Noise-induced hearing loss
||This article's introduction section may not adequately summarize its contents. (March 2014)|
Noise-induced hearing loss (NIHL) is hearing decrease caused by loud sound. Evidences of NIHL include a history of exposure to loud sound and a hearing loss in a narrow range of frequencies, such as those from gunfire, power tools, explosions and night club music. The best, first option for protecting hearing is lowering the volume at the source of the sound.
Currently, hearing loss in mammals is permanent. While frogs, fish, and birds with hearing loss regain their hearing naturally, mammals lost that ability as much as 300 million years ago, and so far scientists have been unsuccessful in solving that problem.
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 (eardrum). 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 overstimulation of the hair cells leads to heavy production of reactive oxygen species, leading to oxidative cell death. In animal experiments, antioxidant vitamins have been found to reduce hearing loss even when administered the day after noise exposure. They were not able to fully prevent it.
Damage ranges from exhaustion of the "hair" (hearing) cells in the ear to loss of those cells
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.
The ear can be exposed to short periods in excess of 120 dB without permanent harm — albeit with discomfort and possibly pain; but long term exposure to sound levels over 80 dB can cause permanent hearing loss.
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, a large drum hit loudly, and firecrackers.
Gradually developing NIHL
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 any source of excessive volume, such as home and vehicle stereos, concerts, nightclubs, excessive noise in the workplace, and personal media players. The U.S. Department of Labor's Occupational Safety and Health Administration (OSHA) states that exposure to 85 dB(A) of noise, known as an exposure action value, for more than eight hours per day can result in permanent hearing loss. 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". 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.
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. As harmful noise exposure continues, the commonly affected frequencies will broaden and worsen in severity. "NIHL usually occurs initially at high frequencies (3, 4, or 6 kHz), and then spreads to the low frequencies (0.5, 1, or 2 kHz)".
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. According to a survey by Lass, Woodford, C. Lundeen, D. Lundeen and Everly-Myers, which examined high school student's 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.
Hearing protection programs have been hindered by people not wearing the protection for various reasons, including the desire to converse, uncomfortable devices, lack of concern about the need for protection, and social pressure against wearing protection. In recent years, Buy Quiet programs and initiatives have arisen in an effort to combat occupational noise exposures. These programs promote the purchase of quieter tools and equipment and encourage manufacturers to design quieter equipment. 
A systematic review of the effectiveness of interventions to promote the use of hearing protection devices such as earplugs and earmuffs among workers found that tailored interventions improve the average use of such devices when compared with no intervention. (Tailored interventions involve the use of communication or other types of interventions that are specific to an individual or a group and aim to change behavior). Mixed interventions such as mailings, distribution of hearing protection devices, noise assessments, and hearing testing are also more effective in improving the use of hearing protection devices compared with hearing testing alone.
In the workplace
About 30 million workers are exposed to hazardous noise, with an additional 9 million exposed to solvents and metals that put them at risk for hearing loss. Occupational hearing loss is one of the most common occupational diseases. 49% of male miners have hearing loss by the age of 50. By the age of 60, this number goes up to 70%. Construction workers also suffer an elevated risk. A screening program focused on construction workers employed at US Department of Energy facilities found 58% with significant abnormal hearing loss due to noise exposures at work.
The following is a list of occupations that are most susceptible to hearing loss:
- Orchestra conductors
Musicians, from classical orchestras to rock groups, are exposed to high decibel ranges. Some rock musicians experience noise-induced hearing loss from their music, and some studies have found that "symphonic musicians suffer from hearing impairment and that the impairment might be ascribed to symphonic music."
Music-induced hearing loss is still a controversial topic for hearing researchers. While some studies have shown that the risk for hearing loss increases as music exposure increases, other studies found little to no correlation between the two. Experts at the 2006 "Noise-Induced Hearing Loss in Children at Work and Play" Conference agreed that further research into this field was still required before making a broad generalization about music-induced hearing loss.
Given the extensive research suggesting that industrial noise exposure can cause sensorineural hearing loss a link between hearing loss and music exposures of similar level and duration (to industrial noise) seems highly plausible. Determining which individuals or groups are at risk for such exposures may be a difficult task. Recent research suggests that despite concerns about the proliferation of personal music players, in fact discos, concerts and live music events may be more hazardous to youth's hearing.
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 eight-hour time period. OSHA also states that "exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level". The United States Department of Defense (DoD) instruction 605512 has some differences from OSHA 1910.95 standard, for example, OSHA 1910.95 uses a 5 dB exchange rate and DoD instruction 605512 uses a 3 dB exchange rate.
There are programs that seek to increase compliance and therefore effectiveness of hearing protection rules; the programs include the use of hearing tests and educating people that loud sound is dangerous
Employees are required to wear hearing protection when it is identified that their eight-hour time weighted average (TWA) is above the exposure action value of 90 dB. If subsequent monitoring shows that 85 dB is not surpassed for an eight-hour TWA, the employee is no longer required to wear hearing protection.
In the European Union, directive 2003/10/EC mandates that employers shall provide hearing protection at noise levels exceeding 80 dB(A), and that hearing protection is mandatory for noise levels exceeding 85 dB(A). Both values are based on 8 hours per day, with a 3 dB exchange rate.
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.[clarification needed] 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 is evidence that hearing loss can be minimized by taking megadoses of magnesium for a few days, starting as soon as possible after exposure to the loud noise. A magnesium-high diet also seems to be helpful as an NIHL-preventative if taken in advance of exposure to loud noises. Consuming sizable amounts of magnesium can be potentially harmful, so this treatment should be followed with caution.
There are currently no medical options for NIHL from noise-exposure which occurred more than a week previously. However, current research for the possible use of drug and genetic therapies look hopeful.
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- http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9735 OSHA 29 CFR 1910.95(b)(2)
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- "Noise-Induced Hearing Loss". National Institute on Deafness and Other Communication Disorders. October 2008.
- Noise-Induced Hearing Loss from the National Institutes of Health
- How to Prevent Noise-Induced Hearing Loss from the American Academy of Family Physicians.
- Dangerous Decibels Includes general information and a "virtual exhibit" as well as resources for teachers.
- NIOSH Noise and Hearing Loss Prevention Topic Page
- NIOSH Power Tools Sound Pressure and Vibrations Database
- New York City construction noise control products and vendor guidance sheet
- Online Audiometric Test Calibrated test, up to 80 dBHL. Confirm your hearing loss and track how it evolves over time.
- An online audiometric test featuring equal loudness curves
- NIOSH Buy Quiet Topic Page