The hearing mechanism, being very sensitive to sound stimuli, is also quite susceptible to injury. Many workplaces are hazardous to hearing health due to exposure to (1) noise, (2) physical trauma, and/or (3) toxic materials. Each of these elements comprises a section in this chapter. The focus is noise damage, however, as it is far and away the most prevalent type of occupational auditory injury. The final section discusses medicolegal aspects of occupational hearing loss.
OCCUPATIONAL HEARING LOSS DUE TO NOISE
Noise-induced hearing loss (NIHL) is a subset of occupational hearing loss caused by chronic overexposure to hazardous levels of workplace noise. This is distinguished from acoustic trauma, another subset of occupational hearing loss, caused by a sudden burst of ultra-high-intensity sound. NIHL and acoustic trauma may both also occur from nonworkplace (eg, recreational) noise exposure. Because the pattern of hearing loss is essentially the same between occupational and recreational noise damage, it can be difficult to distinguish the 2 without a very clear history.
NIHL represents about 15% of the total societal burden of hearing loss among American adults. People with occupational NIHL represent approximately half that total, or about 2.5 million adults; another 2 million suffer NIHLs from nonoccupational or leisure activities such as hunting and target shooting, listening to loud music, or other noisy recreational activities.
In spite of considerable attention aimed at limiting noise overexposure—through wearing of personal hearing protection, engineered reductions in noise levels, and industrial hygiene measures—loss of hearing still occurs. The additive effect of aging upon the auditory system complicates the evaluation process even if serial examinations are available. Hearing loss in the military—both NIHL and acoustic trauma—is an increasing source of disability and compensation in the Veterans Administration.
The goal of this review is to provide guidance for physicians who evaluate and treat people with suspected NIHL. We will focus primarily on diagnosis and evaluation, with updated information on prevention and remediation.
Noise may be defined as unwanted, undesirable, or excessively loud sounds as experienced by an individual. The effects of chronic noise exposure vary with the characteristics of the sound: damage is related to intensity, exposure duration, and exposure pattern (continuous exposures are more damaging than interrupted exposures for the same overall duration and intensity). Daily exposure to hazardous noise over years produces the characteristic loss of high-frequency sensitivity in the 4-kHz to 6-kHz range (noise notch; see Figures 60–1 and 60–2).
Relative consonant and vowel sounds on an audiogram are a function of frequency.
Median noise-induced threshold shift as a function of frequency, for various durations of exposure compared with nonexposed controls. (A) Nine or fewer years of noise exposure. (B) Five or more years ...