH defines material hearing impairment as an average of the HTLs for both ears that exceeds 25 dB at 1000, 2000, 3000, and 4000 Hz. Based on this definition, the excess risk is 8% for workers exposed to an average daily noise level of 85 dBA over a 40-year working lifetime. NIOSH continues to recommend the REL of 85 dBA as an 8-hr TWA on the basis of (1)analyses supporting the 1972 REL of 85 dBA as an 8-hr TWA, (2) reanalyses of the ONHS data, (3) ASHA Task Force positions on preservation of speech discrimination, and (4) analyses of excess risk of ISO, EPA, and NIOSH databases.
For extended work shifts (i.e., greater than 8 hr), lower exposure limits can be extrapolated from the REL of 85 dBA as an 8-hr TWA (see Section 1.1.1 or Table 1-1). Stephenson et al. [1980] studied human responses to 24-hr noise exposures and found that no temporary threshold shift occurred for broadband noise exposures less than 75 to 80dBA. These data are in line with the recommendation that TWA exposures be less than 80 to 81 dBA for durations greater than 16 hr.
Because NIOSH is recommending a 3-dB exchange rate with an 85-dBA REL, a ceiling limit for continuous-type noise is unnecessary. For example, with an 85-dBA REL and a 3-dB exchange rate, an exposure duration of less than 28 sec would be allowed at a 115-dBA level.
The generally accepted ceiling limit of 140 dB peak SPL for impulsive noise is based on a report by Kryter et al. [1966]. Ward [1986] indicated that "this number was little more than a guess when it was first proposed." To date, a proposal for a different limit has not been supported. Henderson et al. [1991] indicated that the critical level for chinchillas is between 119 and 125 dB; and if a 20-dB adjustment is used to account for the difference in susceptibility between chinchillas and humans, the critical level extrapolated for humans would be between 139 and 145 dB. Based on the 85-dBA REL and the 3-dB exchange rate, the allowable exposure time at 140 dBA is less than 0.1 sec; thus, 140dBA is a reasonable ceiling limit for impulsive noise.
Health effects depend on exposure level and duration. The NIOSH recommendation for a 3-dB exchange rate is based in part on the conclusions from a NIOSH contract report [Suter 1992a]. This report involved an exhaustive analysis of the relationship between hearing loss, noise level, and exposure duration. Although the time/intensity relationship is most commonly referred to as the exchange rate, it is also referred to as the "doubling rate," "trading ratio," and "time-intensity tradeoff." The 3-dB exchange rate is also known as the equal-energy rule or hypothesis, because a 3-dB increase/decrease represents a doubling or halving of the sound energy. The most commonly used exchange rates incorporate either 3 dB or 5 dB per doubling or halving of exposure duration [Embleton 1994].
The 3-dB exchange rate is the method most firmly supported by scientific evidence for assessing hearing impairment as a function of noise level and duration. This rate is already used in the United States by the EPA and the U.S.Department of Defense. The 3-dB exchange rate is used worldwide by nations such as Canada, Australia, New Zealand, the Peoples Republic of China, the United Kingdom, Germany, and many others. First proposed by Eldred et al. [1955], the 3-dB exchange rate was later supported by Burns and Robinson [1970]. The premise behind the 3-dB exchange rate is 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. However, Ward and Turner [1982] suggest restricting its use to the sound energy accumulated in 1 day. They distinguish between (1) an interpretation of the total energy theory that would allow an entire lifetime of exposure to be condensed into a few hours and (2) a restricted equal-A-weighted-daily-energy interpretation of the theory. Burns [1976] also cautions against the misuse of the equal-energy hypothesis, noting that it was based on data gathered from workers who experienced 8-hr occupational exposures daily for periods of months to years; thus, extrapolation to very different conditions would be inappropriate.
