Factors affecting laterality of standard threshold shift in occupational hearing conservation programs.

Although noise-induced hearing loss is generally characterized as bilaterally symmetrical, 80% of audiometric shifts meeting Occupational Safety and Health Administration (OSHA) criteria were unilateral in a large industrial sample. The primary factor related to unilateral OSHA shifts was asymmetric baseline hearing level. Better ears at baseline audiogram were more likely to demonstrate OSHA shifts than poorer ears. When hearing levels were symmetric at baseline, left ears were more likely to demonstrate OSHA shifts. Although OSHA shifts were primarily unilateral, cumulative effects of noise exposure were judged to be bilaterally symmetrical due to larger hearing shifts occurring in better ears of subjects with preexisting hearing asymmetries. Recovery from temporary threshold shift in second ears tested was judged not to be a significant determinant for high prevalence of unilateral shifts. Many factors likely interact to confound interpretive value of OSHA shift laterality in occupationally noise-exposed populations. This holds important implications for hearing conservation program professionals considering hearing shift laterality as a factor in shift etiology.