[Adjustment of dose-response relationship of industrial impulse noise induced high frequency hearing loss with different exchange rate].

Objective: To explore a new method to evaluate the dose-response relationship between the noise exposure and prevalence of noise induced high frequency hearing loss.

Methods: Physical examination and questionnaire survey were conducted among 32 mechanical workers, 12 males and 20 females, aged 35.1 +/- 7.2, exposed to industrial impulse noise (impulse noise group), and 163 textile workers, 82 males and 81 females, aged 31.5 +/- 8.7, exposed continuous noise (continuous noise group). SH-126 dosimeter was used to measure the A weighted equal sound level in continuous eight working hours (L(Aeq.8h)) so as to evaluate the individual noise exposure. Cumulative noise exposure (CNE) was calculated with the values of L(Aeq.8h) and noise working year adjusted by different exchange rates (ERs) for each worker. Hearing threshold was measured by audiometer with routine method and adjusted by age and gender according to GBZ49-2002. High frequency hearing loss was diagnosed based on the GBZ49-2002 for each worker.

Results: According to the equal energy rule, with the ER = 3 the CNE of the impulse noise group was [103.2 dB (A).year +/- 4.2dB (A).year], significantly lower than that of the continuous noise group [110.6 +/- 6.0 dB (A).year, P < 0.05]. With the ER = 5.5, the CNE of the impulse noise group was 110.3 +/- 6.6 dB (A).year, not significantly different from that of the continuous noise group [110.6 +/- 6.0 dB (A).year]. The high frequency hearing loss prevalence of the impulse noise group was 68.8%, not significantly different that of the continuous noise group (65%, P > 0.05). Trend chi square test showed significant differences in the relationship between CNE and hearing loss prevalence among the impulse noise group with the ER = 3, the impulse noise group with the ER = 5.5, and continuous noise group (all P < 0.01). With the ER = 3, the high frequency hearing loss prevalence of the impulse noise group with the CNE of 100-104 dB (A).year was 76.9%, significant higher than that of the continuous noise group (30.4%, P < 0.05); and the high frequency hearing loss prevalence of the impulse noise group with the CNE of 105-108 dB (A).year was 90.9%, significantly higher than that of the continuous noise group (50.0%, P < 0.05). With the ER = 5.5, there was no difference in the prevalence of high frequency hearing loss between the continuous noise group and impulse noise group. Logistic regression model showed that with the ER = 3, the dose-response curve of the impulse noise group presented a left shift and with a slope sharper than that of the continuous noise group. With the ER increasing the dose-response curve of the impulse noise group showed a right shift and with a gradually lowering slope. When the ER equaled to 6 in the impulse noise group and equaled to 3 in the continuous noise group, the two dose-response curves were almost superposed.

Conclusion: Impulse noise causes more serious damage in causing high frequency hearing loss than continuous noise according to equal energy rule. Increasing the ER value in the impulse noise group adjusts the dose-response curve to be close to that of the continuous noise group.