In 1973, the U.S. Air Force adopted a 4-dB exchange rate [U.S. Air Force 1973]. This exchange rate is based on an unpublished analysis by H.O. Parrack at the Aerospace Medical Research Laboratory. However, a set of curves based on this analysis was published as Figure 20 in a joint EPA/Air Force report [Johnson 1973]. The 4-dB exchange rate came closest to the curve that best described temporary theshold shift at 1000-Hz audiometric frequency [Johnson 1973]. However, Johnson [1973] also pointed out that according to these curves, the 3-dB exchange rate would best protect hearing at the 4000-Hz frequency, and the 5-dB exchange rate would be a good compromise if hearing were to be protected only at the midfrequencies—500, 1000, and 2000 Hz.
The relationship between the 3-dB exchange rate and energy can be illustrated as follows. The American National Standard for Acoustical Terminology, ANSI S1.1-1994 [ANSI 1994] defines the decibel as a "unit of level when the base of the logarithm is the tenth root of ten, and the quantities concerned are proportional to power. . . . [E]xamples of quantities that qualify are power (in any form), sound pressure squared, particle velocity squared, sound intensity, sound-energy density, and voltage squared. Thus, the decibel is a unit of sound-pressure-squared level; it is common practice, however, to shorten this to sound pressure level, when no ambiguity results from so doing."
Ostergaard [1986] provided a functional elucidation of the relationships pointed to in the ANSI definition:
In acoustics, decibel notation is utilized for most quantities. The decibel is a dimensionless unit based on the logarithm of the ratio of a measured quantity to a reference quantity. Thus, decibels are defined as follows:
L = k log10 (A/B) where L is the level in decibels, A and B are quantities having the same units, and k is a multiplier, either 10 or 20 depending on whether A and B are measures of energy or pressure, respectively. Any time a level is referred to in acoustics, decibel notation is implied. In acoustics all levels are referred to some reverence quantity, which is the denominator, B, of the equation.
Applying this mathematical relationship in the following calculations demonstrates how every doubling of energy yields an increase of 3 dB:
Let X = the exchange rate whereby energy is doubled
10 Log10 (A/B) + X = 10 Log10 (2A/B)
X = 10 Log10 (2A/B) - 10 log10 (A/B)
= 10 Log10 (2)
= 10 (0.301)
= 3.01 dB
This same relationship does not hold true for the 5-dB exchange rate. To derive X = 5dB, 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; it is less protective than the equal-energy hypothesis. The 5-dB exchange rate attempts to account for the interruptions in noise exposures that commonly occur during the workday [40 Fed. Reg. 12336 (1975)], presuming that some recovery from temporary threshold shift occurs during these interruptions and the hearing loss is not as great as it would be if the noise were continuous. The rule makes no distinction between continuous and noncontinuous noise, and it will permit comparatively long exposures to continuous noise at higher sound levels than would be allowed by the 3-dB rule. On the basis of the limited data that existed in the early 1970's, NIOSH [1972] recommended the 5-dB exchange rate; however, 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 Committee on Hearing, Bioacoustics, and Biomechanics (CHABA) for the National Academy of Sciences—National Research Council issued criteria for assessing allowable exposures to continuous, fluctuating, and intermittent noise [Kryter et al. 1966]. The CHABA criteria were an attempt to predict the hazard from nearly every conceivable noise exposure pattern based on temporary threshold shift experimentation. In the development of its criteria, CHABA used the following postulates:
However, these CHABA postulates were not validated. Research has been unable to demonstrate a simple relationship between temporary threshold shift, permanent threshold shift, and cochlear damage [Burns and Robinson 1970; Ward 1970, 1980; Ward and Turner 1982; Hétu 1982; Clark and Bohne 1978, 1986]. The CHABA criteria assumed that worker exposures could be characterized by regularly spaced noise bursts interspersed with periods that were sufficiently quiet to allow hearing to recover. However, this assumption is not characteristic of many typical industrial noise exposures. Workers will always develop temporary threshold shift before sustaining permanent threshold shift, barring an ototraumatic incident. Temporary threshold shift is a useful metric for monitoring the effects of noise exposure; these studies do not imply otherwise.
