Hearing thresholds in an auto assembly plant: prospects for hearing conservation in an Nigerian factory

The hearing of 165 auto assembly workers exposed to noise levels of between 94 and 108 dB was assessed by means of air and bone conductance audiograms obtained during peak and low production periods to determine acute shifts in hearing. Compared to non-industrially exposed and non-production subgroups, the exposed subjects had significantly (P less than 0.001) elevated thresholds at both peak and low production periods. There was an acute and significant (P less than 0.001) drop in the hearing thresholds of the exposed subjects during low production period. This was especially true of cross-sections of the exposed subjects and of a cohort specifically sampled at both high and low production periods. Hearing thresholds for the exposed subjects increased with both age and duration of employment and were significantly correlated with duration of employment (P less than 0.05). Of all the confounders (age, duration of employment, residential density) assessed by stepwise multiple regression analysis, duration of employment was the only significant contributor to the threshold shift. There was no conductive deafness among the study subjects. There was no consistent association between blood pressure and either hearing threshold or noise level. The results indicate good prospects for hearing conservation for Nigeria workers if hearing protection and noise abatement programs, currently not mandated by statutes, can be instituted.