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Recognizing administrative and engineering controls that can reduce noise exposure
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Recommending and fitting hearing protection devices (HPDs)
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Evaluating the effectiveness of HPDs
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Providing education to help children and adults protect their hearing
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Monitoring hearing conservation programs to ensure compliance with occupational noise
regulations
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Advocating for effective hearing loss prevention
Through these services, audiologists work to prevent NIHL in children and adults.
Overview of NIHL
NIHL develops when the sensitive structures of the inner ear are damaged by exposure to high noise
levels for extended periods of time or to intense impulsive sounds. Although exposure to noise in
the workplace is a common cause of NIHL, many nonoccupational activities can also produce noise
levels high enough to damage hearing. As a result, NIHL is seen in people of all different ages and
backgrounds.
Effects of Noise Exposure
Exposure to loud noise can damage the auditory system and cause other nonauditory problems. Some
of these problems may appear during or immediately after the noise exposure, while others can develop
slowly over time with repeated exposure to loud noise. In some cases, the symptoms improve over time
after the noise exposure ceases. However, in other cases, the damage is permanent.
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Temporary threshold shift (TTS): Exposure to high noise levels can cause temporary
metabolic changes to the cochlea, including swelling of the hair cells and auditory nerve
terminals. As a result, a listener may experience decreased hearing, tinnitus, perception
of speech sounding dull or muffled, and aural fullness. These symptoms typically last less
than 1hour to several hours or days. In some cases, the symptoms may completely resolve
(indicating complete recovery), but in other cases, there may be effects that persist long
term (indicating incomplete recovery).
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Permanent threshold shift (PTS): When the effects of a TTS do not completely resolve, the
effect on hearing is considered to be a PTS (i.e., permanent NIHL). A PTS occurs when noise
exposure causes the hair cells to swell to the point of rupturing. Because human hair cells do
not regenerate, the damaged hair cells are replaced by scar tissue. The stereocilia on the tops
of the hair cells can become fused together, thus interfering with the hair cell transduction
process. Over time, the auditory nerve terminals connected to the damaged hair cells will
degenerate. Although NIHL can be prevented by avoiding exposure to loud noise or wearing
effective hearing protection, there is no treatment to reverse the damage once it occurs.
The classic presentation of a PTS includes permanent bilateral sensorineural hearing loss,
with a “noise notch” in the high frequencies. In adults, a noise notch presents as decreased
hearing sensitivity at 3000, 4000, or 6000 Hz, with better hearing at lower and higher
frequencies. In children, the noise notch typically occurs at 6000 Hz (Niskar et al., 2001).
NIHL is often symmetrical between ears for most types of industrial noise exposure,
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whereas musicians and hunters/shooters typically have asymmetrical hearing loss (i.e.,
primarily for those who shoot rifles and shotguns).
In addition to hearing loss, those with permanent NIHL also often experience tinnitus and
hyperacusis.
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Acoustic trauma: Exposure to impulse noise greater than 140 dB SPL can cause instant,
permanent hearing damage called acoustic trauma. At this level, the pressure wave traveling
through the auditory system is so intense that it can cause mechanical damage. The tympanic
membrane may rupture. The ossicles can disarticulate, and the delicate structures of the
cochlea may be torn. The resulting hearing loss is typically severe to profound and permanent.
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Nonauditory effects: Exposure to loud noise can also cause various nonauditory problems,
such as increased stress, anxiety, depression, distractibility, and annoyance, as well as increased
risk of high blood pressure and heart disease (Basner et al., 2014; Dzhambov & Dimitrova,
2016). Communicating in high-noise environments can also cause vocal strain due to the need
to talk over the noise. Working in a high-noise environment can decrease job performance,
especially when the task is complicated or involves multitasking. As a result, a higher incidence of
accidents is seen in high-noise work environments (e.g., Neitzel et al., 2017; Picard et al., 2008).
