Occupational Noise Exposure Guidelines: Protecting Workers' Hearing

Noise is unwanted sound and is an extensive occupational health problem, an occupational hazard. Indeed, NIHL has been observed for centuries. More than 30 million Americans are exposed to hazardous sound levels on a regular basis. Almost all noise-induced hearing loss (NIHL) is preventable. However, once acquired, NIHL is permanent and irreversible.

In addition to hearing loss, evidence shows that prolonged or repeated exposure to high levels of noise is associated with other health effects, such as heart disease, hypertension, and insomnia. The World Health Organization (WHO) and the United Nations Environmental Programme (UNEP) attest that exposure to loud sounds (noises) adversely affects the lives of millions of people at home, at work, and in the community.

Several recommendations for limiting noise exposure to avoid adverse effects have been developed over the years. However, noise exposure guidelines vary. In this article, we describe how these factors, and other considerations, make developing and applying noise exposure guidelines difficult.

Unlike many occupational hazards, exposures to harmful sound levels can also occur in recreational settings, homes, almost anywhere. Thus, the effects of a work exposure may be aggravated by other activities. To protect hearing, people should take measures to limit or minimize all exposures to harmful sound levels. This can be done by either controlling the sound source by reducing the level, by isolating the source or muffling the sound or by the use of personal protective equipment such as ear plugs or muffs.

Understanding Sound and Noise Measurement

The criteria used to define noise limits are complicated by the intrinsic complexities and nuances of sound measurement. Generally, exposure to continuous noise is measured using the frequency dependent A-weighted decibel scale (dB[A]). However, using the C-weighted scale (dB[C]) or unweighted (sound pressure level) scale may be appropriate for noise sources with a large proportion of low-frequency components, very intense levels, and/or high intensity impulse noise.

A-weighted noise levels or dBA is used to predict hearing risk. Decibels (dB) are a scale for measuring noise. A-weighted noise exposure levels. Unit, dB. Few exposures are constant and unvarying; therefore, most exposure limits are based on a Time Weighted Average (TWA) which sums energy over time. This involves specifying an exchange rate.

Time Weighted Average (TWA) is the average noise level during a shift (usually 8 hours). An exchange rate is the increase in allowable exposure level with each halving of exposure duration. Most guidance specifies a 3-dB exchange rate based on the “equal energy” principle (i.e., sound energy doubles with every 3 dB increase in level). Some noise limits use a higher exchange rate (e.g., 5-dB) to allow for intermittency which reduces risk. The exchange rate substantially influences measured exposure and risk. Other exposure characteristics may also be used to quantify exposure. Differences in exposure definitions explain some of the variability across the noise criteria above.

As sounds become louder than 85 dBA, the length of a daily exposure must be reduced. For each 3 dBA increase in noise level, NIOSH recommends reducing the exposure duration by half. This is called the exchange rate. Similarly, if the daily exposure is longer than 8 hours, the allowable noise level is lower.

All are measured on the dB(A) scale and use a 3-dB exchange rate. However, the EPA 70 dB(A) exposure limit is set to cover a 24-hour time period (Leq(24)), the 80 dB(A) recommended by the WHO is for a maximum of 40 hours per week, and the 85 dB(A) NIOSH REL is based on an 8-hour workday. The 100 dB(A) WHO limit for venues and events is averaged over only 15 minutes.

A simple example of a noise map. It can be hand-drawn in practice.

How to do a quick workplace noise assessment

Assessing Noise Exposure in the Workplace

Measure workplace areas with a sound level meter (SLM) and create a noise map of facility areas. If an SLM is not available, sound measurement apps can provide a measure of area noise but may not comply with regulatory requirements. After completing a noise map, identify the loudest areas on the map and the equipment in those areas. Noise levels are likely hazardous if a person must raise their voice to speak with someone is an arm's length away.

