Influence of environmental factors on the evolution of industrial noise-induced hearing loss.

The aim of the study is to investigate how environmental factors, associated with exposure to industrial noise, affect the development of chronic noise-induced hearing loss. The study was conducted on 186 male subjects working in two bottling plants, situated respectively in a small farming community, and in a medium-sized city with significant levels of noise pollution. Levels of occupational exposure were the same for the two groups. The subjects were selected by means of a preliminary medical examination, and exposed to tonal hearing tests and acoustic impedance tests. Statistical analysis was performed on hearing threshold values obtained at the frequencies of 500, 1000, 2000 and 4000 Hz. The comparison between the thresholds obtained in the two groups showed a statistically significant difference, especially at the frequency of 4000 Hz and for occupational exposure exceeding 17 yr. The results led us to conclude that environmental factors, and urban noise in particular, influence the onset and development of occupational acoustic trauma, and that those working in the country are significantly less affected than those in the city. Since occupational exposure was the same for both groups, their different responses must therefore be interpreted as due to differences in non-occupational exposure, in turn dependent on different opportunities for rest from noise and different levels of exposure to noise pollution.