May 1, 2013 Features

Planet of Sound

In a world where noise never stops, hearing threats bombard us every day. Take a tour of some of America’s noisiest environments—and bring your earplugs.

Noise pervades our society. The booms, screeches and reverberations of traffic, manufacturing, construction and airplanes can't be avoided in daily life. And the onslaught is magnified for those whose jobs require noisy tools and tasks: soldiers and police officers firing guns and sounding sirens, farmers and factory workers running heavy machinery, or airport workers directing thundering jets.

Planet%20Sound

But one person's unpleasant noise may be another's sought-after sound: Concerts, restaurants and bars, movies, and sporting events all generate high noise levels—some loud enough to damage hearing, especially with prolonged exposure.

No matter how "noise" is defined—as loud, discordant, unharmonious, unpleasant, undesired, unexpected or simply something that interferes with hearing—none of these definitions truly characterizes noise's effects on human beings. Many offending stimuli affect not only our hearing but also our well-being: Noise exposure has been implicated in cases of sleep disturbance, heart disease and hypertension, among other adverse effects.

In this issue of The ASHA Leader, we present two sets of articles by prominent audiologists, researchers and sound engineers. In the first, we listen in on some of the most prominent sources of noise, and describe the risks of extended exposure. In the second, experts suggest how best to minimize adverse effects, from better acoustic design to otoprotective agents to a simple pair of earplugs. We will never eliminate noise, but we may learn to mitigate its auditory and nonauditory effects.

Robert W. Sweetow, PhD
University Of California, San Francisco

Industrial-Strength Noise

We have all experienced the noise of a construction zone while driving. For most of us, the noise may not seem excessive due to the briefness of exposure, construction barriers and closed car windows. But, as reported by Noah S. Seixas and colleagues in their 2011 study in Occupational & Environmental Medicine, construction workers can suffer hearing loss even at an average exposure level around the "safe" 85 dBA level.

Construction noise levels vary depending on the source: Cranes, cement mixers, welding, hammering and boring are only a few examples. These noise levels can average from below 85 dBA for concrete mixers to a staggering 113 dBA for air track drills. And indoor projects, or larger outdoor projects using earth-moving equipment, increase noise exposure to even more dangerous levels.

An estimated 16 million people—approximately 13 percent of the U.S. workforce—work in the manufacturing sector. In 2012, the Bureau of Labor Statistics reported occupational hearing loss in 20,700 people. More than 68 percent of these were manufacturing-sector workers in the following industries: beverages, tobacco, food, textiles, apparel, wood products (including furniture), rubber, leather, stone, clay, glass, primary metals, fabricated metal products, transportation equipment, industrial machinery, electronic and electric equipment, and chemicals.

Exposure to high noise levels can lead to temporary hearing loss, but after several such instances the loss becomes permanent. Workers often are unaware of their initial, mild hearing loss, which occurs only at higher frequencies and allows sufficient ability to communicate in quiet surroundings. But with years of hazardous noise exposure the hearing loss can progress further, causing difficulties in communication and social isolation.

Industrial noise exposure also is related to other health problems such as tinnitus, high stress levels at work, and cardiovascular ailments. Hazardous noise also reduces job productivity by interfering with work communication and social exchanges. Loud noise can interfere with the ability to concentrate at work and can cause excessive fatigue at the end of the day, with slow recovery. Our review of the literature indicates that reduction of industrial noise exposure can benefit workers, employers and society.

Vishakha W. Rawool, PhD, CCC-A
West Virginia University, Morgantown

Assault From Above

If you live far from an airport, you may find the occasional sound of a plane overhead pleasing or exciting, but certainly not life threatening. Now imagine standing on the tarmac all day as jets scream overhead, departing and arriving. For those who live or work in airplanes' flight paths, it's an everyday occurrence—a layover that never ends.