Measure individual worker noise exposures using a personal dosimeter in areas with high noise. Use personal noise dosimeters to best assess a worker's personal noise exposure. Attach the microphone to the top of the worker's shoulder on the side that is likely to have the highest exposure. If the device does not have a separate microphone this may not be necessary. You should tell workers they are having their noise exposure monitored and provide them with an explanation of the results. Involve workers in the noise monitoring process. They can provide important information about the work environment, machinery operation, and specific job tasks. Encourage workers to tell you when changes in equipment or production occur.

This figure shows the relationship between exposure level and exposure duration under the NIOSH REL.

Hearing Tests and Audiometric Evaluation

Audiometric evaluation is required if employees are exposed to sound levels equal to or exceeding an 8 hour TWA of 85 dB. This evaluation consists of a baseline audiogram obtained within 6 months of an employee’s first exposure to levels of 85dB or greater followed by yearly retesting. The baseline audiogram must be preceded by 14 hours without exposure to workplace noise. The baseline audiogram is then compared to the most recent test to determine if the audiogram is valid and if a standard threshold shift (STS) has occurred in the employee’s hearing. A STS is defined as an average shift from baseline of 10 dB or more in the audiometric frequencies 2000, 3000, and 4000 Hz.

NIOSH recommends annual audiometric testing for workers exposed to noise above the REL. Conduct testing on workers when they are first enrolled in a hearing loss prevention program. This is called baseline testing. NIOSH recommends that employers obtain baseline tests within 30 days of initial exposure for newly exposed workers. Unlike baseline testing, annual hearing tests should be done as close to the end of a worker's shift as possible (no preceding quiet period). Results of annual hearing tests should be compared to results from the baseline hearing test to check for significant threshold shift (STS).

An audiogram is a graph of hearing thresholds at various frequencies in each ear. Thresholds which are 25 dB HL (hearing level) or better (lower) are considered normal for adults. NIOSH considers a 15 dB change in hearing threshold at any frequency to represent an STS. If an STS is noted during an annual hearing test, the worker should be retested within 30 days to confirm. Workers who develop an STS - whether temporary or permanent - should be notified. Steps should be taken to prevent further change in hearing.

This audiogram shows normal hearing thresholds in the right ear. Frequency refers to how low or high the "pitch" of the sound is.

Hearing Loss Prevention Program (HLPP)

All University employees whose noise exposures equal or exceeds an eight-hour time weighted average (TWA) of 85 decibels are enrolled in a hearing conservation program. The employee is to be notified when he/she is exposed at or above an 8-hour time-weighted average of 85 decibels. The University strongly encourages the use of hearing protection devices while working around noisy equipment. For employees who have experienced a standard threshold shift, hearing protectors must attenuate employee exposure to an 8-hour time-weighted average of 85 decibels or below. Signs are to be posted in areas where noise levels are above 85 dBA stating that hearing protection is required. An annual training program is provided for each employee included in the hearing conservation program.

When employees are subjected to sound levels exceeding 85 dBA TWA, feasible administrative or engineering controls are to be utilized. Types of administrative controls are rotation of employees, limiting time of certain operations, or restricting areas or work operations. If feasible engineering or administrative controls cannot be accomplished personal hearing protective devices must be provided and used to reduce sound levels in areas above 85 dBA. The hearing protection used will depend on the operation, employee preference and attenuation required. The safety and health officer is to assist in supplying information on attenuation data and supervise the correct use of hearing protectors. Employees are given the opportunity to select their hearing protectors from a variety of suitable hearing protectors.

Employee exposure measurements and assessments are maintained at the office of Environment, Health and Safety. Employee audiometric test records are kept at the University Employee Occupational Health Clinic. Noise is unwanted sound and is an extensive occupational health problem.

Hearing loss occurs in two basic ways: conductive and sensorineural. Conductive hearing loss is caused by a problem with a part of the outer or middle ear. A breakdown or obstruction interferes with sound transmission. This type of hearing loss is common and can often be reversed and hearing restored with appropriate treatment. Sensorineural hearing loss occurs from changes in the receptor hair cells in the inner ear or in the nerves carrying impulses to the brain. This type of hearing loss is usually permanent. Noise-induced hearing loss (NIHL) is sensorineural and is usually permanent.

