Hearing loss due to continuous exposure to steady-state broad-band noise

An analysis is made of data from a number of field investigations concerning the influence of steady-state broad-band noise on the hearing of workers, exposed to it continuously for eight hours a day. The results relate to exposure times ranging from 10 to 40 years and to noise with noise-rating numbers for 500 to 2000 Hz (NR) ranging from 75 to 98 or sound levels from 80 to 103 dBA. Noise-induced shifts of the median hearing levels as well as shifts indicated at the 90th, 75th, 25th, and 10th percentiles, were determined for seven frequencies between 500 and 8000 Hz as a function of exposure years and sound level. The variability in hearing levels associated with noise exposure was shown to be constant after ten exposure years and an increasing function of sound level at each frequency except 4000 Hz. At this frequency, the variance in hearing levels decreased with increasing sound level. The limit of permissible noise exposure, defined as the maximal level which did not cause measurable noise-induced shift in hearing levels of workers exposed to it irrespective of exposure years, was shown to be NR 75 or 80 dBA. An acknowledgment is made of the need for hearing-conservation measures given exposure conditions which exceed 80 dBA.