The potential effects of aviation noise on people's mental well-being are well documented, particularly the annoyance and stress caused by repeated noise exposure. People living near airports indicate in surveys that they are bothered by aircraft noise, and that it interferes with their daily life. They also perceive themselves as being in poorer health and suffering from increased insomnia, headaches and stress. Additional studies have found that children exposed to chronic noise develop communication issues and tend to speak in incomplete sentences—using shorter utterances to combat the noise. The challenge can be even greater for students with hearing loss.

The word noise is derived from the Latin "nausea" which means discomfort or illness. The definition makes more sense when you consider the modern issues related to aviation noise.

Growing evidence suggests there may be long-term, cumulative health effects for those living near airports, including cardiovascular disease, hypertension and ischemic heart disease, perhaps brought on by sleep disruption and noise-induced stress. Other confounding health effects such as obesity and diabetes may arise.

Researchers are suggesting possible ways to mitigate these effects, based on the many factors that can alter the amount of noise perceived on the ground. For example, departing aircraft are louder than arriving aircraft, and an aircraft arriving at night is often perceived as being louder than the same aircraft arriving in daylight. Weather conditions can also affect aircraft noise levels: Low clouds may increase the noise levels by reflecting sound back to the ground. Noise exposure can also change by varying use of existing runways or constructing new ones.

Understanding these and other factors may provide, for people who live beneath airplanes' flight paths, a less drastic solution than a "For Sale" sign.

Kerri A. Helms, MS, CCC-A
Saint Louis University, Mo.

Raging Roads

Where is traffic noise a pressing problem? It is sometimes a concern of formerly serene, first-world suburbs that have fallen victim to development and now sit beside busy new interstate highways. But this problem is even more severe in the developing world, where environmental noise—mainly from heavily trafficked roads—can reach 75–80 dBA, as the World Health Organization reported in 1999. Clearly, with development continuing around the world, this issue will only grow if we do not address it.

When most clinicians think of noise exposure, we first think of it in terms of noise-induced hearing loss. But road noise generally does not produce levels severe enough to cause NIHL. The Occupational Safety and Health Administration mandates hearing conservation only when a person is exposed to 85 dBA for eight hours. And according to the World Health Organization, exposure to environmental noise below 70 dBA for 24 continuous hours does not put a person at risk for NIHL.

But long-term exposure to noise has been linked to other problems, such as high levels of annoyance and sleep disturbance. In a 2012 study, Minho Kim surveyed the populations of several Georgia counties, including Fulton County and the city of Atlanta. Kim found that 10 percent of respondents reported high daytime annoyance, and 2.8 percent reported nighttime sleep disturbance related to traffic noise.

Kim pointed out that these psychosocial effects have been linked with negative health outcomes, including chronic disease. Some of the more serious implications of long-term traffic noise exposure could include high blood pressure, heart disease and myocardial infarction, to highlight a few. Although most studies point to a need for more research in these areas, a 2009 study by Jenny Selander published in Epidemiology also lends support to the idea that long-term exposure to residential traffic noise greater than 50 dBA increases the risk for myocardial infarction.

As clinicians, being aware of the potential risks of exposure to traffic noise can help us provide appropriate counseling and referral, even when the exposure does not cause measurable damage to the peripheral auditory system.

Maureen Fischer, MS, CCC-A
Saint Louis University, Mo.

Dinner, a Movie ... and Hearing Loss?

It's hard to argue against the idea that it assaults the wallet to eat out, attend a sporting event, sip a martini at an upscale bar or even just go to the movies. But we do these things anyway, because they're pleasurable enough to justify the price. What we may forget is that they also may assault the auditory system with potentially damaging sound levels.

The New York League for the Hard of Hearing reported in 1999 that sporting events can reach levels of 127 dBA, firecrackers have been measured at levels up to 155 dBA, and the noise in health clubs can reach 120 dBA. The intensity levels from movies, particularly during the initial previews or trailers, have been reported as high as 112 dBA. These levels, perhaps intense enough to cause emotional concern or tinnitus, are likely not sufficient to place the moviegoer at risk for permanent hearing loss. Most movies run two or three hours, and even some of the louder movies include periods of relatively quiet dialogue.

