Distribution of risk factors for hearing loss: implications for evaluating risk of occupational noise-induced hearing loss.

This paper presents an analysis of hearing threshold levels among 2066 white male workers employed in various U.S. industries studied in the 1968-72 NIOSH Occupational Noise and Hearing Survey (ONHS). The distribution of hearing threshold levels (HTL) is examined in relation to various risk factors (age, prior occupational noise, medical conditions) for hearing loss among a population of noise exposed and control (low noise-exposed) industrial workers. Previous analyses of a subset of these data from the ONHS focused on 1172 highly "screened" workers. An additional 894 male workers (609 noise-exposed and 285 controls), who were excluded for various reasons (i.e., nonoccupational noise exposure, otologic or medical conditions affecting hearing, prior occupational noise exposure) have been added to examine hearing loss in an unscreened population. Data are analyzed by age, duration of exposure, and sound level (8-h TWA) by individual test frequency. Results indicate that hearing threshold levels are higher among unscreened noise-exposed and control workers relative to screened workers. Analysis of risk factors such as nonoccupational noise exposure, medical conditions, and type of industry among unscreened controls indicated that these factors were not significantly associated with increased mean HTLs or risk of material impairment over and above what is expected due to age. Age-specific mean hearing threshold levels (and percentiles of the distribution) among the unscreened ONHS control population may be used as a comparison population of low-noise exposed white male industrial workers for evaluating the effectiveness of hearing conservation programs for workers less than 55 years of age. To make valid inferences regarding occupational noise-induced hearing loss, it is important to use hearing data from reference (control) populations that are similar with respect to the degree of subject screening, type of work force (blue vs white collar), and the distribution of other risk factors for hearing loss.