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AUDIOLOGY NUGGET
Noise exposure resulting in a TTS can cause cochlear synaptopathy, which is
permanent damage to the synapses between the inner hair cells of the cochlea
and the auditory nerve fibers (e.g., Kujawa & Liberman, 2009). Some studies
have found a significant correlation between cochlear synaptopathy (indirectly
measured by evoked auditory potentials) and hearing in noise performance in
patients with behavioral hearing thresholds that fall within normal limits for 250
to 8000 Hz on the pure-tone audiogram (e.g., Grant et al., 2020). Difficulty
understanding speech in background noise when audiometric thresholds are
clinically normal has been termed “hidden hearing loss.” Cochlear synaptopathy
following noise exposure has been documented in various animal species, with
some species showing spontaneous synaptic recovery (Kauer et al., 2019) while
others do not (Kujawa & Liberman, 2009; Liu et al., 2012). However, much
remains unknown about cochlear synaptopathy in humans because human
studies must rely on indirect electrophysiological measures due to inability to
directly measure cochlear synaptopathy in vivo (i.e., within a living organism).
Human studies have reported inconsistent results, with some finding evidence
of permanent cochlear synaptopathy in listeners with a history of noise exposure
and clinically normal audiometric thresholds (Bramhall et al., 2017) and others
finding no synaptic deficits in similar groups of adults (Grinn & Le Prell, 2022).
Causes of NIHL
As a general rule, exposure to any noise 85 dBA or greater has the potential to damage hearing,
depending on the length of exposure. For occupational noise exposure, mining, manufacturing, construction, farming, military, law enforcement, and forestry are associated with the highest noise levels
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and highest prevalence of NIHL (Masterson et al., 2013). Power tools, guns, and music are among
the most common sources of hazardous nonoccupational noise exposure. In most cases, occupational
and nonoccupational noise sources cause gradual onset of NIHL, with repeated TTSs that eventually
progress to permanent NIHL. However, immediate permanent NIHL is possible with exposure to a
single loud impulse noise, such as a gunshot. Bomb blasts, jet engines, firecrackers detonating near the
ear, and airbag deployment can also generate noise levels above 140 dB SPL, thus creating the potential
for acoustic trauma.
Prevalence of NIHL
NIHL can begin at any age and affects millions of children and adults in the United States. It is
ranked as the second most common cause of sensorineural hearing loss, with presbycusis being the
most common cause (Rabinowitz, 2000). Approximately 24% of adults in the United States have
audiometric notches, but more than 50% of adults with NIHL do not have noisy jobs (Carroll et al.,
2017). Among the general population, an estimated 12.5% of children 6 to 19 years of age have NIHL
(Niskar et al., 2001). Older children (12–19 years) have a higher prevalence at 17.1%. A significantly
higher prevalence has been reported among rural children, with 22.5% of children 6 to 19 years of
age and 26.5% of children 12 to 19 years of age having NIHL (Renick et al., 2009). Together, these
findings highlight the need for hearing conservation efforts, not only for workers in high-noise job
settings but also for children and adults exposed to nonoccupational noise.
Industrial Audiology
Most recommendations regarding safe versus hazardous noise exposures, when hearing protection
should be worn, and when a hearing conservation program should be implemented come from consensus noise standards developed for occupational safety based on basic and applied research. These
recommendations are often also applied to nonoccupational noise exposure due to the lack of specific
recommendations related to prevention of nonoccupational NIHL.
OSHA Noise Regulation/Standard
The Occupational Safety and Health Administration (OSHA) Noise Regulation 29 CFR 1910.95 (last
revised in 1983) includes OSHA’s requirements for hearing conservation programs. OSHA (2022,
December 6) covers most, but not all, private-sector workers and employers. It is worth noting that
although some sources (e.g., Bruce et al., 2014) differentiate between a “standard” and a “regulation,
OSHA (2023, May 24) uses the terms interchangeably, defining both as “a regulatory requirement
established and published by the agency to serve as criteria for measuring whether employers are in
compliance with the OSH Act laws” (“What Is a Standard/Regulation?” section).
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Purpose: OSHA creates and enforces the federal noise regulation/standard and issues citations
when there are violations (Bruce et al., 2014). OSHA takes into consideration the economic
effects of the noise regulations/standards issued.
