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Preventing Noise-Induced Hearing Loss

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Preventing Noise-Induced Hearing Loss INVITED COMMENTARY Preventing Noise-Induced Hearing Loss Julia Doswell Royster Noise-induced hearing loss (NIHL) is second only to presby- each higher audiometric test frequency (3 kHz, 4 kHz, cusis, age-related hearing loss, as a cause of hearing impair- 6 kHz, 8 kHz, which correspond to increasingly higher ment; yet, it is preventable. This commentary summarizes pitches); NIHL grows most rapidly during early years of the effects of noise on hearing, relevant workplace regula- exposure, with the majority of damage occurring in the tions, and ways to minimize excessive noise’s deleterious first 10 years of exposure; NIHL initially presents as high- impact on individuals’ quality of life on and off the job. frequency notches in the audiogram, and the audiometric frequency with maximum notching depends on noise spec- trum and the ear canal’s size and resonance. Initial notches he risk of noise-induced hearing loss (NIHL) depends usually affect 3 kHz, 4 kHz, or 6 kHz with recovery at Ton noise exposure, which is the combination of duration 8 kHz; with additional years of noise exposure NIHL spreads and sound level at the ear. Gradual hearing damage corre- to lower frequencies, but 2 kHz and below are affected lates best with sound level in decibels measured using the less than higher frequencies; tinnitus often accompanies A-weighting network (dBA), which over-emphasizes the pri- NIHL and may be as debilitating as the hearing loss itself; mary speech frequencies and de-emphasizes sounds below concurrent exposure to chemicals may potentiate NIHL. 500 Hz and above 6000 Hz. Exposures to steady or fluctu- [3] For additional details, see a guidance statement from ating noise less than 85 dBA averaged over an 8-hour day the American College of Occupational and Environmental pose little risk of hearing damage over a lifetime. Gradual Medicine (ACOEM) [4]. NIHL develops through repeated temporary threshold shifts in hearing that incrementally fail to recover back to baseline. Nonoccupational Noise In contrast to NIHL, acoustic trauma results in immediate Many people are not exposed to significant nonoc- permanent hearing loss due to impulse noise (eg, gunfire cupational noise frequently enough and long enough to or explosive blasts) or short durations of exposure to sound incur NIHL, but potentially damaging noise sources include levels exceeding 115 dBA. Both NIHL and acoustic trauma gunfire, fireworks, chain saws, power tools, woodwork- are permanent sensorineural hearing losses. NIHL is gener- ing equipment, and attendance at sporting events, motor ally a bilateral, fairly symmetrical, high-frequency emphasis races, and loud concerts. Personal stereo players can be loss, but acoustic trauma may cause markedly asymmetrical hazardous if the user wears the device regularly enough at thresholds if the sound levels at the ears differ due to the high in-ear sound levels. Children are susceptible to acous- head shadow effect. For example, gunfire from long-barrel tic trauma if they activate sound sources (for example a arms causes greater loss in the ear closer to the barrel. bicycle horn or cap pistol) close to their ears. A recent Individuals vary in their susceptibility to both age- review [5] discusses challenges in assessing nonoccupa- related hearing loss and noise damage. The International tional noise damage. What is clear is that if people expe- Organization for Standardization standard ISO 1999:2013 rience temporary threshold shift or tinnitus after a noise [1] and the corresponding American National Standards exposure, or if they need to shout to communicate at arm’s Institute standard ANSI 3.44 (R 2006) [2] permit esti- length, then they need to avoid that exposure unless wear- mation of the total hearing loss from the combination of ing hearing protection devices (HPDs). aging and NIHL from years of 8-hour noise exposures of 85 to 100 dBA for population fractiles differing in suscep- Occupational Noise tibility. These standards include choices of presbycusis According to the National Institute for Occupational and age-effect reference populations that differ in terms of Safety and Health (NIOSH), over 22 million workers are whether subjects were screened for otological disease and exposed to potentially damaging occupational noise in the nonoccupational noise exposure. Figure 1 illustrates com- bined hearing loss from age and noise. Electronically published March 31, 2017. Important characteristics of the relationship between Address correspondence to Dr. Julia Doswell Royster, 4706 Connell Dr, age-related hearing loss and gradual NIHL include the Raleigh, NC 27612 ([email protected]). N C Med J. 2017;78(2):113-117. ©2017 by the North Carolina Institute of following: age-related hearing loss grows slowly at young Medicine and The Duke Endowment. All rights reserved. ages, accelerates with advancing