Hearing loss and intermittent noise exposure.

Previous studies of occupational hearing loss have led to the belief that a typical hearing loss pattern results regardless of the noise source. Classically, the frequencies between 3,000 and 6,000 Hz are affected first. Subsequently, as the exposure continues, the hearing loss spreads to involve lower frequencies (speech) and 8,000 Hz. Although it has been assumed that this progression is typical for occupational hearing loss, it must be remembered that these studies were based chiefly on continuous exposure to noise in areas such as textile mills and paper factories. Practical experience in industry and review of millions of audiograms have shown that the classic audiogram is not as common as generally thought, especially in workers subjected to intermittent noise exposure such as chipping. In this investigation, 12,000 workers were screened to find 295 who met the strict criteria, the most important of which was intermittency of noise exposure. Most of the subjects were exposed to jackhammer noise that peaked at about 118 dBA. The study reveals a substantially different pattern of hearing loss from that reported previously. Intermittent exposure to intense noise results in very severe loss in high frequencies but relatively little or no hearing loss in the lower frequencies, even after many years of exposure. It remains to be determined whether this pattern of hearing loss results from intermittent exposure to all types of noise or only from those kinds investigated in this study. It is speculated that frequent rest periods permit the ear to protect itself from damage in the speech frequencies. This concept opens many possible avenues for preventing hearing handicap in industrial situations.