Noise-induced hearing loss

Overview
Noise-induced hearing loss (NIHL) is an increasingly prevalent disorder that results from exposure to high-intensity sounds, especially over a long period of time.

Description
NIHL is a preventable hearing disorder that affects people of all ages and demographics. According to a position statement released by the American Academy of Audiology in 2003, "The average, otherwise healthy, person will have essentially normal hearing at least up to age 60 if his or her ears are not exposed to high noise levels" (American Academy of Audiology [AAA], 2003). Unfortunately, around 30 million adults in the United States are exposed to hazardous sound levels in the workplace (National Institute for Occupational Safety and Health [NIOSH], 2000). Among these 30 million people, one in four will acquire a permanent hearing loss as result of their occupation (AAA, 2003). Even though NIHL primarily affects the adult working population, NIHL can be identified in the adolescent and young adult population as well. “The Hearing Alliance of America reports that 15 percent of college graduates have a level of hearing loss equal to or greater than their parents” (Fausti, Wilmington, P.V. Helt, W.J. Helt, and Konrad-Martin, 2005, p. 51). The incidence of NIHL in young adults is not surprising, due to the popularity of portable music devices (Walkmans and iPods), concerts, and nightclubs within this population. However, through education, audiological evaluations, hearing conservation programs in the workplace, and the proper use of ear protectors, NIHL is preventable.

Mechanism of Causes
NIHL occurs when too much sound intensity is transmitted into and through the auditory system. An acoustic signal from an energy source, such as a radio, enters into the external auditory canal and is funneled through to the tympanic membrane. The tympanic membrane acts as an elastic diaphragm and drives the ossicular chain of the middle ear system into motion. Then the middle ear ossicles transfer mechanical energy to the cochlea by way of the stapes footplate hammering against the oval window of the cochlea. This hammering causes the fluid within the cochlea (perilymph and endolymph) to push against the stereocilia of the hair cells, which then transmit a signal to the central auditory system within the brain. When the ear is exposed to excessive sound levels or loud sounds over time, the force placed on the stereocilia of the hair cells becomes damaging, producing abnormalities of the cells.

Some of the abnormalities include metabolic exhaustion of the hair cells, structural changes and degeneration of structures within the hair cells, morphological changes of the cilia, ruptures of cell membranes, and complete degeneration and loss of hair cells, neural cells and supporting cells. (Gelfand, 2001, p. 202)

NIHL is therefore the consequence of overstimulation of the hair cells and supporting structures. Structural damage to hair cells (primarily the outer hair cells) will result in hearing loss that can be characterized by an attenuation and distortion of incoming auditory stimuli.

Types
There are two basic types of NIHL: NIHL caused by acoustic trauma and gradually developing NIHL. NIHL caused by acoustic trauma refers to permanent cochlear damage from a one-time exposure to excessive sound pressure. This form of NIHL commonly results from exposure to high-intensity sounds such as explosions, gunfire, and firecrackers. Gradually developing NIHL refers to permanent cochlear damage from repeated exposure to loud sounds over a period of time. Unlike NIHL from acoustic trauma, this form of NIHL does not occur from a single exposure to a high-intensity sound pressure level. Gradually developing NIHL can be caused by multiple exposures to musical concerts, nightclubs, excessive noise in the workplace, and personal music devices. The U.S. Department of Labor’s Occupational Safety and Health Administration (OSHA) states that exposure to 90 dB(A) of noise for more than eight hours per day can result in permanent hearing loss (Occupational Health and Safety Administration [OSHA], 2002). Since decibels are based on a logarithmic scale, every increase of 3 decibels SPL results in a doubling of intensity, meaning hearing loss can occur at a faster rate. Therefore, gradually developing NIHL occurs from the combination of sound intensity and duration of exposure. Both NIHL caused by acoustic trauma and gradually developing NIHL can often be characterized by a specific pattern presented in audiological findings. NIHL is generally observed to affect a person’s hearing sensitivity in the higher frequencies, especially at 4000 Hz. “Noise-induced impairments are usually associated with a notch-shaped high-frequency sensorineural loss that is worst at 4000 Hz, although the notch often occurs at 3000 or 6000 Hz, as well” (Gelfand, 2001, p. 202). Doctoral students at the University of Iowa have termed this notch, specific to a noise-induced etiology, a "muna." The symptoms of NIHL are usually presented equally in both ears (Gelfand). Not all audiological results from patients with NIHL match the above description. Often a decline in hearing sensitivity will occur at frequencies other than at the typical 3000-6000 Hz range. Variations arise from differences in people’s ear canal resonance, the frequency of the harmful acoustic signal, and the length of exposure (Rösler, 1994). As harmful noise exposure continues, the commonly affected frequencies will broaden and worsen in severity (Gelfand). “NIHL usually occurs initially at high frequencies (3k, 4k, or 6k Hz), and then spreads to the low frequencies (0.5k, 1k, or 2k Hz)” (Chen, 2003, p. 55).

