Noise exposure, hearing protection and hearing loss. A long-term study at an automobile sheet-metal pressing plant.

The design and implementation of a hearing conservation programme in a heavy industry with high noise levels is described. Action was taken on several points, including noise measurements, noise reduction, education on hearing protector use, and audiometric monitoring. It was found that the noise at the plant has been greatly reduced during a period of 25 years. The reduction in noise was made possible by identifying predominant sources of noise, changing the structure of the buildings and by changing production methods. The results are presented in the form of noise zone maps.

By means of noise dosimetry, one case of automation was studied on a production line for large sheet metal punchpresses. It was found that the introduction of automation reduced the noise immission to the workers in spite of the fact that new sources of noise had been introduced. Also, the number of workers exposed to noise was reduced.

The use of hearing protectors was studied through a questionnaire. A device was constructed which measured the use of earmuffs during the working day. It was found that 30% of the workers reported problems due to heat, sweat, itching and other local problems. Few complaints regarding communic-ation difficulties and accident hazards were reported. The workers who had problems due to sweat were also found to have a higher production of sweat than other workers, under the same conditions.

The audiograms in each age group were studied by means of a retrospective investigation covering 25 years. During the studied period, the median hearing thresholds in each age group improved by 17-20 dB HL. These improvements will have a major influence on the number of workers entitled to economic compensation as a consequence of noise-induced hearing loss.

In a longitudinal investigation covering up to 15 years, an analysis was made of the audiograms of workers who had been employed at the plant for many years. It was found that the rate of threshold shift was no faster than in ISO7029 (1984) database A.