Evaluation of physical/chemical synergistic influence of the working environmental factor on hearing, and the application to the comfortable environment.

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To clarify of evaluation of physical/chemical synergistic influence of the working environmental factor on hearing, and the application to the comfortable environment, animal experiments were carried out, and the male workers were examined.The subjects were male guinea pigs. Hearing threshold was measured by auditory brainstem response. The first experiment was only for methanol exposure, and methanol inhalation levels were 500 ppm, 900 ppm, 1500 ppm, 1900 ppm, 2400 ppm and 3000 ppm. The results of the first experiment demonstrated that methanol inhalation more than 1900ppm dose-dependently increased the hearing threshold. The second experiment was only for noise exposure, and noise exposure levels were 90, 100 and 105 dB SPL. The detection threshold in the 105 dB group was significantly increased. In the third experiment, guinea pigs were exposed to 1500 ppm or 1900 ppm methanol as sub-ototoxic concentration following 100 dB SPL white noise as sub-ototoxic level. In the combined group … More , the marked increase in the detection threshold was observed. The increase detection threshold was the greatest in the group of 1900 ppm methanol combined with noise.We measured hearing level and upper limit of hearing in 93 male workers occupationally exposed to organic solvents and noise. Environmental monitoring, biological monitoring and noise level measurement in workplaces were done. Most exposure level of organic solvents and/or noise were within the occupational exposure limits. The prevalence rate of upper limit of hearing below 75th percentile curve was higher in workers exposed to organic solvents and noise, and the upper limit of hearing was reduced in workers who were exposed for 5 years or more. This reduction was dose-dependent and was related to styrene concentrations in breathing-zone air and mandelic acid concentrations in urine. The results suggest that the probable synergistic effect of organic solvents and noise on hearing should be considered even when the exposures are within the occupational exposure limits. Less