Hearing Conservation

Since the publication in 1989 of an earlier position statement by the American College of Occupational and Environmental Medicine (ACOEM),¹ noise-induced hearing loss remains one of the most prevalent occupational conditions, partly due to the fact that noise is one of the most pervasive occupational hazards found in a wide range of industries. ACOEM believes that occupational clinicians need to become increasingly proficient in the early detection and prevention of noise-induced hearing loss. This requires clarification of current best practices, as well as additional research into certain aspects of noise-induced hearing loss that remain poorly understood.

Definition
Occupational noise-induced hearing loss, as opposed to occupational acoustic trauma, is hearing loss that develops slowly over a long period of time (several years) as the result of exposure to continuous or intermittent loud noise. Occupational acoustic trauma is a sudden change in hearing as a result of a single exposure to a sudden burst of sound, such as an explosive blast. The diagnosis of noise-induced hearing loss is made clinically by a medical professional and should include a study of the noise exposure history.

Characteristics
The principal characteristics of occupational noise-induced hearing loss are as follows:

• It is always sensorineural, affecting hair cells in the inner ear.
• Since most noise exposures are symmetric, the hearing loss is typically bilateral.
• Typically, the first sign of hearing loss due to noise exposure is a “notching” of the audiogram at 3000, 4000, or 6000 Hz, with recovery at 8000 Hertz (Hz).² The exact location of the notch depends on multiple factors including the frequency of the damaging noise and the length of the ear canal. Therefore, in early noise-induced hearing loss, the average hearing thresholds at 500, 1000, and 2000 Hz are better than the average at 3000, 4000, and 6000, and the hearing level at 8000 Hz is usually better than the deepest part of the “notch.” This “notching” is in contrast to age-related hearing loss, which also produces high frequency hearing loss, but in a down-sloping pattern without recovery at 8000 Hz.³
• Noise exposure alone usually does not produce a loss greater than 75 decibels (dB) in high frequencies, and 40 dB in lower frequencies. However, individuals with superimposed age-related losses may have hearing threshold levels in excess of these values.
• The rate of hearing loss due to chronic noise exposure is greatest during the first 10-15 years of exposure, and decreases as the hearing threshold increases. This is in contrast to age-related loss, which accelerates over time.
• Most scientific evidence indicates that previously noise-exposed ears are not more sensitive to future noise exposure and that hearing loss due to noise does not progress (in excess of what would be expected from the addition of age-related threshold shifts) once the exposure to noise is discontinued.⁴
• In obtaining a history of noise exposure, the clinician should keep in mind that the risk of noise-induced hearing loss is considered to increase significantly with chronic exposures above 85 dBA for an 8-hour time-weighted average (TWA). In general, continuous noise exposure over the years is more damaging than interrupted exposure to noise which permits the ear to have a rest period. However, short exposures to very high levels of noise in occupations such as construction or firefighting may produce significant loss,^5,6 and measures to estimate the health effects of such intermittent noise are lacking. When the noise exposure history indicates the use of hearing protective devices, the clinician should also keep in mind that the real world attenuation provided by hearing protectors may vary widely between individuals.⁷

Evaluation of the Effectiveness of a Hearing Conservation Program
To date, there is no universally accepted method of evaluating the effectiveness of a hearing conservation program. Hearing conservation programs include aspects of administrative controls, engineering controls, audiometric surveillance, and training. Occupational physicians can actively participate with employers in improving all these aspects of hearing conservation programs through ongoing evaluation of program outcomes and processes.