Estimation of Occupational Noise–Induced Hearing Loss Using Kurtosis-Adjusted Noise Exposure Levels

Studies have shown that in addition to energy, kurtosis plays an important role in the assessment of hearing loss caused by complex noise. The objective of this study was to investigate how to use noise recordings and audiometry collected from workers in industrial environments to find an optimal kurtosis-adjusted algorithm to better evaluate hearing loss caused by both continuous noise and complex noise. DESIGN: In this study, the combined effects of energy and kurtosis on noise-induced hearing loss (NIHL) were investigated using data collected from 2601 Chinese workers exposed to various industrial noises. The cohort was divided into three subgroups based on three kurtosis (β) levels (K(1): 3 ≤ β ≤ 10, K(2): 10 <β ≤ 50, and K(3): β > 50). Noise-induced permanent threshold shift at test frequencies 3, 4, and 6 kHz (NIPTS(346)) was used as the indicator of NIHL. Predicted NIPTS(346) was calculated using the ISO 1999 model for each participant, and the actual NIPTS was obtained by correcting for age and sex using non-noise-exposed Chinese workers (n = 1297). A kurtosis-adjusted A-weighted sound pressure level normalized to a nominal 8-hour working day (L(Aeq,8h)) was developed based on the kurtosis categorized group data sets using multiple linear regression. Using the NIPTS(346) and the L(Aeq.8h) metric, a dose-response relationship for three kurtosis groups was constructed, and the combined effect of noise level and kurtosis on NIHL was investigated. RESULTS: An optimal kurtosis-adjusted L(Aeq,8h) formula with a kurtosis adjustment coefficient of 6.5 was established by using the worker data. The kurtosis-adjusted L(Aeq,8h) better estimated hearing loss caused by various complex noises. The analysis of the dose-response relationships among the three kurtosis groups showed that the NIPTS of K(2) and K(3) groups was significantly higher than that of K(1) group in the range of 70 dBA ≤ L(Aeq,8h) < 85 dBA. For 85 dBA ≤ L(Aeq,8h) ≤ 95 dBA, the NIPTS(346) of the three groups showed an obvious K(3) > K(2) > K(1). For L(Aeq,8h) >95 dBA, the NIPTS(346) of the K(2) group tended to be consistent with that of the K(1) group, while the NIPTS(346) of the K(3) group was significantly larger than that of the K(1) and K(2) groups. When L(Aeq,8h) is below 70 dBA, neither continuous noise nor complex noise produced significant NIPTS(346). CONCLUSIONS: Because non-Gaussian complex noise is ubiquitous in many industries, the temporal characteristics of noise (i.e., kurtosis) must be taken into account in evaluating occupational NIHL. A kurtosis-adjusted L(Aeq,8h) with an adjustment coefficient of 6.5 allows a more accurate prediction of high-frequency NIHL. Relying on a single value (i.e., 85 dBA) as a recommended exposure limit does not appear to be sufficient to protect the hearing of workers exposed to complex noise.