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Noise-induced hearing loss

Every day, we experience sound in our environment, such as the sounds from television and radio, household appliances, and traffic. Normally, we hear these sounds at safe levels that do not affect our hearing. However, when we are exposed to harmful noise—sounds that are too loud or loud sounds that last a long time—sensitive structures in our inner ear can be damaged, causing noise-induced hearing loss (NIHL). These sensitive structures, called hair cells, are small sensory cells that convert sound energy into electrical signals that travel to the brain. Once damaged, our hair cells cannot grow back.
What sounds cause NIHL?
NIHL can be caused by a one-time exposure to an intense “impulse” sound, such as an explosion, or by continuous exposure to loud sounds over an extended period of time, such as noise generated in a woodworking shop.
Sound is measured in units called decibels. On the decibel scale, an increase of 10 means that a sound is 10 times more intense, or powerful. To your ears, it sounds twice as loud. The humming of a refrigerator is 45 decibels, normal conversation is approximately 60 decibels, and the noise from heavy city traffic can reach 85 decibels. Sources of noise that can cause NIHL include motorcycles, firecrackers, and small firearms, all emitting sounds from 120 to 150 decibels. Long or repeated exposure to sounds at or above 85 decibels can cause hearing loss. The louder the sound, the shorter the time period before NIHL can occur. Sounds of less than 75 decibels, even after long exposure, are unlikely to cause hearing loss.
Although being aware of decibel levels is an important factor in protecting one’s hearing, distance from the source of the sound and duration of exposure to the sound are equally important. A good rule of thumb is to avoid noises that are “too loud” and “too close” or that last “too long.”
What are the effects of NIHL?
Exposure to harmful sounds causes damage to the hair cells as well as the auditory, or hearing, nerve (see figure). Impulse sound can result in immediate hearing loss that may be permanent. This kind of hearing loss may be accompanied by tinnitus—a ringing, buzzing, or roaring in the ears or head—which may subside over time. Hearing loss and tinnitus may be experienced in one or both ears, and tinnitus may continue constantly or occasionally throughout a lifetime.
Continuous exposure to loud noise also can damage the structure of hair cells, resulting in hearing loss and tinnitus, although the process occurs more gradually than for impulse noise.
Exposure to impulse and continuous noise may cause only a temporary hearing loss. If a person regains hearing, the temporary hearing loss is called a temporary threshold shift. The temporary threshold shift largely disappears 16 to 48 hours after exposure to loud noise. You can prevent NIHL from both impulse and continuous noise by regularly using hearing protectors such as earplugs or earmuffs.
Scientists believe that, depending on the type of noise, the pure force of vibrations from the noise can cause hearing loss. Recent studies also show that exposure to harmful noise levels triggers the formation of molecules inside the ear that damage hair cells and result in NIHL. These destructive molecules play an important role in hearing loss in children and adults who listen to loud noise for too long.
What are the symptoms of NIHL?
When a person is exposed to loud noise over a long period of time, symptoms of NIHL will increase gradually. Over time, the sounds a person hears may become distorted or muffled, and it may be difficult for the person to understand speech. Someone with NIHL may not even be aware of the loss, but it can be detected with a hearing test.
How can a person tell if a noisy situation is dangerous to their hearing?
People may differ in their sensitivity to noise. Nevertheless, as a general rule, noise is probably damaging to the hearing if the noise:
Makes it necessary to shout to be heard over the background noise,
Causes ear pain,
Makes the ears ring, or
Causes a loss of hearing for several hours or more after exposure to the noise.
In contrast to popular belief, there is no truth to the idea that a person is able to "toughen up" the ears by frequent exposure to loud noise. In reality, cumulative noise in the past has probably damaged the ears to such a degree that a person doesn't hear the noise as much. Unfortunately, no treatment is available for noise-induced hearing loss once the damage has occurred.
How loud can a sound get before it affects hearing?
Many experts agree that continual exposure to more than 85 decibels (dB) is dangerous to the ears. As already mentioned, the decibel is a measure of the intensity of sound. For example:
The faintest sound the human ear can detect is labeled 0 dB, whereas the noise at a rocket pad during launch approaches 180 dB;
A quiet whisper is approximately 30 dB;
Normal conversation is 60 dB;
A lawnmower is 90 dB; and
The sound from an iPod Shuffle has been measured at 115 dBs.
Decibels are measured logarithmically, which means that the sound energy of noise increases by units of 10. Therefore, a dB increase of a sound from 20 to 30 dB is an increase of 10 times, and a dB increase of a sound from 20 to 40 dB corresponds to increase of 100 times (10 times 10).
Do the duration and closeness of exposure to loud noise relate to hearing damage?
There is a direct correlation between the duration of exposure to a loud noise and the damage to hearing. This means that the longer the exposure, the greater the damage. Furthermore, the closer one is to the source of the intense noise, the more damaging it is. An example of this is iPod hearing loss. There is an alarming increase in hearing loss in children and young adults due to listening to loud music through earplugs so close to the eardrum.
Another example is discharging firearms. The loud blast of a gun so close to the ear can cause problems for anyone not wearing ear protection.
What factors increase a person's susceptibility to noise-induced hearing loss?
The following factors have been associated with an increased susceptibility to noise-induced hearing loss:
Blue eyes,
Light skin,
Family history of hearing loss,
Diabetes mellitus,
Meniere disease,
Iron deficiency,
Vitamin A deficiency,
Older age,
Atherosclerosis (hardening of the arteries), and
Smoking tobacco.
How else can noise affect a person?