In general, the CHABA hearing damage risk criteria proved too complicated for general use. Botsford [1967] published a simplified set of criteria based on the CHABA criteria. One of the simplifications inherent to the Botsford [1967] method was the assumption that interruptions would be of "equal length and spacing so that a number of identical exposure cycles would be distributed uniformly throughout the day." These interruptions would occur during coffee breaks, trips to the washroom, lunch, and periods when machines were temporarily shut down.
During the same period, another related development led to the 5-dB exchange rate. Simplifying the criteria developed by Glorig et al. [1961] and adopted by ISO [1961], the Intersociety Committee [1970] published its criteria, which 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 of less than 8 hr, the Committee recommended maximum exposure levels based on a 5-dB exchange rate. The only field study that has been repeatedly cited as supporting the 5-dB rule is one study of coal miners by Sataloff et al. [1969].
In 1969, the U.S. Department of Labor promulgated a noise standard [34 Fed. Reg. 790 (1969a)] under the authority of the Walsh-Healey Public Contracts Act. The standard contained a PEL of 90 dBA for continuous noise. Exposure to varying or intermittent 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 was used for short-duration, high-level bursts. This standard was withdrawn after a short period. Later in 1969, the Walsh-Healey noise standard that is in effect today was issued [34 Fed. Reg. 7948 (1969b)]. In this version, any special criteria for varying or intermittent 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 it.
Beginning with the study of Burns and Robinson [1970], the credibility of the 3-dB rule has been increasingly supported by numerous studies and by national and international consensus [EPA 1973, 1974; 39 Fed. Reg. 43802 (1974b); ISO 1971; von Gierke et al. 1981; ISO 1990; U.S. Air Force 1993; U.S. Army 1994; ACGIH 1995].
Data from a number of field studies correspond well to the 3-dB rule (equal-energy hypothesis), as Passchier-Vermeer [1971, 1973] and Shaw [1985] have demonstrated. In Passchier-Vermeer's [1973] portrayal of the data, the Passchier-Vermeer [1968] and the Burns and Robinson [1970] prediction models for hearing losses as a function of continuous-noise exposure level fit the data on hearing losses from varying or intermittent 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 hypothesis.
Some older field data from occupations such as forestry and mining show less hearing loss than expected when compared with equivalent levels of continuous noise [Sataloff et al. 1969; Holmgren et al. 1971; Johansson et al. 1973; INRS 1978]. However, these findings have not been supported by the two NIOSH [1976, 1982] studies of intermittently exposed workers or the analyses conducted by Passchier-Vermeer [1973] and Shaw [1985].
Data from animal experiments support the use of the 3-dB exchange rate for single exposures of various levels within an 8-hr day [Ward and Nelson 1971; Ward and Turner 1982; Ward et al. 1983]. Nevertheless, several animal studies have demonstrated that some recovery may occur during the "quiet" periods of an intermittent noise exposure [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 quiet periods are considerably higher and where interruptions are not evenly spaced.
The possible ameliorative effect of intermittency does not justify the use of the 5-dB exchange rate. For example, although Ward [1970] noted that some industrial studies have shown lower permanent threshold shifts 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 PEL for certain intermittent noise exposures, as suggested by EPA [1974] and Johansson et al. [1973]. This response 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 should be determined by the temporal pattern of the noise and the levels of quiet between noise bursts. At this time, however, little quantitative information is available about these parameters in industrial environments. Therefore, the need for an adjustment should be clarified by further research. Although the 3-dB rule may be somewhat conservative in truly intermittent conditions, the 5-dB rule will be underprotective in most others. 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 duration, whether or not an adjustment is used for certain intermittent exposures.