Noise levels in restaurants also can exceed safety standards. Although a study I conducted with colleagues in 2000 indicated average integrated sound level (L-avg) ranging from 50 dBA to 90 dBA and maximum sound levels from 85 dBA to 109 dBA, peak unweighted levels reached as high as 142 dB SPL. Because the typical patron doesn't spend eight hours a day in restaurants—similar to the movie situation—restaurant noise is unlikely to cause permanent hearing damage. However, restaurant noise levels certainly can be high enough to be annoying—for people with hearing loss and those with normal hearing alike.

Zagat Surveys, publishers of guidebooks that include reviews of about 15,000 restaurants nationwide, lists noise as the second most reported problem cited by diners (the first is poor service). Based on these findings, in 1998 we persuaded the San Francisco Chronicle to rate background noise—along with food quality and service—in their restaurant reviews. Since then, other news outlets across the country have followed suit. The Chronicle's system rates restaurants on a scale from one bell ("pleasantly quiet" and less than 65 dBA) up through four bells, and finally to "bomb," which indicates a restaurant "too noisy for normal conversation," with noise levels of 80 dBA and higher.

This system serves a useful purpose not only for patrons with hearing impairment, but also for those with normal hearing, in selecting where they want to satisfy their gustatory and auditory cravings: They can do so without paying too high a price.

Robert W. Sweetow, PhD
University of California, San Francisco

'Rock 'n' Roll Ain't Noise Pollution'—Or Is It?

I recently received an e-mail from one of my patients—a musician in a reggae band—at the Boston Children's Hospital Musicians' Hearing Program (bit.ly/music-hearing) expressing great pleasure with her musicians' earplugs. She now uses them regularly to prevent tinnitus following band practices and performances. But unlike this patient, many musicians still reject hearing protection despite the risks of sustained exposure to loud rock and the benefits of protection.

Why do musicians and music enthusiasts need earplugs? I have made sound-level measurements at many concert venues, from stadium shows to small clubs. Sound levels vary, but they most often average 104–106 dBA at the mixer board. According to the National Institute for Occupational Safety and Health's damage-risk criteria for maximum recommended noise exposure levels, a person should not be exposed to more than 3.5 to 6.0 minutes at these levels. If a concertgoer were to listen to a 45-minute show without any hearing protection, he or she would sustain 7.5 to 12 times the allowable sound exposure in a given day, just in that short 45-minute exposure.

In 2010, I recorded sound levels for the audience and crew at a rock festival, The Bamboozle Road Show, and documented the results in a 2011 Audiology Today article co-authored with AuD extern Frank Wartinger. The four-hour show's average level—accounting for louder and softer songs and breaks—was 105 dBA at the sound engineer's location (at the center of the audience). A concertgoer who never left his or her seat for the duration would be exposed to roughly 50 times what is considered allowable for occupational noise exposure. Repeated regularly, this concertgoer is at very high risk for noise-induced hearing loss and tinnitus.

Music is not "noise." It is a desired signal that people experience by choice. But when experienced without regard to hearing health, music can be harmful. It is vital that hearing health care providers focus not on devices, but on developing relationships with our consumers. Devices are not the cure. We are.

Brian J. Fligor, ScD, CCC-A
Boston Children's Hospital; Harvard Medical School, Boston, Mass.

Dialing Down the Din

We see the problems, but what's being done to make our work spaces and public places easier on our ears?

Buildings That Connect and Protect

The acoustic fabric of our daily lives—particularly in the places we teach, learn and work—is changing. You might chalk this change up to the explosion of interactive multimedia devices in our pockets, the coffee-house sensory preferences of an emerging generation, or even to a real or perceived rise in attention disorders. But one thing you aren't likely to do is reverse the trend: Our brains are increasingly bombarded with more signals and more noise.

