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We are exposed continuously to sound in our environment. Typically, these sounds are produced or maintained at low levels in the case of home appliances, an air conditioning system, the television or radio, and running water from the bathroom or kitchen faucet. There are times, however, when sound may reach a level of intensity that is potentially harmful to hearing as in the case of a hammer striking metal, the blast from a rifle, a motorcycle engine, and the thrust of a jet engine during take off. The potential for hearing to be temporarily or permanently affected by the sudden exposure to an intense sound is well understood by most individuals. It is just as important to understand that long-term exposure to certain levels of sound can affect sensitive structures within the inner ear over time and cause noise-induced hearing loss (NIHL). When hearing loss occurs as a result of noise exposure it can lead to problems such as understanding others when they speak, particularly on the phone or in noisy environments. It may also lead to the development of a constant ringing, humming, or rushing noise (tinnitus) in one or both ears.
Definition of noise-induced hearing loss
NIHL is a hearing disorder that occurs as a result of either sudden high-level noise exposure leading to a sudden decline in hearing in one or both ears or long-term excessive noise exposure leading to gradual, progressive loss of high frequency hearing over time. The damage is primarily to the hair cells in the inner ear (cochlea). Hearing loss due to long-term noise exposure is typically not profound and rarely exceeds 75 decibels (dBHL) in the high frequencies (4-6000 Hertz (Hz)) and 40 dBHL in the low frequencies (1-3000 Hz). The rate of further loss tends to lessen as the hearing sensitivity declines over time. The typical pattern of hearing loss shows a “notching” of the audiogram at high frequencies of 3000, 4000, and 6000 Hz with recovery at 8000 Hz. (see Figure 1). The initial hearing loss is centered at 4000 Hz with an affect on adjacent frequencies with continued exposure. NIHL usually occurs in both ears, but the degree of loss can be different between the right and left ears due to differences in the cumulative exposure between the two ears. For instance, the ear that is closest the muzzle of a rifle may be affected more than the opposite ear in hunting enthusiasts. With blast exposures (i.e., explosions), the ear closest to the blast may be severely affected while the opposite ear remains normal or experiences only a mild loss of hearing.
Fifteen percent of Americans aged 20-69 (26 million) have hearing loss secondary to noise exposure.1 The types of exposures include impact (e.g., hammer against metal) or impulse (e.g., blast from a firearm, jet engine) noise, constant high-level noise in the workplace, and recreational noise (e.g., personal listening devices). The National Health and Nutrition Examination Survey (NHANES) report has estimated that 12.5% of school-aged children (approximately 5.2 million) are affected by NIHL, the majority of these cases related to recreational exposures.2 Although recreational exposures (e.g., personal music devices) in ages 12-19 have increased from 19.8% (1988-94) to 34.8% (2005-06), the rate of NIHL has not changed significantly.3
Noise is a common occupational hazard that leads to one of the most common complaints in adults visiting an Otolaryngologist (Ear, Nose and Throat specialist). The National Institute for Occupational Safety and Health (NIOSH) estimates that approximately 30 million workers in the US are exposed to hazardous noise and that 12% of work-related accidents were cases of NIHL.4 While any worker exposed to loud noise is at risk for developing NIHL, certain occupations have a greater risk. These include construction workers, factory workers, police, firefighters, military personnel, farmers, truck drivers, and musicians. An estimated 242 million is spent annually on worker’s compensation for hearing loss disability.5 The Department of Veterans Affairs spent an estimated 1.4 billion dollars in calculated compensation for hearing loss in fiscal year 2010.6 The World Health Organization describes noise exposure as a major cause of permanent hearing loss around the world with 16% of the disabling hearing loss in adults attributed to occupational noise.7 These data certainly underscore the impact of noise exposure on hearing health and the economy.
How does noise damage hearing
In order to understand how noise can affect hearing, it is important to understand how the ear works. The ear has the unique capability of transforming sound waves into electrical signals that are presented to the brain. When sound waves enter the ear canal they are transmitted to the eardrum (see Figure 2). The eardrum will vibrate and transfer the vibrations to the three middle ear bones (malleus, incus and stapes). The middle ear bones then transfer the air-conducted vibrations to the fluid chamber within the inner ear (cochlea). The transformation of the air-conducted signal to vibrational signals in the cochlea act upon a membrane (basilar membrane) that runs a continuous winding path through the “snail shell-like” configuration of the cochlea. Resting upon this membrane are specialized sensory cells (hair cells) that respond to different frequencies depending on their location in the cochlea. The hair cells move up and down and microscopic hair-like projections (stereocilia) on top of the cells contact a plate-like structure that causes the stereocilia to bend. When this happens, certain chemicals (electrolytes) move into the cell creating an electrical signal. The electrical signal is then carried by the auditory nerve to the brain where sound is perceived and understood.
