Evaluation of noise-induced hearing loss by reference to the upper limit of hearing.

We have defined the upper limit of hearing as the maximum audible frequency measured with fixed intensity and changing frequency. We have previously established the standard upper limit ageing curves from the normal age variation in the upper limit of hearing. In the present study, we sought to clarify the effects of occupational noise on the upper limit of hearing. We measured the upper limit of hearing in 239 healthy male workers (478 ears) exposed to intensive occupational noise. Their age variation in the upper limit of hearing was compared with the standard upper limit ageing curves in males. There were statistically significant deteriorations. Even if the ears that had normal hearing levels (35 dB or less) were selected, deterioration in the upper limit of hearing was noticeable. The upper limit of hearing may serve as clinically useful information on the hearing impairment that precedes noticeable hearing impairment in conventional audiometry for workers exposed to intensive occupational noise.