In particular, classrooms are changing. The tide of "active learning" is sweeping through higher education and lapping at the shores of secondary and elementary schools. This collaborative teaching and learning model clusters students in small groups around interactive display screens, engaged in project-based peer interaction, and studies suggest promising effects on learning. But the trade-off is more noise.

Possibly as a result of this change in higher education classrooms, the workplace is also changing. Virtually gone are private offices and closed conference rooms. The open-source, open-office loft phenomenon challenges even the lowly cubicle's padded walls as barriers to communication and collaboration. Packed into higher-density and more flexible workstation configurations like "benching" and "hoteling," office workers are not just online—they're increasingly onstage. Privacy and quiet are endangered species.

A critical challenge facing architects is how to design buildings and spaces to reap the obvious benefits of a more engaged, connected and collaborative world without sending it over the edge into sensory chaos and acoustic overload. A handful of design strategies guide that balance:

  • Allow choice and variety. Provide spaces with a full range of sensory exposures, from high-energy and high-stimulation to soft, low and quiet.
  • Zone and buffer. Protect quieter spaces by placing them away from noisier ones and locating support spaces like file, storage and work rooms in between.
  • Absorb. Use acoustically absorptive wall and ceiling surfaces to kill ambient noise on the "first bounce."
  • Mask and cover. Use ventilation systems and white noise generators—devices that produce a constant sound, such as rushing air—to balance speech privacy and speech intelligibility.
  • Careful separation. Use glass walls to marry visual connection with acoustical separation. Translucent panels provide even more privacy, while allowing daylight to circulate.
  • Manage transitions. Corridors, entryways and other connectors that link noisy spaces with quieter ones should be designed to signal the change. Lighting, color and texture of finishes can all be used to provide visual and tactile transition cues.
  • Switch gears. Create break room and lounge spaces that offer sensory contrast from the work areas they support. A stimulating coffee bar with TV screens can be a welcome jolt from a quiet office, just as a softly lit quiet lounge can be an oasis from a noisy trading floor.

Fortunately, architects and acousticians are armed with new virtual reality tools that can accurately predict and even simulate the acoustics of any new environment before it's built. Most everything we design now is built first in a computer as a three-dimensional digital model. New software allows us to inform that model with detailed and measurable acoustical properties of every wall, floor, ceiling and door we build into it. Running auralization algorithms—used to model room acoustics virtually via computer simulation—can help predict background noise levels, and help us understand how to better soften and separate spaces acoustically. We can analyze the mathematical data as sound pressure levels in decibels, or we can use sophisticated headphones and amplifiers to simulate the eventual acoustical effects of the future environment audibly. That ability empowers not only designers, but also our clients in understanding the acoustical balance between communication, collaboration, stimulation and noise.

Marcus Adrian, AIA
Mackey Mitchell Architects, Saint Louis, Mo.

Operation Hearing Health

Since the beginning of mechanized warfare, noise in the military has become a routine part of our lives. Hazardous noise is expected, but not always predictable during conflict, in training environments and off-duty. Despite consistent efforts to lessen noise exposure and damage, the invisible injuries of tinnitus and hearing loss continue to plague service members and veterans as the two most prevalent wounds of war. Noisy environments unique to the military are encountered during combat, in combat training and from the logistics of war. Although the threat of noise can't always be predicted, acoustic injury largely is still preventable. Our focus—along with hearing preservation—is to enhance the ability to communicate effectively in a time-critical, lethal and often chaotic environment. This ability helps to reduce combat confusion and increase mission effectiveness.

In 2009, due to the cost and prevalence of hearing loss and tinnitus, Congress established the Hearing Center of Excellence to focus on the prevention, diagnosis, mitigation, treatment and rehabilitation of hearing loss and auditory system injury. Accordingly, we have begun to see the successful transition of Department of Defense hearing conservation programs to more comprehensive hearing health programs, and are developing data-sharing with the Department of Veterans Affairs to track and trend injury patterns and provide continuity of care.

