Objective To study the tendency of high frequency hearing threshold in noise exposure workers under in dividual hearing protection and discuss relevant regulations in order to provide epidemiological evidence for occupational noise protection practice and making relative criteria.
Methods A total of 698 ears of 355 workers received 4 times of pure tone audiometer tests during the period of 3 years in 5 enterprises in Shanghai, where occupational noise levels were over the national limitation≥ 85dB(A) and hearing conservation programs were carried out. The test results were analyzed statistically.
Results The average of low-frequency hearing threshold in all selected workers was below 25dB(HL) over the 4 tests, and no significant change was observed. The average of high-frequency hearing threshold increased with the length of service, and the yearly average of high frequency hearing threshold increased within the groups with different length of service over the 3 years' observation. Comparing with the first hearing test result, the averages of high frequency hearing threshold of the subjects with normal high frequency hearing, mild high frequency hearing impairment, or subjects under surveillance were increased by year and that of the normal group increased (4.14& #177;3.98) dB(HL) in 3 years.
Conclusion The yearly variation of average high frequency hearing threshold among all subjects was less than 2 dB(HL), which indicated that individual hearing protection device assisted noise exposure workers' hearing loss more controllable. So the period of health surveillance for noise exposure workers in service may extend to 2 years or longer.
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