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Exchange rate: Also known as the time-intensity tradeoff or doubling rate, the exchange
rate refers to the relationship between noise levels and their allowable exposure times. Stated
differently, the allowed exposure time is cut in half for every increase in exposure level of
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5dBA or doubled for every decrease in exposure level of 5 dBA. OSHA (1983) uses a 5-dB
exchange rate. Therefore, exposure to 90 dBA for 8 hours is expected to cause the same
amount of auditory damage as exposure to 85 dBA for 16 hours or to 95 dBA for 4 hours.
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Action level (AL): Workers must be enrolled in a hearing conservation program, and HPDs
must be made available when noise exposure is an 8-hour time-weighted average (TWA) of
85dBA or greater (or the equivalent). A TWA combines all of the sound levels measured
during a work shift into one integrated overall exposure value.
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Permissible exposure level (PEL): Workers exposed to an 8-hour TWA of 90 dBA or
greater (or the equivalent) must be enrolled in a hearing conservation program, and feasible
administrative or engineering controls must be used. If these controls fail to reduce noise
exposure to the PEL or less, HPDs must be worn.
NIOSH Criteria for a Recommended Standard
The National Institute for Occupational Safety and Health (NIOSH) makes noise standard recommendations (last revised in 1998) to protect workers from developing occupational NIHL.
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Purpose: NIOSH conducts research and makes recommendations for best practice based
on current research, without factoring in economic considerations (Themann et al., 2013).
NIOSH recommendations for noise exposure and hearing conservation are generally more
conservative (i.e., offer greater protection) than the OSHA standard.
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Exchange rate: NIOSH (1998) uses a 3-dB exchange rate. As such, every 3-dB increase in
the noise exposure level results in a halving of the allowable exposure time, and every 3-dB
decrease in the noise level results in a doubling of the allowable exposure time.
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Recommended exposure level (REL): Workers exposed to an 8-hour TWA of 85 dBA or
greater (or the equivalent) must be enrolled in a hearing conservation program, and feasible
administrative and engineering controls must be used to reduce noise exposure below the
REL. If these controls fail to reduce noise exposure below the REL, HPDs must be worn.
See Appendix 8–A for a summary of key components of the OSHA regulation/standard and NIOSH
noise recommendations.
MSHA Regulations
The purpose of the Mining Safety and Health Administration (MSHA, 2000) Noise Regulation 30
CFR Part 62 is to prevent NIHL among workers in the underground and surface mining industry.
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Exchange rate: MSHA uses a 5-dB exchange rate.
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Action level (AL): Miners must be enrolled in a hearing conservation program, and HPDs
must be made available when noise exposure is an 8-hour TWA of 85 dBA or greater (or the
equivalent).
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PEL: Miners exposed to an 8-hour TWA greater than 90 dBA (or the equivalent) must be
enrolled in a hearing conservation program, and all feasible engineering and administrative
controls must be used to reduce noise exposure to the PEL. If the noise exposure cannot be
reduced below the PEL using all feasible engineering and administrative controls, the miner
must wear hearing protection.
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Other Noise Regulations
Other U.S. agencies and organizations have their own noise regulations for workers. Examples include:
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Federal Railroad Association: Applies to employees exposed to noise in the locomotive cab
(U.S. Department of Transportation, 2006)
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Department of Defense: Applies to U.S. military personnel and civilian personnel exposed to
hazardous occupational and operational noise (U.S. Department of Defense, 2019)
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Department of Transportation: Applies to truck and bus drivers through the Federal Motor
Carrier Safety Administration (1994)
AUDIOLOGY NUGGET
The OSHA maximum PEL and the NIOSH REL are not designed to protect
all workers from any degree of NIHL (Johnson, 2018; Neitzel, 2008; NIOSH,
1998). Excess risk is the “percentage of people in a noise-exposed population
who develop a material hearing impairment (as defined by OSHA or NIOSH)
above and beyond the percentage of people in a non-noise-exposed population
who develop a material hearing impairment” (Johnson, 2018, p. 5). Calculation
of excess risk is based on an 8-hour workday, 5 days per week, over a 40-year
working lifetime. Using the NIOSH definition of material hearing impairment
(1000, 2000, 3000, and 4000 Hz), an average exposure level of 90 dBA over
a working lifetime will result in an estimated 25% excess risk of developing
hearing loss, compared to an excess risk of only 8% if the average exposure
is 85 dBA (NIOSH, 1998). While neither limit protects all workers from all
hearing loss, the NIOSH REL of 85 dBA is more protective and results in fewer
workers developing significant hearing loss. In order to protect the most sensitive
individuals, NIOSH recommends hearing protection be worn any time noise
exposure exceeds 85 dBA for any duration (Kardous et al., 2016).
