[Industrial noise and presbyacusia in the determination of hearing damage: studies in chemical industry workers].

An audiometric investigation was carried out on 710 workers of a chemical plant in order to ascertain the role of industrial noise and presbysocio-acusia in causing hearing loss. The workers underwent an audiometric test both aerial and via bone, in a sound-proof chamber, after a 16-hour rest from noise. The subjects also answered a questionnaire on nonoccupational noise exposure and previous hearing and dysmetabolic disorders. Noise exposure was assessed on the basis of a cumulative exposure index obtained for each subject by multiplying the mean daily exposure level by seniority in the job. The results showed that in workers not exposed to high noise levels, industrial noise and presbysocio-acusia induce, via a direct mechanism, either separately or together, only slight damage to social hearing frequencies, i.e., those of normal conversation. At frequencies of 4 and 6 kHz, however, presbysocio-acusia seems to play a more important role than industrial noise in causing hearing loss. The practical implications arising from this study concern the frequencies that should be used for a medicolegal assessment of industrial noise induced hearing loss and the the threshold for these frequencies above which assessment of hearing loss should begin. As regards the first point, the results obtained in this study suggest the use of medicolegal methods based on normal conversation frequencies since presbysocio-acusia should not in any case be taken from these frequencies as it affects them only marginally. As regards the second point, the results show that assessment of hearing loss should begin when the hearing threshold at the above frequencies exceeds 25 dB. The study also showed that in epidemiological investigations on subjects exposed to industrial noise, the cumulative exposure index is better correlated with the rise in hearing threshold at the various frequencies than with seniority.