Our efforts remain focused on improving the health and quality of life of service members and veterans, highlighting hearing loss and tinnitus as both a military readiness and a population health issue, with risk and reward manifest both on and off duty, at peacetime and at war. We are achieving our goals by means of an established hearing health improvement network collaborating to improve prevention and care of auditory-vestibular issues through data management, research and outreach. Also, by providing efficient clinical care and coordinating our efforts throughout the DoD/VA system, we are able to monitor our service members more closely and understand their diagnosis and treatment from the time they join the military to separation, retirement and beyond.

With the continued efforts of this DoD/VA partnership, we hope to build on the framework of the hearing health platform so hearing loss and tinnitus are no longer accepted consequences of military service.

Jeffrey L. Wisneski, MAJ, USAF, AuD, CCC-A
Air Force Medical Operations Agency

Rx for Rock 'n' Roll

We know listening to loud music carries a risk of hearing loss (see "'Rock 'n' Roll Ain't Noise Pollution'—Or Is It?"). Short of attending a concert and listening to only one song, the best way to lower this risk is hearing protection.

Some earplugs perform better than others for reducing sound levels while maintaining reasonable sound quality. Custom earplugs and earphone sleeves are relatively low-cost and accessible. Music lovers should also take listening breaks, allowing ears to rest, and avoid loud music for 24–48 hours after a loud concert so their hearing can recover.

When using music players, listeners should invest in headphones that fit snugly into the ear (either one-size-fits-most eartips or custom "sleeves"). Research suggests that blocking out ambient noise allows the listener to keep the volume at a moderate level.

Brian J. Fligor, ScD, CCC-A
Boston Children's Hospital; Harvard Medical School, Boston, Mass.

Can't Stop It? Block It ... or Repair It

People who work in high-noise environments often say, "Hearing loss is expected in my job." Reducing noise at the source is the best strategy for protecting hearing, but in some settings—military combat, for example—noise levels cannot be controlled.

The Occupational Health and Safety Administration recommends workers use hearing protection when noise exposure levels are equal to or greater than an eight-hour, time-weighted average of 90 dBA. The National Institute for Occupational Safety and Health recommends hearing protection when sound levels are at or above 85 dBA. When noise exposure levels cannot be controlled, hearing protection devices designed to reduce the noise level reaching the eardrum can minimize the possibility of noise-induced hearing loss, tinnitus, annoyance, irritability and hypertension.

HPDs are available in various physical styles, including earplugs, semi-inserts, earclips, earmuffs, helmets and earphones. Many commonly used HPDs provide more reduction of high-pitched noises compared to low-pitched noises. Other devices designed for musicians attempt to provide relatively equal reduction of high- and low-pitched sounds to maintain the quality of music. Some modern HPDs can check their own fitting adequacy and sound an alert when the fit becomes loose; others can provide hearing enhancement for soft and critical sounds, automatically reduce hazardous noise levels, allow radio communication without the use of a boom microphone near the worker's mouth (a microphone inside the HPD picks up speech signals through the skull), measure protected and unprotected noise exposures, or alert the wearer when noise levels become hazardous.

To prevent hearing loss and improve worker acceptance and continued use, consider several factors when selecting HPDs, including accurate amount of noise reduction, comfort, ease of fit, ease of maintenance, convenience and availability, ability to hear important signals, compatibility with other work or protective gear, and many other factors specific to individual workers, such as the severity of existing hearing loss and work conditions.

Pharmacological defenses are also visible on the hearing horizon. When the auditory system suffers temporary damage from noise exposure, the effect typically lasts several days or weeks. Soon, this may prove a critical window to repair the damage and prevent permanent hearing loss.