Noise Measurement and Exposure Analysis
In occupational settings, measurement of noise levels can serve different purposes (Moritz, 2014).
While audiologists may perform basic noise measurements, industries typically engage the services of
industrial hygienists to measure and identify noise exposures as part of comprehensive assessment
of worksite health and safety risks. The results of noise monitoring can be used to:
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Identify workers with exposures that exceed occupational standards who must therefore be
enrolled in a hearing conservation program
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Determine if administrative and/or engineering controls are needed
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Decide how much attenuation is needed from HPDs
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Educate and motivate workers to protect their hearing
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Predict if a worker’s hearing loss could be caused by their noise exposure in workers’
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compensation cases
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Determine if ambient noise levels in rooms where audiometric testing is performed meet
recommended standards
Equipment Used for Noise Measurement
Noise levels are measured using a SLM or a noise dosimeter. SLMs can be held by the user or attached
to a stand/tripod and are used mainly for area noise monitoring to determine if noise levels in specific
work locations could be potentially hazardous to hearing. The SLM should be calibrated before and
after each use by placing the acoustic calibrator on top of the microphone. SLMs must meet the
American National Standards Institute Specification for Sound Level Meters (ANSI S1.4, 1983, as
cited in Gelfand, 2009) if used for measuring noise for compliance with occupational noise requirements. There are several types of SLMs (ANSI, 2006, as cited in Gelfand, 2009):
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Type 0: Also called a laboratory standard SLM, a Type 0 SLM has an accuracy of ± 0.7 dB.
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Type 1: Known as a precision SLM, a Type 1 SLM has an accuracy of ± 1 dB.
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Type 2: Considered a general-purpose SLM, a Type 2 SLM has an accuracy of ± 2 dB. This is
the minimum required by OSHA and NIOSH for occupational noise measurements.
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Type S (Special purpose): A Type S SLM has limited capabilities/settings and is not used for
occupational noise measurements.
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Integrating: An integrating SLM has the same functions as a Type 1 or Type 2 SLM, but it
averages the measured noise levels over a period of time.
A noise dosimeter is a small, integrating SLM that can be worn for extended periods of time (i.e.,
a work shift) to determine a worker’s noise exposure. The noise dosimeter is typically clipped to the
listener’s shirt at the shoulder to be closer to ear level. Since SLMs are rather unwieldly for measuring
the noise exposure of individual workers who change locations and tasks during their work shift,
noise dosimeters are preferred for such scenarios. Noise dosimeters can be set to measure according to
different occupational noise standards, with some capable of measuring using different settings simultaneously (e.g., measuring using the OSHA and NIOSH settings at the same time). Noise dosimeters
should be calibrated before and after each use, and they must meet the American National Standards
Specification for Personal Noise Dosimeters (ANSI S1.25, as cited in Moritz, 2014) if used for measuring noise for compliance with occupational noise standards.
Settings and Features
SLMs and noise dosimeters have various settings and features that can be changed depending on the
purpose of the noise measurement. For occupational noise measurement, it is important that these
settings comply with recommendations or regulations specified by the governing safety organization.
Examples of the most common settings include:
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Threshold: The threshold represents the lowest sound level to be measured by an integrating
SLM/noise dosimeter. OSHA requires that occupational noise measurements include all
continuous, intermittent, and impulse noise levels between 80 dBA and 130 dBA. NIOSH
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recommends including all continuous, intermittent, and impulse noise levels between 80 dBA
and 140 dBA.