Effect of noise on hearing
Most people have had the experience of developing ear pressure, fullness, tinnitus and muffled hearing after being subjected to sudden impact (hammer on metal) or impulse (firearm blast) noise, prolonged work with noisy equipment in the garage, or after attending an outdoor rock concert. Fortunately, these symptoms tend to be temporary and resolve over a period of 12-48 hours. If a hearing test (audiogram) is done shortly after the onset of symptoms, it is likely that the audiogram would show a “dip” in high frequency hearing. Repeat testing after a few days may then show recovery of hearing sensitivity to normal levels. However, repeated exposures may lead to a permanent decrease in hearing. This permanent shift in hearing thresholds is associated with irreversible hair cell damage in the cochlea. Once a permanent hearing loss is established, there is no medical or surgical treatment that can restore hearing to normal. NIHL has its greatest affect on communication skills that require speech understanding in complex listening conditions (i.e., background noise). Many of us have had the experience of attempting to understand a one-on-one conversation in a crowded room filled with background noise even though our hearing may be normal. This scenario is extremely difficult for those with hearing loss. High frequency hearing loss that occurs in NIHL is characterized by a loss of consonant discrimination. Consonant sounds such as f, s, t, d, sh and k are high frequency sounds that are critical in the intelligibility of speech.
Chronic noise exposure
The amount of sound that is capable of producing hearing loss is related to a concept called the cochlear damage risk criteria. In other words, the total sound energy delivered to the inner ear over time is predictive of injury and hearing loss. Therefore, an intense sound over a short period of time and a less intense sound over a longer period will produce equal damage to the ear. An increase in sound intensity of 3 dB is associated with a doubling of sound pressure in the ear. Therefore, for each 3 dB increase in sound exposure the time exposed must be cut in half in order to deliver the equal sound energy. The Occupational Safety and Health Administration (OSHA) has established guidelines for industrial hearing conservation programs that adhere to the equal energy concept. The OSHA standard uses a 5 dB rule because of anticipated fluctuations in exposure levels throughout the time of exposure. The measurement used to establish noise levels emphasizes the middle and high frequencies more than the low frequencies. Therefore, decibel measurements use an A-weighted scale (dBA) to establish exposure levels. For example, a 90 dBA exposure (e.g., lawn mower) is allowed for 8 hours, a 95 dBA exposure (e.g., table saw) for 4 hours, and so forth, until a maximum allowable intensity of 115 dBA (e.g., Hammer drill) for 15 minutes. It is essential that employers monitor noise exposure levels if there is potential for employee noise exposure to reach or exceed 85 dBA during an 8-hour work shift. Baseline hearing tests are done within the first six months of employment and testing repeated at least on an annual basis. Furthermore, the potential for hearing loss due to noise may be heightened by the additional exposure to chemicals (i.e., organic solvents, inhalants, metals) in the workplace. Research evidence is clear that occupational exposures to both chemicals and noise can cause a greater effect on hearing than either alone. Despite enhanced awareness of the impact of noise exposure and the stringent policies published by OSHA, occupational NIHL remains a significant health concern.
In industry, employers are obligated to establish a hearing conservation program if noise exposure levels meet or exceed the OSHA 85 dBA rule. The most effective way for employers to reduce the noise hazard is through engineering and administrative controls. Engineering controls are used to reduce the noise output from machinery or equipment and to block the transmission of noise through the use of barriers situated between the employee and the noise hazardous source. Administrative controls, such as reducing the time exposure to noise hazardous equipment by rotating employees, may also further reduce the potential for overexposure. The use of hearing protection devices is an additional and necessary measure for minimizing noise exposure. Whether addressing work-related or recreational exposures, personal hearing protection will further reduce the potential for hearing loss over time. Basically, there are two types of hearing protections devices- earplugs or earmuffs. Commercially available hearing protection devices are labeled with a Noise Reduction rating (NRR) that is based on performance obtained under ideal laboratory conditions. Although disposable foam earplugs (foamies), reusable earplugs, hearing bands, and custom-fitted earplugs may reduce noise exposure by 25-33 dB, it is not uncommon for the user to wear the device improperly or to use them only periodically despite continuous noise exposure. Therefore, the effective noise reduction may be only 15 dB. Earmuffs, active noise reduction earmuffs, and electronic headsets have an NRR of 25-31 dB. The electronic headsets (noise-cancelling headsets), like those used by the military in combat situations, are effective for reducing impulse noise (e.g., firearm blast, explosions) exposure and allow for communication while blocking outside noise. Double hearing protection (wearing both earplugs and earmuffs) may extend the total NRR to 36 dB. Therefore, the potential for total noise reduction is not based on simply adding the individual NRRs for the two devices. Fit and comfort are essential factors in maximizing the NRR potential of any personal hearing protection device.