Prevention
The good news is that NIHL can easily be prevented through the use of some of the most simple, widely available and economical tools. This includes but is not limited to ear protection (i.e. earplugs and earmuffs), education, and hearing conservation programs. Earplugs and earmuffs can provide the wearer with at least 5 to 10 dB SPL of attenuation (Gelfand, 2001). According to a survey by Lass, Woodford, C. Lundeen, D. Lundeen and Everly-Myers (1987), which examined high school students’ attitudes and knowledge concerning hearing safety, 66% of the subjects reported a positive response to wearing hearing protection devices if educated about NIHL. Unfortunately, more often than not, individuals will avoid the use of ear protection due to embarrassment, lack of comfort, and reduced sound quality.

However, the effectiveness of hearing protection programs is hindered by poor compliance in the use of hearing protection devices due to communication difficulties, comfort issues, individuals’ attitudes about protecting themselves from noise-induced hearing loss, and individuals’ perceptions about how others who do not use hearing protection will view them if they choose to use hearing protection. (Fausti et al., 2005, p. 51)

Workplace Standards
The Occupational Safety and Health Administration (OSHA) describes standards for occupational noise exposure in articles 1910.95 and 1926.52. OSHA states that an employer must implement hearing conservation programs for employees if the noise level of the workplace is equal to or above 85 dB(A) for an averaged 8-hour time period (Gelfand, 2001). OSHA also states that "exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level" (CFR 1910.95(b)(2)). The United States Department of Defense (DoD) instruction 605512 has some differences from OSHA 1910.95 standard. OSHA 1910.95 has a Permissible Exposure Limit of 90 dBA for an hour eight hour period, while the DoD has a Permissible Exposure Limit of 85 dBA for an eight hour period. Additionally, OSHA 1910.95 uses a 5 db exchange rate and DoD instruction 605512 uses a 3 db exchange rate.

“Hearing conservation programs in the workplace and in the general population seek to increase compliance and effectiveness of hearing protection protocols through audiometric screening tests and education on the dangers of noise exposure” (Fausti et al., 2005, p. 51). Employees are required to wear hearing protection when it is identified that their 8-hour TWA is above 85 dB. If subsequent monitoring shows that 90 dB is not surpased for an 8-hour TWA the employee is no longer required to wear hearing protection (OSHA 3074, 2002 (Revised)).

Few companies have emerged offering solutions for employers wishing to comply with the myriad regulations regarding hearing protection. One such company is Tomorrow's Safety Today.

Mitigation
For people living with NIHL, there are several management options that can improve the ability to hear and effectively communicate. Management programs for people with NIHL include counseling and the use of hearing aids and FM systems. With proper amplification and counseling, the prognosis is excellent for people with NIHL. The prognosis has improved with the recent advancements in digital hearing aid technology, such as directional microphones, open-fit hearing aids, and more advanced algorithms. Annual audiological evaluations are recommended to monitor any changes in a patient’s hearing and to modify hearing-aid prescriptions. There are no medical options at present for a person with NIHL. However, current research for the possible use of drug and genetic therapies look hopeful (National Institute on Deafness and Other Communication Disorders [NIDCD], 2006).