A cohort of over 450 construction apprentices and unexposed graduate student controls was enrolled and assessed annually for hearing damage over the past five years. Results demonstrated small but significant reductions in Distortion Product Otoacoustic Emissions (DPOAEs) in relation to hearing protection-adjusted noise exposure, especially around 4 kHz. We propose to continue studying approximately 300 cohort members who have had at least two previous examinations for noise exposure and hearing damage by measuring air conduction hearing threshold levels (HTL.s) and DPOAEs. An additional five years of evaluation of the early progression of noise-induced hearing damage among young construction workers exposed to high but variable noise levels, and prospective documentation of the relationship between DPOAEs and standard HTLs, will provide much greater quantification of the risk and development of Noise-Induced Permanent Threshold Shift (NIPTS). We will enhance our previous efforts by improving the accuracy of both hearing damage assessment and noise exposure estimation, while collecting data that is consistent with our previous efforts. Annual examinations will include standard audiometry, wide-band eminence acoustic reflex (AR), and DPOAEs, including a DP-Gram with L1=65 dB and I/O growth functions at 2, 3, 4 and 6 kHz. We will include audiometric testing with a 2 dB stepsize in order to improve the resolution of the measurement. AR measurement will allow for consideration of the AR as a potential modifier of NIPTS risk. DPOAE measurements will be collected using an improved protocol, and DPOAE I/O functions will be modeled using a segmented regression technique for better estimation of a DPOAE 'threshold'. Exposure estimation will continue using a task-based approach, but will be enhanced by obtaining activity and task updates from subjects quarterly, improving the specificity of task and tool definitions, and asking subjects to subjectively rate each of their exposures with reference to expected levels for the specific task. Sub-studies will be conducted to quantify the effects of each of these methodological changes and ensure continuity with the first five-year study. By continuing to follow this cohort, we will obtain much needed data concerning the natural history of early noise-induced hearing loss and the potential use of DPOAEs as markers of early hearing damage and predictors of later hearing loss. Continued prospective study of this established and well characterized cohort of construction workers is a unique opportunity to further our understanding of NIPTS, arid to identify potential approaches to hearing loss prevention.
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