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. [1966] and later acknowledged by Ward [1986] as little more than a guess. NIOSH [1972] did not address the hazard of impulsive (i.e., impulse or impact) noise, although NIOSH stated that the provisions of the recommended standard in the criteria document were intended to apply for all noise. Although there is yet no unanimity as to which criteria best describe the relationship between NIHL and exposure to impulsive noise, either by itself or in the presence of continuous-type (i.e., continuous, varying, or intermittent) noise, there is an international standard that has become widely used by most industrial nations. This standard, ISO 1999, Acoustics—An Estimation of Noise-Induced Hearing Impairment [ISO 1990], integrates both impulsive and continuous-type noise (and uses the 3-dB exchange rate of the equal-energy rule) when calculating sound exposures over any specified time period. NIOSH concurs with this approach and recommends that noise exposure levels be calculated by integrating all noises (both impulsive and continuous-type) over the duration of the measurement.
Despite its simplicity, the equal-energy rule is not universally accepted as a method for characterizing exposures that consist of both impulsive and continuous-type noises. Another approach favors evaluating impulsive noise separate from that of continuous-type noise. Studies that would argue for this approach will be discussed first, followed by a discussion of studies elucidating the rationale for the NIOSH position on the equal-energy rule.
In her evaluation of the effects of continuous and varying noises on hearing, Passchier-Vermeer [1971] 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 by Ceypek et al. [1973], Hamernik and Henderson [1976], and Nilsson etal. [1977] also indicated that continuous and impulsive noises have a synergistic rather than additive effect on hearing.
Comparing the studies of Passchier-Vermeer [1973] and of Burns and Robinson [1970], Henderson and Hamernik [1986] 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 that 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 etal. [1984] and Kuzniarz et al. [1976] on workers exposed to impulsive noise environments, Henderson and Hamernik [1986] indicated that exposure to continuous and impulsive noises in combination may be more hazardous than exposure to continuous noise alone.
Voight et al. [1980] studied noise exposure patterns in the building construction industry and related the equivalent continuous sound level for 8 hr (LAeq8hr)to audiometric records of more than 81,000 construction workers in Sweden. They found differences in hearing loss among groups exposed to noise of the same LAeq8hr but with different temporal characteristics. Groups exposed to impulsive noise had more hearing loss than those exposed to continuous noise of the same LAeq8hr.
Sulkowski and Lipowczan [1982] conducted noise measurement and audiometric testing in a drop-forge factory. The HTLs of 424 workers in the factory were compared with the predicted values according to the Burns and Robinson equation [1970]. The observed and predicted values differed in that the observed hearing loss was smaller than predicted at the lower audiometric frequencies, but the observed hearing loss was greater than predicted at the higher audiometric frequencies. In their study of hearing loss in weavers, who were exposed to continuous noise, and drop-forge hammer men, who were exposed to impact noise of equivalent energy, Sulkowski et al. [1983] found that the hammer men had substantially worse hearing than the weavers.
Thiery and Meyer-Bisch [1988] conducted a cross-sectional epidemiologic study at an automobile manufacturing plant. The automotive workers were exposed to continuous and impulsive noises at LAeq8hr ranging from 87 to 90 dBA. When their HTLs were compared with those of workers exposed to continuous noise at LAeq8hr 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 9 years of exposure.
Starck et al. [1988] compared at the 4000-Hz audiometric frequency the HTLs of forest workers using chain saws and shipyard workers using hammers and chippers. The forest workers were exposed to continuous-type noise, whereas the shipyard workers were exposed to impact noise. Starck et al. [1988] also used the immission model developed by Burns and Robinson [1970] to predict the HTLs for both groups. They found that the Burns and Robinson model was accurate at 4000 Hz for the forest workers; however, it substantially underestimated the HTLs at 4000 Hz for the shipyard workers.
The studies described here provide evidence that the effects of combined exposure to impulsive and continuous-type 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. Noise energy does not appear to be the only factor that affects hearing. The amplitude, duration, rise time, number of impulses, repetition rate, and crest factor also appear to be involved [Henderson and Hamernik 1986; Starck and Pekkarinen 1987; Pekkarinen 1989]. The criteria for exposure to impulsive noise based on the interrelationships of these parameters await the results of further research.
