Page 20 - Fall 2007

of cochlear neurotransmitters (e.g., glutamate), alterations in intracellular calcium balances, and other changes related to metabolic exhaustion. The free radicals injure a wide variety of essential structures (e.g., nuclear DNA, membranes, mito- chondria) in the cochlea causing cell damage and cell death through several cell-death sequences including apoptosis and oncosis (Bohne et al., 2007). Eventually, widespread cell death caused by noise over-exposure leads to NIHL.
Within the cochlea, different groups of hair cells are tuned to unique frequencies, which, for the healthy human ear, span a frequency range from 20-20,000 Hz (i.e., a meas- ure of vibration rate per second). Typically, the high-fre- quency area of the cochlea is damaged initially by loud nois- es, which makes it more difficult to listen to the high-pitched voices of children and many women as well as to discern words that contain certain consonant sounds (e.g., ‘ess’ and ‘ch’), particularly if they are spoken in the presence of back- ground noise. Unfortunately, once damaged or decimated by unmanageable levels of toxic free radical molecules, hair cells cannot repair themselves or grow back thus causing a per- manent hearing loss. Frequently, this type of NIHL is also accompanied by tinnitus.
The federal agency of the National Institutes of Health (NIH) that funds research into the causes, diagnosis, treat- ment, and prevention of NIHL, i.e., the National Institute on Deafness and Other Communication Disorders (NIDCD), estimates that about 10% of Americans between 20-69 years old (or 22 million Americans) already have suffered perma- nent damage to their hearing from exposure to excessive noises including those in the workplace, recreational settings, and at home. The NIDCD also speculates that some 30 mil- lion people are at risk for NIHL in these same settings. In fact, NIHL is the most common work-related disease. Earlier, Niskar et al. (2001) estimated from the database of the Third National Health and Nutrition Examination Survey (NHANES III) representing a national, population- based, cross-sectional survey that some 5.2 million 6-19 year olds (12.5%) have hearing loss directly related to noise expo- sure.
NIHL is a preventable hearing problem. By practicing good hearing health in everyday life, the hazards of being exposed to excessive noise can be avoided. You simply need to know that sounds above 85 decibels (dB sound pressure level or SPL) can damage the ear. Because of the occupation- al risk of NIHL, specific government standards regulate allowable noise exposures above this level, at least, in the workplace. Also, protective earplugs that fit snugly into the outer ear canal, special earmuffs that fit over the entire outer ear, or other hearing protective devices, which are usually available at pharmacies and hardware and sporting goods stores, should be worn when involved in a loud activity. Properly fit earplugs or earmuffs reduce sound by 15-30 dB. Further, if you suspect a NIHL, consult a physician, i.e., oto- laryngologist or otologist/neurotologist, who specializes in diseases of the ear, or arrange for a hearing test by an audiol- ogist. If you believe that you have grown used to loud noise, it probably has already damaged your ears. And, as stressed by the American Hearing Research Foundation (AHRF), to
date, there is no approved treatment, i.e., no medication and no surgery to reverse a permanent hearing loss like NIHL. Not even a hearing aid can truly correct your hearing once it is damaged by noise.
However, several research groups have begun testing various chemicals for their safety and effectiveness in pre- venting or countering NIHL. Each of these candidate reme- dies for NIHL controls oxidative damage from free radicals either by directly blocking their creation or by removing free radicals from the cell before they damage critical intracellu- lar structures. There is no doubt that a ‘hearing pill’ that could be taken in conjunction with a noisy event (e.g., before or after) would make a more amenable option than wearing hearing protection devices or limiting the time spent exposed to loud sounds. At present, three potentially otoprotective chemicals (i.e., D-methionine, ebselen, and N-acetylcysteine) are being tested in humans (e.g., Kopke et al., 2007). All three, which have been shown to have excellent safety pro- files in preliminary human studies, enhance the natural antioxidant, glutathione, which is found in hair cells and which battles chemical stress. Each of these agents can be simply dissolved in water or can be taken orally as pills. If these tests are successful and the United States Food and Drug Administration (FDA) approves the drugs, they will be the first pharmacologic products of their kind to combat NIHL. FDA approval, claim the developers of these drugs, likely means that an over-the-counter remedy for NIHL will be available within the next 5-10 years.
In the meantime, a number of professional organizations provide helpful information on the dangers of portable music players. For example, over the past several years, the American Speech-Language-Hearing Association (ASHA), an organization of hearing, speech, and language profession- als, has conducted a consumer awareness campaign called ‘Listen To Your Buds’ that alerts individuals to the potential risk of hearing loss from the unsafe usage of personal audio devices. The NIDCD also has formed a coalition with the National Institute for Occupational Safety and Health (NIOSH) and more than 60 diverse national organizations [e.g., National Hearing Conservation Association (NHCA)] to conduct the WISE EARS!® campaign aimed at preventing NIHL over a life time. Also, academic institutions like the Oregon Health and Science University have partnered with public state-government agencies such as the Oregon Museum of Science and Industry to mount consumer cru- sades, in this case, the Dangerous Decibels® campaign, aimed at reducing the prevalence of NIHL and tinnitus through exhibits, educational outreach, and research. The Dangerous Decibels® program maintains that NIHL can be prevented through three highlighted messages: ‘turn it down,’ ‘walk away,’ and ‘protect your ears.’
The underlying mechanisms of NIHL are still not fully understood, although one notion is that intense noise over- stimulation leads to metabolic changes that compromise the cellular elements of the inner ear. For certain, over the past 100 years, the knowledge that loud noise may result in hear- ing loss has been well established. And, the phenomenon of NIHL has clearly been linked to recreational noise and
18 Acoustics Today, October 2007