An Investigation of the Adjustment of Retrospective Noise Exposure for Use of Hearing Protection Devices

Background: A previous study in the same working population showed a stronger relation for noise and acute myocardial infarction among those who did not wear HPD. Because accurate noise exposure assessment is complicated by the use of HPD, we previously developed a multilevel model of the likelihood of HPD use for British Columbia (Canada) lumber mill workers. Historical estimates of noise exposure can be adjusted according to models predictions and a reduction in misclassifying workers, exposure is expected. Methods: Work history and exposure information were obtained for 13 147 lumber mill workers followed from 1909 until 1998. Audiometric data for the cohort, including hearing threshold levels at several pure tone frequencies, were obtained from the local regulatory agency for the period from 1978 to 2003. Following the modeling of HPD use, noise estimates were adjusted according to models predictions and attenuation factors based on existing research and standards. Adjusted and unadjusted noise metrics were compared by investigating their ability to predict noise-induced hearing loss. Results: We showed a 4-fold increase in the noise exposure and hearing loss slope, after adjusting for HPD use, while controlling for gender, age, race, as well as medical and non-occupational confounding variables. Conclusion: While the relative difference before and after adjustment for use of HPD is considerable, we observed a subtle absolute magnitude of the effect. Using noise-induced hearing loss as a 'gold standard' for testing the assessment of retrospective noise exposure estimates should continue to be investigated.