Investigators are exploring a host of pharmacological agents and strategies, each of them addressing an underlying physiological mechanism of hearing loss:

  • Agents to minimize the death of tiny hair cells in the inner ear (anti-apoptotics).
  • Agents to reduce oxidative stress or antioxidents including D-methionine (D-Met), Ebselen and neurotrophic growth factors.
  • Agents to reduce excessive release of glutamate, which can bind to the postsynaptic receptors leading to neuronal degeneration. These include glutamatergic neurotransmission blockers such as riluzole or glutamate receptor anatagonists such as caroverine.
  • Metabolites like cobalamin (vitamin B12) to stabilize neural activity and improve vascular endothelial function.
  • Vasodilators—such as magnesium—to improve blood flow to the inner ear.
  • A combination of some of the above agents, including beta-carotene, vitamins C and E, and magnesium.
  • Gene therapy involving the injection of Atoh1, a gene critical for hair cell differentiation in the cochlea, to induce repair or regeneration of stereocilia located on top of the hair cells. Mutation of some genes (for example, P2RX2) makes some individuals more susceptible to hearing loss. Thus, therapy targeting the mutant P2X2 receptor may also prevent hearing loss in these people.

Vishakha W. Rawool, PhD, CCC-A
West Virginia University, Morgantown


Resources for Hearing Protection

The U.S. Department of Health and Human Services launched Healthy People 2020 in December 2010, announcing the new 10-year goals and objectives for health promotion and disease prevention. This decade there is a new section identified as "Hearing and Other Sensory or Communication Disorders," created with input from ASHA.

These patient education materials from ASHA’s Audiology Information Series provide information for your patients that can help improve outcomes and influence behavior changes:

 



Sources

Industrial-Strength Noise

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Department of Health and Human Services (National Institute of Occupational Safety and Health). (2010). Publication No. 2010-136. Manufacturing sector, occupational safety and health research needs and partnerships for the second decade of NORA: Occupationally-induced hearing loss. Retrieved from www.cdc.gov/niosh/docs/2010-136/pdfs/2010-136.pdf [PDF].

Eaton, S. (2000). Worker's compensation board of BC. Engineering Section report. ARCS Reference No: 0135-20. Construction noise. Retrieved from www2.worksafebc.com/pdfs/hearing/ConstructionNoise.pdf [PDF].

Rawool, V. W. (2012). Hearing conservation: In occupational, recreational, educational, and home settings. New York: Thieme.

Seixas, N. S., Neitzel, R., Stover, B., Sheppard, L., Feeney, P., Mills, D., & Kujawa, S. (2012). Ten-year prospective study of noise exposure and hearing damage among construction workers. Occupational & Environmental Medicine, 69, 643–650.

Assault From Above

Rosenlund, M., Berglind, N., Pershagen, G., Jarup, L., & Bluhm, G. (2001). Increased prevalence of hypertension in a population exposed to aircraft noise. Occupational & Environmental Medicine, 58, 769–773.

Standfeld, S. A., Berglund, B., Clark, C., et al. (2005). Aircraft and road traffic noise and children's cognition and health: a cross-national study: Ranch study team. Lancet, 365, 1942–1949.

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Swift, H. (2010). A review of literature related to potential health effects of aircraft noise. Partner project 19, final report. Retrieved from http://web.mit.edu/aeroastro/partner/reports/proj19/proj19-healtheffectnoise.pdf [PDF].

Raging Roads

Jenny Selander, M. E. (2009). Long-term exposure to road traffic noise and myocardial infarction. Epidemiology, 20(2), 272–279.

Minho Kim, P. S. (2012). Road traffic noise annoyance, sleep disturbance, and public heatlh implications. American Journal of Preventative Medicine, 43(4), 353–360.

World Health Organization. (1999). Guidelines for community noise. Geneva: Author.

Dinner, a Movie ... and Hearing Loss?

Chase, M. (1997). Pardon? Speak up? I've just been to the movies. Wall Street Journal, July, 1997.

