[Permanent shift of auditory thresholds caused by continuous and impulse noise: comparative studies]

The studies included two groups of workers, only men, of maximally similar structure of age and length of employment but different exposure conditions. 112 cotton weavers worked under continuous noise and 64 forgers under impulse noise. Noise measurements demonstrated that in both groups the equivalent levels were almost identical (101.8 dB-A in the weaving room and 100.4 dB-A at the forge). The permanent hearing threshold shift in both types of exposure was found to exhibit similar time course, developing within 10-15 years of exposure, then stabilizing, and reaching the asymptote within about 20 years. Despite the same dose of acoustic energy, the hearing impairment induced by impulse noise was greater and is characterized by the maximum loss at 6 kHz frequency, as opposed to the notch at 4 kHz typical of continuous noise.