age, and increases at 0029-2559/2017/78210 NCMJ vol. 78, no. 2 113 ncmedicaljournal.com figure 1. Hearing loss from age alone compared to combined hearing loss from aging and noise exposure as predicted using ANSI S3.44 [2] for typical ears (median susceptibility) of the white male nonindustrial noise-exposed population from Annex C at age 30 (panel A) and at age 60 (panel B) at noise exposures of 85-100 dBA for 10 versus 40 years. US [6], and additional personnel are noise-exposed in mili- is steady and the worker is exposed for 8 hours, then the tary service. Workers in general industry receive some pro- choice of exchange rate makes no difference. The 3-dB tection from NIHL via the Occupational Safety and Health exchange rate overestimates the hazard of intermittent Administration (OSHA) Noise Standard and Hearing noise which, because it is not continuous, allows ears to Conservation Amendment, 29 CFR §1910.95 [7]. This regu- recover some during quiet breaks. For a discussion, see lation defines the daily permissible exposure level without Dobie and Clark [9]. enforced use of hearing protection devices (HPDs) as an The required phases of an HCP under 29 CFR 1910.95 8-hour time-weighted average exposure (TWA) of 90 dBA are as follows: sound surveys and employee exposure mon- and mandates that workers be included in a hearing con- itoring; engineering and/or administrative noise controls servation program (HCP) if their TWA is 85 dBA or higher. to reduce employee exposures; hearing protection device OSHA TWAs are calculated using a 5-dB exchange rate, availability at a TWA of 85 dBA and mandatory wear if the which assumes that if the daily duration of noise is cut in TWA exceeds 90 dBA; employee education about effects half, then a sound level 5 dBA higher will deliver an equiva- of noise and use of HPDs; annual audiometric monitoring; lent daily noise dose and equal potential for NIHL. Many and recordkeeping, to report occupational hearing loss, countries and the ISO 1999 model use a 3-dB exchange on OSHA’s Form 300 (log of occupational injuries and rate, which is also recommended by NIOSH [8]. Table 1 illnesses). illustrates the OSHA and NIOSH criteria. If the sound level Workers not covered by OSHA’s general industry standard fall under separate standards for mining [10], table 1. railroads [11], construction [12], and the Department of Durations of Noise Exposure Allowed without Wearing Defense [13]. The regulation for construction workers is Hearing Protection at Selected A-Weighted Sound Levels much less detailed, and there is no standard specific to Under the Criteria of Osha [7] And Niosh [8]. agricultural workers, who are also exposed to hazardous OSHA criteria, NIOSH criteria, noise. The military standard includes provisions regarding 5-dB exchange rate, 3-dB exchange rate, impulse noise from weapons. For an encyclopedic volume Allowed unprotected PEL=90dBA for PEL=85dBA for exposure duration 8 hours 8 hours on noise and HCPs, see The Noise Manual 5th Edition [14]. 16 hours 85 dBA 82 dBA Unfortunately, hearing loss continues to be preva- 8 hours 90 dBA 85 dBA lent in many industries [15]. HCPs are often not effective 4 hours 95 dBA 88 dBA because management frequently prefers not recording 2 hours 100 dBA 91 dBA occupational hearing loss to preventing that hearing loss 1 hours 105 dBA 94 dBA in the first place. The following are OSHA’s recordability .5 hours 110 dBA 97 dBA criteria for occupational hearing loss [16]: OSHA standard .25 hours 115 dBA 100 dBA threshold shift (STS), a change from an ear’s audiometric PEL = permissible exposure level. baseline of 10 dB averaged over 2 kHz, 3 kHz, and 4 kHz 114 NCMJ vol. 78, no. 2 ncmedicaljournal.com after application of age corrections; an absolute thresh- if properly fitted and consistently worn. Unfortunately, in old average at 2 kHz, 3 kHz, and 4 kHz of 25 dB or more; many workplaces HPD use is poorly enforced and employ- determination of work-relatedness by a physician or other ees are not well trained on how to achieve adequate atten- licensed health care professional such as an audiologist; if uation. Moreover, the noise reduction rating (NRR) labeled the hearing loss occurred by acoustic trauma, it is recorded on HPD packaging greatly overestimates real-world atten- as an injury; if gradual, it is recorded as an illness. Both the uation and does not even rank different HPDs in terms of National Hearing Conservation Association [17] and the relative real-world attenuation [20], so purchasers do not ACOEM [18] have guidance statements relevant to work- have any readily available means of comparison shopping. relatedness and recording of hearing loss. OSHA deems The ability of HPDs to reduce noise at the eardrum this defined shift recordable if occupational exposure con- depends largely on whether the wearer attains a seal either tributed to the shift. around the ear (for earmuffs) or in the ear canal (for ear- Because the 3 frequencies averaged for STS differ in plugs).
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