After exposure to noise, tinnitus, which is a ringing or another sound in the ears, occurs commonly. The tinnitus is a sign that inner ear damage or nerve destruction has occurred. Initially the tinnitus is temporary, lasting only several hours. As more cumulative exposure and damage occur, the tinnitus will last longer until it eventually becomes permanent. Loud noise will also cause some people to have anxiety and irritability, an increase in heart rate and blood pressure, or an increase in stomach acid. In addition, very loud noise can reduce efficiency in performing difficult tasks by diverting attention from the job.
How effective are hearing protection devices?
Hearing protection devices decrease the intensity of sound that reaches the eardrum. They come in two forms: earplugs and earmuffs.
Earplugs: Earplugs are small inserts that fit into the outer ear canal. To be effective they must totally block the ear canal with an air-tight seal. They are available in a variety of shapes and sizes to fit individual ear canals and can be custom made. For people who have trouble keeping them in their ears, they can be fitted to a headband.
Earmuffs: Earmuffs fit over the entire outer ear to form an air seal. They are held in place by an adjustable band. Earmuffs must be snugly sealed so the entire circumference of the ear canal is blocked.
Do hearing protectors prevent a person from communicating with others?
The answer is no, at least for people with normal hearing. In fact, just as sunglasses help vision in very bright light, hearing protectors enhance speech understanding in very noisy places. Even in a quiet setting, a normal-hearing person wearing hearing protectors should be able to understand a regular conversation.
Hearing protectors do slightly reduce the ability of those with damaged hearing or poor comprehension of language to understand normal conversation. Nevertheless, it is essential that persons with impaired hearing wear earplugs or earmuffs to prevent further inner ear damage from noise.
How is hearing loss identified?
Hearing loss usually develops over a period of several years. Since the hearing loss is painless and gradual, many people may not notice it. What someone may notice is tinnitus, which is a ringing or another sound in the ear. The tinnitus could be the result of long-term exposure to noise that has damaged the hearing nerve. Or, a person may have trouble understanding what people are saying or may hear everyone as mumbling. Such hearing difficulties are especially apt to occur when one is trying to hear in a noisy place such as in a crowd or at a party. These difficulties could be the beginning of high-frequency hearing loss.
A typical hearing test (audiogram) of a person with noise induced hearing loss will initially show only a high frequency loss at 4000 Hz. (Hertz or Hz is the measure of sound frequency or pitch. Four thousand Hz is high frequency, while 250 or 500 Hz would be low frequency). With continued noise exposure and hearing loss, the audiogram will show a broader loss to include lower (deeper) frequencies.
Noise-induced hearing loss will almost always affect both ears equally, but in some situations, especially with firearm usage, it may be worse in one ear than in the other. For example, firing a rifle tends to injure the ear opposite the side of the trigger finger due to the shadow (blocking the sound) effect of the shooter's head.
What can be done to treat hearing loss?
If a person has any of these symptoms that suggest hearing loss, he or she should consult a physician with special training in ear and hearing disorders (an otolaryngologist or otologist). This type of doctor can diagnose hearing problems and recommend the best way to manage them.
While there is no cure for noise-induced hearing loss, there is some promising research being done. The National Institute on Deafness and Other Communication Disorders (NIDCD) is looking at the use of antioxidants to prevent hearing loss and restore hearing. NIDCD-funded researchers have shown that aspirin and vitamin E can reduce hearing loss if used before exposure to loud noise.
Researchers at the University of Michigan used vitamins A, C, E and magnesium prior to loud noise exposure to prevent hearing loss in animal studies. Studies on people are in progress.

Management, Including Medicolegal Aspect
Prevention
From a clinical perspective, once a diagnosis is made, further noise exposure should be kept away from as far as possible. This may be by avoiding the excessive noise altogether or, if this is not possible, by the use of ear protection in the form of earplugs or earmuffs.
Personal hearing protection
As regards hearing protection, the choice is between earplugs, earmuffs and active noise reduction. The choice will often depend on the performance of the protector and the environment in which it will be used.
Nonspecific Management
Unfortunately, there is no way to replace the hearing that has been lost. In those individual with a mild high-tone hearing loss, hearing aids tend to provide little benefit. The best that can be done in this situation is to give advice regarding the optimization of their acoustic environment. This involves the reduction of background noise (as far as possible), face-to-face conversation to maximize exposure to nonverbal communication cues and an explanation of the problem, to allow the legitimization of their hearing loss.
Specific Management
As the hearing loss becomes more severe, a hearing aid takes on an increasingly beneficial role. A recent National Institute of Clinical Excellence report has suggest that there is significant benefit to patients in being fitted with binaural hearing aids.
Tinnitus should be managed as part of the overall care package. Modern neurophysiological methods (such as tinnitus retraining therapy) utilize a combination of cognitive, directive counseling and sound therapy (including hearing aids and/or white-nose generators) and report useful success rates in the region of 60-70 percent.
Pursuing a claim
In the clinical setting, once a diagnosis of noise-induced hearing loss is made, the patient may well ask advice about pursuing a claim for damages.
More common option is to pursuer a civil claim where the burden of proof is on the claimant.
There has been exposure to excessive noise levels
The hearing loss has been a consequence of that exposure
There was a foreseeable risk of injury from the exposure
The case was brought in time.
Conclusion
Hearing loss is painless
Hearing loss is not reversible
The best type of hearing protection is the type that is worn correctly
Key points
In noise-inducted hearing loss cases there are usual three components to the hearing loss: age, noise and an idiopathic component.
The diagnosis is clinical and requires sufficient noise exposure and the presence of a hearing loss greater than expected from age alone.
Tinnitus and hyperacusis are frequent accompanying symptoms and nowadays often present without any evidence of a hearing loss or only a modest hearing loss.
Treatment is essentially preventative and involves personal hearing protection (earplugs or earmuffs) or reduction of noise levels at source.
This is a diagnosis that frequently enters the legal arena.