Noise Reduction Rating

The Noise Reduction Rating (NRR) is a single-number rating method that describes the protection provided by a hearing protector. It indicates how much the overall noise level is reduced by the hearing protector. Field studies by the National Institute for Occupational Safety and Health (NIOSH) have shown that employees normally achieve only about 1/2 of the noise reduction provided by hearing protection. To correctly estimate the noise reduction provided by hearing protection devices, it is important to remember to first subtract 7 from the NRR and then subtract the remainder from the A weighted noise level.

Key Definitions

Understanding the terminology used in occupational noise exposure guidelines is crucial for effective implementation and compliance. Here are some key definitions:

• Action Level: A specific noise exposure level that triggers certain actions, such as enrollment in a hearing conservation program.
• Audiologist: A healthcare professional specializing in the diagnosis, treatment, and prevention of hearing disorders.
• Baseline Audiogram: The initial hearing test used as a reference point for comparison with subsequent annual audiograms.
• Criterion Level: The maximum permissible noise exposure level for a given duration.
• Decibel (dB): A unit used to measure the intensity of sound.
• Dual Hearing Protection Level: The noise exposure level at which the concurrent use of both earplugs and earmuffs is required.
• Exchange Rate: The amount of increase in sound level, in decibels, which requires halving the allowable exposure time.
• Hearing Protector: A device designed to reduce the amount of noise reaching the ear.
• Hertz (Hz): A unit used to measure the frequency of sound.
• Permissible Exposure Level (PEL): The maximum allowable noise exposure level over a specified time period, as defined by regulatory agencies.
• Reportable Hearing Loss: A significant change in hearing threshold that warrants further action.
• Standard Threshold Shift (STS): A change in hearing threshold, relative to the baseline audiogram, that indicates a potential hearing loss.
• Time-Weighted Average (TWA): The average noise level during a shift, typically 8 hours.

Regulatory Framework

The Occupational Noise Exposure Standard requires that protection against the effects of noise exposure be provided when sound levels exceed those shown in the following table when measured on the “A” scale of a standard sound level meter at slow response. The standard requires that feasible administrative or engineering controls be utilized when levels exceed those listed. If these controls fail to reduce the sound level to those listed, personal protective equipment must be provided and used to reduce sound levels to within the levels of the table.

The PEOSH Occupational Noise Exposure Standard, 29 CFR1910.95, states that “When information indicates that any employee’s exposure may equal or exceed an 8-hour time-weighted average of 85 decibels (dB) the employer shall develop and implement a monitoring program (29 CFR 1910.95(d)).

Noise Exposure Monitoring Methods

The three most common methods of noise monitoring are area surveys, personal dosimetry and engineering surveys. In an area survey, a sound level meter is used to measure environmental noise levels. This information may be used to identify areas where more thorough noise exposure evaluation is needed. Personal dosimetry, a method of evaluating an employee’s individual exposure, is used to determine the noise levels experienced by employees throughout the work shift. A noise measuring instrument with the microphone positioned in the employee’s hearing zone is used to measure sound levels. OSHA defines the “hearing zone” as a two-foot sphere around the employee’s head. This type of monitoring result can also represent the exposure of other workers whose exposures result from the same tasks. During engineering surveys, octave band analyzers and sound level recorders are used to obtain information on the frequency/intensity composition of noise.

Administrative and Engineering Controls

Administrative and engineering controls can be used to reduce exposures to the point where the hazard is eliminated or reduced to manageable proportions. These control measures consist of removing the worker or removing the hazard. This may be accomplished by substituting less noisy equipment for instance. Administrative controls include work scheduling and task assignment. Individual employee exposure is reduced by decreasing the amount of time spent in a noisy environment. Engineering controls include mufflers, sound barriers, enclosures, and sound absorbing materials.