While the majority of hearing loss in North American adults is age related, 5% to 10% is caused by occupational noise exposure.[1] Acoustic trauma and noise-induced hearing loss (NIHL) account for the majority of work-related hearing impairment.

As part of hearing conservation programs, BC employers are required to provide eight components, including annual hearing screenings for noise-exposed workers. Still, WorkSafeBC currently has about 30 000 ac­tive occupational (NIHL) claims, with another 3000 or so new claims added each year. Annual health care costs associated with these claims are more than $20 million, with $2 million for hearing aid batteries alone. Given the financial implications and significant social consequences, it is important to remember that most NIHL is preventable.

Acoustic trauma
Acoustic trauma refers to mechanical injury to the ear following exposure to sudden intense sound. The critical sound pressure level (SPL) needed to cause acoustic trauma is approximately 140 dB SPL.[2-4] Conductive hearing loss rarely results from noise, but a severe blast can rupture the eardrum, dislocate the middle ear ossicles, or both.[5]

Acoustic trauma usually results in temporary hearing loss and tinnitus, with full recovery in days or weeks. However, very intense exposures can produce permanent sensorineural hearing impairment.[6,7] Progressive hearing loss is not expected with this type of injury, and any permanent hearing impairment should stabilize within 6 to 12 months of the incident.[5,8,9]

Traumatic hearing loss may be asymmetrical, with greatest hearing loss in the ear closest to the noise source.[2,5,6,10]

NIHL
Exposure to noise of moderate intensity—85 to 130+ dBA—over many years can result in NIHL,[11] which is caused by metabolic and/or structural damage to the inner ear. Since the inner ear is affected, the loss is always sensorineural, rather than conductive.

NIHL presents as a gradual, symmetrical decline in hearing, even where the noise source is consistently on one side. The decline is symmetrical because of reverberation and head movement, and because very little energy is lost as sound travels from one side of the head to the other.[10,12]

Earliest deterioration is typically in the 3000 to 6000 Hz range.[4,5,13] However, a 4000 Hz audiometric notch can also be caused by non-occupational conditions, and has been demonstrated in individuals who have never been exposed to noise.[4,13]

Hazardous noise contributes to hearing loss most rapidly in the first few years of exposure.[14,15] However, NIHL does not progress once the noise exposure has ceased.[14,16] In patients who show signs of NIHL, further loss can be prevented with appropriate hearing protection or noise control measures.

Recommendations
If occupational hearing loss is suspect­ed, please submit a Form 8 to Work­SafeBC and then proceed as you would with any other patient. That may in­clude referral to an audiologist or hearing instrument practitioner for further follow-up. Since non-occupational pathologies, such as vestibular schwan­noma, otosclerosis, and sudden senori­neural hearing loss can also be present in noise-exposed individuals, otologic referrals for red flag conditions (e.g., asymmetry > 30 dB, unilateral tinnitus, sudden hearing change) would be appropriate regardless of noise exposure history. These tests may or may not be covered by WorkSafeBC.

For more information
For more information about acoustic trauma or NIHL, or WorkSafeBC’s hearing loss diagnostic or treatment services, please contact a medical advisor in your nearest WorkSafeBC office. For worker handouts and other information related to hearing protection and hearing conservation programs, please visit WorkSafeBC.com [11].
—Ronette Haboosheh, MSc, RAUD, RHIP, Audiologist Advisor, WorkSafeBC
—Sasha Brown, MSc, RAUD, Aud(C), Occupational Audiologist, WorkSafeBC

This article is the opinion of WorkSafeBC and has not been peer reviewed by the BCMJ Editorial Board.

References

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2.    Stewart M, Pankiw R, Lehman M, et al. Hearing loss and hearing handicap in users of recreational firearms. J Am Acad Audiol 2002;13:160-168.
3.    Melnick W. Hearing loss from noise exposure. In: Harris C (ed). Handbook of acoustical measurements and noise control. Woodbury, NY: Acoustical Society of America; 1998:18.1-18.19.
4.    Wilson RH. Some observations on the nature of the Audiometric 4000 Hz notch: Data from 3430 veterans. J Am Acad Audiol 2011;22:23-33.
5.    Dobie R. Medical-legal evaluation of hearing loss. San Diego: Singular; 2001. 393 pp.
6.    Alberti P. Traumatic sensorineural hearing loss. In: Ludman H, Wright T (eds). Diseases of the ear. New York: Oxford University Press; 1998:483-494.
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9.    Sataloff R, Sataloff J. Occupational hearing loss. New York: Marcel Dekker; 1993.  833 p.
10.    Clark W. Five myths in assessing the effects of noise on hearing. St Louis, MO: Central Institute for the Deaf, 2000. Accessed 1 January 2001. www.audiologyonline.com [12]. 
11.    International Organization for Standardization. Determination of occupational noise exposure and estimation of noise-induced hearing impairment, database B. ISO 1999:1990. Geneva: International Organization for Standardization; 1990.
12.    Chung DY, Mason K, Willson GN, et al. Asymmetrical noise exposure and hearing loss among shingle sawyers. J Occup Med 1983;25:541-543.
13.    Sataloff R, Sataloff J. Occupational hearing loss: Basic concepts. J Occup Hear Loss 1993;1:7-15. 
14.    American Academy of Otolaryngology. Head and Neck Surgery Foundation, Inc. The Subcommittee on the Medical Aspects of Noise. Evaluation of people reporting occupational hearing loss. J Occup Hear Loss 1998;2:1-10.
15.    Albera R, Lacilla M, Piumetto E, et al. Noise-induced hearing loss evolution: Influence of age and exposure to noise. Eur Arch Otorhinolaryngol 2010;267:665-671.
16.    Kirchner D, Evenson E, Dobie R, et al. Occupational Noise-Induced Hearing Loss. ACOEM Task Force on Occupational Hearing Loss. J Occup Med 2012;54:106-108.
17.    Lee F, Matthews L, Dubno J, et al. Longitudinal study of pure-tone thresholds in older persons. Ear Hear 2005;26:1-11.