Ferguson, M., Davis, A., & Lovell, E. (2000). Cinemas: Do they pose a risk to hearing? Noise Health, 2, 55–58.

Graham, B. (1998). An earful of sound. San Francisco Chronicle, August 9.

League for the Hard of Hearing. (1999). Recreational noise fact sheet. New York City: Author.

Smith B., Dancer, J., Montague, J., & Highley, P. (1999). Thrill or threat? The sound of movies. Hearing Review, 6(11), 50–54.

Sweetow, R., & Tate, L. (2000). I'll have a side order of earplugs, please. Audiology Today, 12(3), 40.

Rock 'n' Roll Ain't Noise Pollution

Fligor, B. J., & Wartinger, F. (2011). Musicians' hearing program. Audiology Today, 23(3), 30–40.

Portnuff, C. D., Fligor, B. J., & Arehart, K. H. (2011). Teenage use of portable listening devices: A hazard to hearing? Journal of the American Academy of Audiology, 22(10), 663–677.

Can't Stop It? Block It ... or Repair It

Attias, J., Sapir, S., Bresloff, I., Reshef-Haran, I., & Ising, H. (2004). Reduction in noise-induced temporary threshold shift in humans following oral magnesium intake. Clinical Otolaryngology and Allied Sciences, 29, 635–641.

Campbell, K., Claussen, A., Meech, R., Verhulst, S., Fox, D., & Hughes, L. (2011). D-methionine (D-met) significantly rescues noise-induced hearing loss: Timing studies. Hearing Research, 282, 138–144.

Chambers, J. C., Ueland, P. M., Obeid, O. A., Wrigley, J., Refsum, H., & Kooner, J. S. (2000). Improved vascular endothelial function after oral B vitamins: An effect mediated through reduced concentrations of free plasma homocysteine. Circulation, 102, 2479–2483.

Chen, Z., Ulfendahl, M., Ruan, R., Tan, L., & Duan, M. (2004). Protection of auditory function against noise trauma with local caroverine administration in guinea pigs. Hearing Research, 197, 131–136.

Fredelius, L. (1988). Time sequence of degeneration pattern of the organ of Corti after acoustic overstimulation. A transmission electron microscopy study. Acta Otolaryngologica, 106, 373–385.

Harris, K. C., Hu, B., Hangauer, D., & Henderson, D. (2005). Prevention of noise-induced hearing loss with Src-PTK inhibitors. Hearing Research, 208, 14–25.

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Lynch, E. & Kil, J. (2009). Development of ebselen, a glutathione peroxidase mimic, for the prevention and treatment of noise-induced hearing loss. Seminars in Hearing, 30, 47–55.

National Institute for Occupational Safety and Health. (1998). Criteria for a recommended standard: Occupational noise exposure—Revised criteria 1998. (Publication No. 98–126). Atlanta, GA: Author.

Occupational Safety and Health Administration. (1983). 29 CFR 1910.95. Occupational Noise Exposure; Hearing Conservation Amendment; Final Rule, effective 8 March 1983. Federal Register, 48, 9738–9785.

Quaranta, A., Scaringi, A., Bartoli, R., Margarito, M. A., & Quaranta, N. (2004). The effects of 'supra-physiological' vitamin B12 administration on temporary threshold shift. International Journal of Audiology, 43, 162–165.

Rawool, V. W. (2012a). Hearing protection and enhancement devices. In V. W. Rawool (Ed.), Hearing conservation: In occupational, recreational, educational, and home settings (pp.136–173). New York: Thieme.

Rawool, V. W. (2012b). Future trends in hearing conservation. In V. W. Rawool (Ed.), Hearing conservation: In occupational, recreational, educational, and home settings (pp. 296–300). New York: Thieme.

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Yamashita, D., Shiotani, A., Kanzaki, S., Nakagawa, M., & Ogawa, K. (2008). Neuroprotective effects of T-817MA against noise-induced hearing loss. Neuroscience Research, 61, 38–42.

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