Modeling Individual Noise-Induced Hearing Loss Risk with Proxy Metrics of External-Ear Amplification
Grinn, Sarah K
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Noise-induced hearing loss (NIHL) can be incurred from occupational or recreational noise exposure. Regardless, NIHL is a preventable form of hearing loss; with accurate knowledge of risk and protective strategies, it can be avoided in almost all cases. Nonetheless, it presents with astounding global incidence, prevalence, and financial toll; there is an immediate need to redesign and strengthen hearing conservation efforts. Two primary factors categorize high-risk NIHL populations: 1) over-exposure to dangerous noise, and 2) individual vulnerability to auditory injury from dangerous noise. At present, established NIHL-risk criteria exist to identify noise over-exposure, accomplished by measuring the sound-level and duration of the exposure; however, there is currently no way to identify individuals who are inherently more vulnerable to auditory injury, as variable vulnerability has never been sufficiently explained. There is a strong possibility that the contribution of external-ear amplification could influence individual vulnerability in NIHL-risk, as amplification is likely to vary across individuals due to ear size and shape. This research tested the hypothesis that 1) participant’s NIHL-risk estimation will considerably differ after accounting for individual external-ear amplification, and 2) external-ear amplification can be estimated by non-invasive and less-technical proxy metrics (e.g., pinna size, earcanal size, body height). 158 participants (age 4-8, 13-17, and 18+ years) completed a demographic questionnaire, otoscopy, tympanometry, pinna measurement, and external-ear amplification measurements during noise stimuli. Noise stimuli included a 58 dB-A pink-noise presented in the free-field and a 53 dB-A pink-noise presented via insert-earphones. Two external-ear measurements (one for each stimulus, respectively) were obtained: 1) amplification derived from combined external-ear structures (i.e., pinna, concha, earcanal), and 2) amplification derived solely from the earcanal structure. External-ear amplification measurement #1 was obtained by placing a probemicrophone near the eardrum, and calculating the pink-noise sound-level difference between the microphone inside the ear and another microphone which measured the sound-level outside the ear; external-ear amplification measurement #2 was obtained by comparing the sound-level recorded by the microphone inside the participant’s ear to the sound-level observed inside a 2.0 cc reference coupler. Individual variability in noise-dose and subsequent NIHL-risk was estimated in hypothetical scenarios after adjusting for the participant’s external-ear amplification added to the sound-level of the exposure. Correlations between external-ear amplification and hypothesized proxy metrics were evaluated. Amplification derived from combined external-ear structures (i.e., pinna, concha, and earcanal) was highly variable (5-19 dB-A), confirming large variability in estimated individual NIHL-risk for free-field exposures; participants with highest amplification were estimated to accrue up to 7x higher in-ear noise-dose than participants with lowest amplification. This type of amplification can be reliably estimated using the proxy metric of pinna height (p < .05). Further, external-ear amplification derived solely from the earcanal structure was highly variable (8-19 dB-A), confirming considerable variation in in-ear NIHL-risk estimation in an ear-level exposures (e.g., earbud music-listening) (safe-listening durations ranging from 6-28 hours). This type of amplification can be reliably estimated using the proxy metrics body height or earcanal size (p <.05). Proxy metric accuracy was confirmed in a separate dataset. External-ear amplification was observed to be highly variable in this study; it is likely that this degree of variability is a source of individual vulnerability to NIHL, given that more noise means more risk. The data from this thesis identify potential non-technical proxy metrics that could reliably estimate external-ear amplification. Taken together, these results make it possible to imagine the inclusion of external-ear amplification as a risk-factor for identification of individuals at risk for NIHL.