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Criterion: The criterion level is the maximum allowable “safe” noise level. For OSHA, the
maximum PEL of 90 dBA is considered the criterion. In contrast, NIOSH recommends
85dBA as the REL or criterion.
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Exchange rate: Based on the equal energy principle, the exchange rate represents the
relationship between the permissible exposure time and different noise levels. OSHA uses a
5-dB exchange rate, and NIOSH uses a 3-dB exchange rate.
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Response time: The response time refers to the amount of time needed for the SLM/noise
dosimeter to reach 63% of its maximum reading (Gelfand, 2009). OSHA and NIOSH require
use of a slow response time, which has a time constant of 1 second, allowing sound level
fluctuations to be averaged out. A fast response time has a time constant of 0.125 seconds,
which is more appropriate for measuring variability in noise levels.
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Frequency weighting scales: The weighting given to the frequencies measured by a SLM/noise
dosimeter can be changed by selecting different weighting scales (Figure 8–1). A linear scale
applies no weighting and measures the overall SPL for all sound detected by the microphone
(Gelfand, 2009).
The dBA scale deemphasizes the low frequencies considerably below 1000 Hz and thus
mimics the ear’s response to low-intensity sounds (i.e., less sensitivity to low frequencies).
The dBA scale is required by OSHA and NIOSH for occupational noise exposure
measurements to assess hearing damage risk.
The dBB scale deemphasizes the low frequencies but not as much as the dBA scale, thus
mimicking the ear’s response to moderate-intensity sounds. This scale is rarely used.
FIGURE 8–1. Sound level meter weighting.
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Very similar to the linear response, the dBC scale only slightly deemphasizes very low
and very high frequencies. The dBC scale is designed to mimic the ear’s response to
high-intensity sounds, so it is used when assessing peak sound pressures from very loud
impulsive sources.
Finally, newer SLMs and noise dosimeters include the dBZ scale as an option. The dBZ
scale provides a flat frequency response (±1.5 dB) for 10 Hz to 20,000 Hz. This replaces
the older “linear” response (which did not define the frequency range over which the meter
would be linear) (NoiseMeters Inc., 2022).
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Octave and one-third octave band analysis: Some SLMs and noise dosimeters have octave-
band or one-third octave band filters that allow measurement of noise in specific frequency
ranges/bands. Octave band filters span one octave (e.g., 2800 to 5600 Hz with a center
frequency of 4000 Hz), whereas one-third octave band filters span one-third of an octave
(e.g., 450 to 560 Hz with a center frequency of 500 Hz). These features are necessary
when noise must be measured in specific frequency bands, such as when determining if
background noise levels meet requirements for audiometric test rooms (ANSI, S3.1–1999,
as cited in Frank, 2000; OSHA, 1983) or when acoustical engineers consider options for
effective noise controls.
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Q & A
Question: Why do OSHA and NIOSH require use of the dBA scale?
Answer: The human ear is not equally sensitive to all frequencies. Sensitivity
is best at about 4000 Hz and poorest in the low frequencies. SLMs can be set
to different frequency weighting scales that represent responses of the human
ear at different intensity levels. The dBA scale mimics the human ear’s response
to moderate sound levels. OSHA and NIOSH require use of the dBA scale for
noise measurements because it most closely correlates with the risk of developing
NIHL.
Quantification of Noise Exposure
SLMs and noise dosimeters provide a variety of numerical values that can help quantify a person’s noise
exposure and subsequent risk of NIHL. Fortunately, current integrating SLMs and noise dosimeters
can calculate these values automatically. However, the same values can be calculated manually if an
integrating SLM or noise dosimeter is not available. Some of the most commonly used values include:
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Peak level: The peak level is the highest instantaneous sound level that the SLM/noise
dosimeter measures without using a frequency weighting scale or response time.
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Maximum level: The maximum level is the highest sound level measured using the selected
frequency weighting scale and response time.
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Lavg (or LAVG): The Lavg represents the logarithmic average sound level measured, but when
a threshold is set, the Lavg does not include any sound at levels below the threshold.