Treatment options for acute hearing loss due to noise exposure
Although most individuals experience spontaneous recovery of hearing within hours to days following acute hearing loss due to noise exposure, some may experience a prolonged effect. In order to understand the potential treatments for acute hearing loss due to noise, it is beneficial to understand the possible mechanisms that may be responsible for injury to the inner ear. Excessive noise can cause a cascade of events that leads to metabolic stress in the cochlea and/or direct mechanical damage to the fine hair cells in the cochlea. The inner ear will respond to excessive noise by producing substances that are both harmful (reactive oxygen and nitrogen species, free radicals) and helpful or potentially protective (natural antioxidants). The harmful substances may affect the ability of the cells within the cochlea to exchange electrolytes and nutrients properly (i.e., metabolic stress). Excessive metabolic stress may then trigger a process called apoptosis or programmed cell death. When this occurs or when there is direct mechanical damage to the hair cells, hearing loss will occur that may be irreversible. Some recovery may occur, however, either naturally or with treatment intervention. This is because certain locations within the cochlear and the hair cell population in those locations may be partially compromised. Although the “window of opportunity” for treatment is unclear, it is reasonable to consider that prompt treatment could “salvage” those areas of the cochlea that are partially compromised. Steroid therapy (either delivered orally or injected into the middle ear space) may have some value in individual cases of acute hearing loss due to sudden high intensity or prolonged (exposure for several hours) excessive noise exposures. There are receptors widely distributed in the cochlea that bind steroids. The body will naturally release steroid substances (glucocorticoids) into the circulation when the inner ear is subjected to intense noise. The use of steroids as a treatment measure may reduce the inflammatory response in the inner ear, stabilize the circulation of critical substances that allow for proper hair cell function, and improve blood flow to the cochlea. Although the treatment outcome in any individual case cannot be guaranteed, it is best to seek the advice of an otolaryngologist as soon as possible. Although research in the animal model has shown some benefit to the use of antioxidant therapy, there is no clear evidence of benefit in humans. Some of the substances that have been studied include vitamins C and E, resveratrol (grapes, raspberries, blueberries), glutathione (garlic, onions, certain vegetables), D-methionine (beef, fish, eggs, soybean, sesame seeds, brazil nuts), coenzyme Q10 (beef, fish, sesame seeds, canola oil), N-acetyl cysteine (chicken, pork, garlic, yogurt, eggs, broccoli), and acetyl–L-carnitine (beef, lamb, chicken, fish, dairy products). Many of these substances are available in a tablet form sold over-the-counter in drug stores and grocery stores. The Food and Drug Administration (FDA) does not regulate these products, benefit has not been demonstrated in human studies, and proper dosing has not been established.
Preventing noise-induced hearing loss
NIHL is the only type of hearing loss that is potentially completely preventable. Understanding the hazards of noise exposure, taking steps to prevent exposures, and protecting your ears will help to maintain good hearing health. Here are a few suggestions for protecting and preserving your hearing:
• Know the relative noise levels that can cause NIHL (levels at or above 85 dB). Here are a few examples:
o Normal conversation: 60 dB
o Lawn mower, heavy city traffic: 90 dB
o Motorcycles: 95 dB
o Electric drill: 95 dB
o MP3 player (at maximum volume): 105 dB
o Fire alarm, firecrackers: 125 dB
o Military jet takeoff with afterburners: 150 dB
o Firearms: 130-160 dB
At a noise level of 150 dB, the maximum daily exposure time with current technology hearing protection that is worn properly is 8.9 seconds. The distance from the source of the sound and the length of time of exposure are important factors for determining the potential risk for hearing loss.
• Avoid or reduce the time of exposure to high levels of noise.
o Avoid high noise, high risk situations
o Turn down the volume of personal listening devices and music systems
o Distance yourself from excessively loud noise when possible
o Give your ears a rest by limiting the time of exposure to loud noise
• Use powered equipment with limited noise output. Technological advancements in the manufacturing of powered home equipment (power tools, lawn maintenance equipment) has lowered the noise output of certain products.
• Always use hearing protection devices in noisy recreational and work environments.
• Seek the advice of a licensed audiologist and/or otolaryngologist, particularly if there is concern for potential hearing loss.
1. NIH, National Institute on Deafness and Other Communication Disorders, Quick Statistics, http://www.nidcd.nih.gov/health/statistics/Pages/quick.aspx
2. Niskar AS, Kieszak SM, Holmes AE, et al. Estimated prevalence of noise induced hearing threshold shifts among children 6 to 19 years of age: The third national health and nutritional examination survey. 1988-1994, U.S. Pediatrics 2001;108:40-43.
3. Henderson E, Testa MA, Hartnick, C. Prevalence of noise-induced hearing-threshold shifts and hearing loss among US youths. Pediatrics 2011 Jan;127(1):e39-46.
4. National Institute for Occupational Safety and Health (NIOSH). 2001. Work-related Hearing Loss. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2001-103.
5. Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, http://www.cdc.gov/niosh/topics/noise/about.html
6. Department of Defense Hearing Center of Excellence, Stats and Figures. http://hearing.health.mil/HearingLoss101/StatsandFigures.aspx
7. Nelson DI, Nelson RY, Concha-Barrientos M, et al. The global burden of occupational noise-induced hearing loss. Am J Ind Med 2005 Dec;48(6):446-58.