The Effect of Noise Exposure and Age on the Changes of Group Mean Hearing Threshold Level: Annual Follow-up Studies for Seven Years

Lee, Ji Ho; Lee, Choong Ryeol; Yoo, Cheol In; Yaang, Seung Rim; Kim, Ok Hyun; Cho, Byung Mann; Lee, Su Ill; Kim, Don Kyoun

Abstract

This study was carried out to evaluate the effect of noise exposure and age on the changes of group mean hearing threshold level(HTL) over 7-year period. For this purpose, consecutive annual audiometric tests were performed from 1991 to 1997. among 718 male employees of a metal product manufacturing factory. The subjects were divided four groups as follows according to noise level category(NLC). NLC-I officer, exposed noise level was under 60 dB(A) of time weighted average(TWA) NLC-JI technical assistant or engineer, they exposed to workplace noise occasionally NLC-IJJ worker, exposed noise level was below 85 dB(A) of TWA. wore hearing protection device(earmuff or earplug) NLC-IV worker, exposed noise level was over 85 dB(A) of TWA, wore hearing protection device(earmuff and earplug). The results were as follows 1. The improvement of group mean HTL was continued until the fifth year, showing the peak at the third year. The magnitudes of this learning effect were 1.5 - 4.6 dB. 2. The mean HTL of each age group tended to increase after the third year and the tendency was more prominent at 4000 Hz. 3. In noise exposed group(NLC-II, III, and IV), mean HTL increased from the third year after decreasing, meanwhile, in noise free group(NLC-I). it was few changed for the entire period. Among the noise exposure group, the mean HTL of NLC-IV was lower than that of NLC-III and NLC-IV. 4. After learning effect, the mean increase of HTL in noise free group(NLC-I) was 0.4-1.7 dB that suggests aging effect, and that in noise exposure group(NLC-ll, III, and IV) was 0.9 -4.1 dB that suggests noise effect. 5. Statistical analysis of the general linear model implicated that the effect of age was statistically significant at 500, 3000, 4000 and 6000Hz. and the effect of noise exposure was statistically significant at all frequencies except 6000Hz. However, the age * noise interaction was not significant at all frequencies. From these results, it was concluded that the effect of age and noise exposure seems to affect the mean HTL independently and these two factors contribute to an additive effect for the mean HTL change. Furthermore, more concerns should be needed for hearing conservation of low level exposures without any specific protection.