Five-Year Longitudinal Cohort Study Determines the Critical Intervals for Periodic Audiometric Testing Based on 5070 Tests of Metallurgical Workers Exposed and Nonexposed to Noise

To compare the progression of 3-, 4-, and 6-kHz thresholds (pure-tone average) over 5 years and determine the most critical period for occupational risk among workers exposed and nonexposed to noise.

Metallurgical workers were divided into 2 groups: noise-exposed and non-noise-exposed groups. The 6 initial audiometric tests of each worker were analyzed as baseline test and annual tests 1 to 5.

A total of 845 workers were included, 748 in the noise-exposed group and 97 in the non-noise-exposed group, resulting in 5070 tests analyzed. The nonexposed group showed no significant difference in the mean pure-tone averages between any of the annual tests in either ear. In the exposed group, a significant difference was observed in mean pure-tone averages between baseline and Test1 (p = 0.001 right ear; p = 0.001 left ear), between Test3 and Test4 (p = 0.002 right ear; p = 0.005 left ear), and between Test4 and Test5 (p = 0.003 right ear; p = 0.001 left ear). There was no difference between Test1 and Test2 or between Test2 and Test3 in either ear.

The progression of pure-tone averages at 3, 4, and 6 kHz differed between workers exposed and nonexposed to noise. Noise-exposed workers had a significant progressive worsening of audiometric thresholds after 3 years of employment. This study identified, in an unprecedented way, two critical periods of noise exposure: in the first year and after the third year of employment in a noisy environment.