In all subjects, increasing trend of hearing threshold levels (HTLs) was observed after three years of follow-up at all testing frequencies in audiometry and this increase was statistically significant at 8KHz(p<0.05). Also, HTLs increased significantly(p<0.05) in the workers who complained tinnitus(at 0.25, 8KHz), who ws below 90 dB (at 8KHz). Effectiveness of the activities for hearing conservation, assessed by sequential test-retest comparisons, was lower in the workers who complained tinnitus, who had past history of ear disease, and whose workplace noise level was below 90 dB.

These findings suggest that the activities for hearing conservation at workplace may have been unsatisfactory until now and more stringent measures may be necessary, especially in the workers who complain tinnitus, who have past history of ear disease, and whose workplace noise level is below 90dB.