Hearing loss caused by exposure to recreational and occupational noise results in devastating disability that is virtually 100 percent preventable. Noise-induced hearing loss is the second most common form of sensorineural hearing deficit, after presbycusis (age-related hearing loss). Shearing forces caused by any sound have an impact on the stereocilia of the hair cells of the basilar membrane of the cochlea; when excessive, these forces can cause cell death. Avoiding noise exposure stops further progression of the damage. Noise-induced hearing loss can be prevented by avoiding excessive noise and using hearing protection such as earplugs and earmuffs. Patients who have been exposed to excessive noise should be screened. When hearing loss is suspected, a thorough history, physical examination and audiometry should be performed. If these examinations disclose evidence of hearing loss, referral for full audiologic evaluation is recommended.

Noise-induced hearing loss is a sensorineural hearing deficit that begins at the higher frequencies (3,000 to 6,000 Hz) and develops gradually as a result of chronic exposure to excessive sound levels.[1] Although the loss is typically symmetric, noise from such sources as firearms or sirens may produce an asymmetric loss. Acoustic trauma, a related condition, results from an acute exposure to short-term impulsive noise.

Noise is perhaps the most common occupational and environmental hazard. As many as 30 million Americans are exposed to potentially harmful sound levels in their workplaces.[3] Outside of work, many persons pursue recreational activities that can produce harmful noise. Sixty million Americans own firearms, and many use them without adequate hearing protection.[4] Other nonoccupational sources of noise include chain saws and other power tools, amplified music,[5] and recreational vehicles such as snowmobiles and motorcycles. Some types of toys for children can produce sounds capable of causing permanent hearing damage.[6]

Noise can be described in terms of intensity (perceived as loudness) and frequency (perceived as pitch). Both the intensity and the duration of noise exposure determine the potential for damage to the hair cells of the inner ear. Even sounds perceived as “comfortably” loud can be harmful.

Sound intensity is measured as sound pressure level (SPL) in a logarithmic decibel (dB) scale (Table 1). Noise exposure measurements are often expressed as dB(A), a scale weighted toward sounds at higher frequencies, to which the human ear is more sensitive. Noise can cause permanent hearing loss at chronic exposures equal to an average SPL of 85 dB(A) or higher for an eight-hour period.[7] Based on the logarithmic scale, a 3-dB increase in SPL represents a doubling of the sound intensity. Therefore, four hours of noise exposure at 88 dB(A) is considered to provide the same noise “dose” as eight hours at 85 dB(A), and a single gunshot, which is approximately 140 to 170 dB(A), has the same sound energy as 40 hours of 90-dB(A) noise.[8]

Epidemiology

Noise-induced hearing loss is the second most common sensorineural hearing loss, after age-related hearing loss (presbycusis). Of the more than 28 million Americans with some degree of hearing impairment, as many as 10 million have hearing loss caused in part by excessive noise exposure in the workplace or during recreational activities.[9] The economic costs of occupational hearing loss have been estimated to be in the billions of dollars.[10] Noise-induced hearing loss has been well recognized since the industrial revolution. An early term for the condition was “boilermakers’ disease,” because so many workers who made steam boilers developed hearing loss.[11] In today’s noisy society, even children and young adults are at risk. A recent study found evidence of high-frequency hearing loss in nearly one third of a cohort of college students.[12]

Pathophysiology

To be perceived, sounds must exert a shearing force on the stereocilia of the hair cells lining the basilar membrane of the cochlea. When excessive, this force can lead to cellular metabolic overload, cell damage and cell death. Noise-induced hearing loss therefore represents excessive “wear and tear” on the delicate inner ear structures. Concurrent exposure to ototoxic substances, such as solvents and heavy metals, may increase the damage potential of noise.7 Once exposure to damaging noise levels is discontinued, further significant progression of hearing loss stops. Individual susceptibility to noise-induced hearing loss varies greatly, but the reason that some persons are more resistant to it while others are more susceptible is not well understood.[13]

Recent animal experiments suggest that free oxygen radicals may mediate noise damage to hair cells.[14] In the future, use of chemoprotective agents such as antioxidants as well as identification of host risk factors for susceptibility to noise-induced hearing loss, may enhance prevention and treatment efforts.