In 1968, CHABA published damage risk criteria for impulsive noise based on the equal-energy hypothesis [Ward 1968]. Over the years, individuals and organizations have supported treating impulsive noise on an equal-energy basis [Coles et al. 1973; EPA 1974; Coles 1980; ISO 1990].
Burns and Robinson [1970] proposed the concept of immission, which is based on the equal-energy hypothesis, to describe the total energy from a workers exposure to continuous noise over a period of time (i.e., months or years). Atherley and Martin [1971] modified this concept to include impulsive noise in the calculation of the LAeq8hr.
In a study of 76 men who were exposed to impact noise in two drop-forging factories, Atherley and Martin [1971] calculated each man's noise exposure (immission level) during his employment period and plotted it against his age-corrected HTLs over six audiometric frequencies. They found that the observed HTLs of the population came close to the predicted HTLs according to Robinson [1968] and concluded that the equal-energy hypothesis was applicable to impact noise. Similarly, Atherley [1973] 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 HTLs.
Guberan et al. [1971] compared the HTLs of 70 workers exposed to impact noise in drop-forging workshops with the predicted HTLs according to Robinson [1968] at the 3-, 4-, and 6-kHz audiometric frequencies. Again, the observed HTLs were in close agreement with the predicted HTLs.
A study of 716 hammer and press operators in 7 drop forges by Taylor et al. [1984] indicated that hearing losses resulting from impact and continuous noises in the drop-forging industry were as great or greater than those resulting from equivalent continuous noise. Using noise dosimetry, Taylor et al. [1984] found that the hammer operators were exposed to an average LAeq8hr of 108 dBA, whereas the press operators were exposed 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 [1968] immission model. The median HTLs of younger press operators (aged 15 to 34) also corresponded closely with the predicted values; however, those of older press operators (aged 34 to 54) were significantly higher than predicted. These results indicate that, up to certain limits, the equal-energy hypothesis can be applied to combined exposure to impact and continuous noises.
In many industrial operations, impulsive noise occurs in concert with a background of continuous-type noise. In some animal studies the effects of combined exposure to continuous-type and impulsive noises appear to be synergistic at high exposure levels [Hamernik et al. 1974]. But the synergism disappears when the exposure levels are comparable with those found in many common industrial environments [Hamernik et al. 1981]. Whether the effects of combined exposure are additive or synergistic, exposure to these noises causes hearing loss; thus the contribution of impulse noise to the noise dose should not be ignored. If the effects are additive, the 85-dBA REL with the 3-dB exchange rate would be sufficiently protective. If the effects are synergistic, the same would still be protective to a smaller extent. NIOSH therefore recommends that the REL of 85 dBA as an 8-hr TWA be applicable to all noise exposures, whether such exposures are from continuous-type noise, impulsive noise, or combined continuous-type and impulsive noises.
No single method or process exists for measuring occupational noise. Hearing safety and health professionals can use a variety of instruments to measure noise and can choose from a variety of instruments and software to analyze their measurements. The choice of a particular instrument and approach for measuring and analyzing occupational noise depends on many factors, not the least of which will be the purpose for the measurement and the environment in which the measurement will be made. In general, measurement methods should conform to the American National Standard Measurement of Occupational Noise Exposure, ANSI S12.19-1997 [ANSI 1996a]. However, it is beyond the scope of this document to serve as a manual for operating equipment and making sound measurements. Rather, this chapter will be limited to concise remarks relevant to operating the two most commonly used instruments for measuring noise exposures: the sound level meter and the noise dosimeter. More detailed discussions about instrumentation and measurement protocols appear in reference sources such as NIOSH [1973], Earshen [1986], Johnson et al. [1991], and Harris [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. At a minimum, it measures sound level in dB SPL. An integrating function may be included to automate the calculation of the TWA or the noise dose.