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Leq: The Leq represents the true equivalent sound level. The Leq is equivalent to the Lavg,
but it is only used when the exchange rate is 3 dB and the threshold is 0 dB.
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Time-weighted average (TWA): The TWA represents a constant sound level lasting 8 hours
that would result in the same amount of sound energy as the noise that was measured.
The TWA is less than the Lavg for a duration less than 8 hours and is equal to the Lavg at
8hours.
Using OSHA’s (1983) 90 dBA exposure limit and a 5-dB exchange rate, the TWA is
calculated as:
TWA = 16.61 × log(D/100) + 90
Where:
D = dose
Using NIOSH’s (1998) 85 dBA exposure limit and a 3-dB exchange rate, the TWA is
calculated as:
TWA = 10.0 × log(D/100) + 85
Where: D = dose
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Noise dose: The noise dose describes the amount of actual noise exposure relative to the
amount of allowable exposure, where 100% and above represents noise-hazardous exposures
(NIOSH, 1998). Table 8–1 shows OSHA’s (1983) maximum PELs and NIOSH’s (1998)
RELs expressed as an 8-hour TWA and a noise dose. The noise dose is calculated as:
D = [C1/T1 + C2/T2 + Cn/Tn] × 100
Where:
Cn = total time of exposure at a specified noise level
Tn = exposure time at which noise for this level becomes hazardous
The total time of exposure at each noise level and the exposure time at which noise at that level
becomes hazardous can be reported in hours or minutes as long as the same unit is used for each noise
level in the calculation.
TABLE 8–1. OSHA’s (1983) Maximum Permissible Exposure Levels (PELs)
and NIOSH’s (1999) Recommended Exposure Levels (RELs) Expressed
as an 8-Hour Time-Weighted Average (TWA) and a Noise Dose
DOSE OSHA 8-HOUR TWA NIOSH 8-HOUR TWA
25% 80 dBA 79 dBA
50% 85 dBA 82 dBA
100% 90 dBA 85 dBA
200% 95 dBA 88 dBA
400% 100 dBA 91 dBA
800% 105 dBA 94 dBA
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CASE EXAMPLE
A worker is exposed to the following noise levels during his 8-hour work shift:
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94 dBA for 3 hours
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88 dBA for 2 hours
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97 dBA for 3 hours
One can use the formulae described above to determine the worker’s noise
dose and his 8-hour TWA using the NIOSH recommendations. First calculate the noise dose. Remember each numerator (i.e., the top number in each
fraction) represents how long the worker was exposed at a specific noise level.
The denominator (i.e., the bottom number in each fraction) represents how
long the worker can safely be exposed at that level using the NIOSH recommendations. These values can be reported in hours or minutes as long as the
same unit is used for all of the fractions. In this example, the length of time the
worker can safely be exposed at each level can be determined either by using
the table provided in the NIOSH noise standard recommendations or by using
the quick rule of decreasing the exposure time by half for every 3-dB increase in
noise level, with an exposure of 85 dBA for 8 hours as the reference point.
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D = [180 minutes/60 minutes + 120 minutes/240 minutes
+ 180 minutes/30 minutes] × 100
= 950%
Therefore, the worker is exposed to a daily noise dose of 950%, which is
well above the maximum allowable 100% dose. To calculate the 8-hour TWA
associated with a 950% dose, simply use the following NIOSH formula:
TWA = 10.0 × log (950/100) + 85
= 94.78 dBA
The worker is exposed to an 8-hour TWA of 94.78 dBA. This is above the
NIOSH REL of 85 dBA. As a result, the worker would need to wear HPDs
at work, have an annual audiometric evaluation, and participate in a hearing
conservation program.
Noise Control
Results of noise monitoring can be used to determine the most appropriate options for reducing
workers’ noise exposure. Noise can be controlled at the source, the path, and/or the receiver.
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Source: Reducing noise at the source involves implementing engineering controls. Examples
include replacing older equipment with newer, quieter equipment or modifying existing equipment to be quieter. This is the best long-term option for noise control, but it can be expensive.
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