The human ear is not equally responsive to all frequencies; it is most sensitive around 4000 Hz and least sensitive in the low frequencies. The responses of the sound level meter are modified with frequency-weighting networks that represent some responses of the human ear. These empirically derived networks approximate the equal loudness-weighting networks or scales; some also have a B-scale. The A-scale, which approximates the ears response to moderate-level sounds, is commonly used in measuring noise to evaluate its effect on humans and has been incorporated in many occupational noise standards. Table 4-1 shows the characteristics of these scales.
A sound level meters response is generally based on either a FAST or SLOW exponential averaging. FAST corresponds to a 125-millisecond (ms) time constant; SLOW corresponds to a 1-s time constant. The meter dynamics are such that the meter will reach 63% of the final steady-state reading within one time constant. The meter indicator reflects the average SPL measured by the meter during the period selected. In most industrial settings, the meter fluctuates less when measurements are made with the SLOW response compared with the FAST response. A rapidly fluctuating sound generally yields higher maximum SPLs when measured with a FAST response. The choice of meter response depends on the type of noise being measured, the intended use of the measurements, and the specifications of any applicable standard. For typical occupational noise measurements, NIOSH recommends that the meter response on a sound level meter be set at SLOW.*
*Meters that are set to integrate or average sound do not use either the FAST or SLOW time constant; they will sample many times each second. For a more detailed description of exponential time weighting, refer to Yeager and March [1991].
The correct use of the microphone is extremely important in obtaining accurate measurements. Microphones come in many types and sizes. A microphone is typically designed for use in a particular environment across a specific range of SPLs and frequencies. In addition, microphones differ in their directionality. For example, some are intended to be pointed directly at the sound; and others are designed to measure sound from a "grazing" angle of incidence. Thus users should follow the sound level meter manufacturers instructions regarding the type and size of microphone and its orientation toward a sound. Also, care should be taken to avoid shielding the microphone by persons or objects [ANSI 1996a]. When measuring a diffuse sound field, the person conducting the measurement should hold the microphone as far from his or her body as practical [Earshen 1986].
Measuring noise with a sound level meter is relatively simple when the noise levels are continuous and when the worker remains essentially stationary during the work shift. A noise dosimeter is preferred for measuring a worker's noise exposure when the noise levels are varying or intermittent, when they contain impulsive components, or when the worker moves around frequently during the work shift.
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 percent dose or TWA. Many dosimeters available today can provide an output in dose or TWA using various exchange rates (e.g., 3, 4, and 5 dB), 8-hr criterion levels (e.g., 80, 84, 85, and 90 dBA), and sound measurement ranges (e.g., 80 to 130dBA). The choice of FAST or SLOW meter response on the dosimeter does not affect the computed noise dose or TWA when the 3-dB exchange rate is used, but it will when other exchange rates are used [Earshen 1986].
In noise dosimetry, the microphone is attached on the worker whose exposure is being measured. The placement of the microphone is important in estimating the worker's exposure, as Kuhn and Guernsey [1983] have found large differences in the sound distribution about the body. ANSI [1996a] specifies that the microphone be located on the midtop of the worker's more exposed shoulder and that it be oriented approximately parallel to the plane of this shoulder.
OSHA requires that, for the purposes of the Hearing Conservation Amendment, all sound levels from 80 to 130 dBA 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. 4135 (1981b)].
To measure all sound levels from 80 to 140 dBA, a noise dosimeter should have an operating range of at least 63 dB and a pulse range of the same magnitude. In contrast, the ANSI S1.25B1991 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 with 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.
Whenever hazardous noise exists in the workplace, measures should be taken to reduce noise levels as much as possible to protect exposed workers 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. 4078 (1981a); 48Fed. Reg. 9738 (1983)]. In addition, research has shown that implementing effective HLPPs (also known as hearing conservation programs) has numerous other benefits in the workplace [NIOSH 1996]. For example, Cohen [1976] found reduced employee absenteeism f