Noise-Induced Hearing Loss and Its Prevention

Picture of the Inner Ear Structure

The industrial and technological revolution may have propelled society to higher levels of achievement, but this progress has also made the world a noisier place in which to live. Noise pollution is a growing health hazard and can be found almost everywhere. Car alarms, leaf blowers, gunshots, boom boxes, and traffic congestion fill our cities with decibels (the measure of sound intensity). Escaping to the country may not provide a quiet refuge, and even farmers are at high risk of exposure to noise from their farm machinery.

Unpleasant or unwanted sounds are not the only harmful noises to which we may be exposed. For example, the music at a concert and the pounding of a jackhammer on the street can be equally damaging to the inner ear. Loud sounds (acoustic energies) delivered with equal intensity or over extended periods, regardless of their source, are equally dangerous. Eventually, continued or repeated exposures to high-intensity sound can cause acoustic trauma to the ear. This trauma can result in hearing loss, ringing in the ears (tinnitus), and occasional dizziness (vertigo), as well as non-auditory effects, such as increases in heart rate and blood pressure.

One-third of the 30 million Americans with hearing loss have an impairment that is at least partially attributed to excessive noise exposure. Noise remains the most common preventable cause of irreversible sensorineural (involving the ear's sensory nerve) hearing loss.

Acoustic trauma occurs when any excessive sound energy strikes the inner ear. If it is brief, the noise may cause a reversible, temporary hearing loss, technically known as a temporary threshold shift. For example, after a loud rock concert, it is common to experience hearing dullness and ringing in the ears for several hours. In this situation, if symptoms persist beyond several days, oral steroids (cortisone-type medications) may help the inner ear to recover. If the noise is loud enough and the duration of exposure long enough, however, it may cause a permanent threshold shift. This condition is called noise-induced hearing loss, which has no cure and is irreversible.

Sudden hearing loss produced by a sudden and very loud noise (blast injury) is called acute acoustic trauma. If the sound is loud enough, it can cause the eardrum to rupture or the person to have a complete loss of hearing. Sometimes, particularly if the sudden loss is total and occurs combined with dizziness, immediate surgical exploration of the ear may be necessary. In this circumstance, the ear surgeon may need to locate and patch a hole (perilymphatic fistula) between the inner ear fluid space and the middle ear space.

People may differ in their sensitivity to noise. Nevertheless, as a general rule, noise is probably damaging to hearing if the noise does the following:

• Makes it necessary to shout to be heard over the background noise
• Causes ear pain
• Makes the ears ring
• 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 can "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, limited treatment is available for noise-induced hearing loss once the damage has occurred.

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 whisper is approximately 30 dB
• Normal conversation is 60 dB
• A lawnmower is 90 dB
• 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 an increase of 100 times (10 times 10).

There is a direct correlation between the duration of exposure to loud noise and hearing damage. 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.

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)
• Smoking tobacco

After exposure to noise, tinnitus, which is a ringing or another sound in the ears, occurs commonly. 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.

Habitual exposure to noise above 85dB will cause gradual hearing loss in a significant number of individuals. Moreover, noise greater than 85dB will accelerate this damage. Accordingly, the U.S. Occupational Safety and Health Administration (OSHA) has imposed regulations nationwide regarding on-the-job noise exposure. For unprotected ears, the allowed exposure time decreases by one-half for each 5 dB increase in the average noise level. For instance, exposure is limited to 8 hours at 90 dB, 4 hr at 95 dB, and 2 hr at 100 dB. The highest permissible noise exposure for the unprotected ear is 115 dB for 15 minutes per day. Any noise above 140 dB is not permitted.

OSHA, in its Hearing Conservation Amendment of 1983, required the institution of a hearing conservation program in noisy workplaces. Such a program must include a yearly hearing test for workers exposed to an average of 85 dB or more of noise during their 8-hour workday. It turns out that approximately 25% of the American industrial workforce is exposed to this level of noise.

Ideally, noisy machinery and workplaces should be designed to be quieter, and/or the worker's time in the noise should be reduced. The cost of reducing noise exposure in these ways, however, is often prohibitive. As an alternative, individual hearing protectors are required when noise averages more than 90 dB during an 8-hour day.

When noise measurements indicate that hearing protectors are needed, the employer must offer at least one type of earplug and one type of earmuff without cost to employees. If the yearly hearing test reveals a hearing loss of 10 dB or more in the higher sound frequencies (pitch) in either ear, the worker must be informed. (The higher frequencies of sound are the most sensitive to noise damage.) Also, the worker must wear hearing protectors when noise averages more than 85 dB for an 8-hour day. Greater losses of hearing or the possibility of ear disease necessitates referral to an ear doctor (otolaryngologist).

Hearing loss usually develops over several years. Since 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. A person may have trouble understanding what people are saying or may hear everyone 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.

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 before loud noise exposure to prevent hearing loss in animal studies. Studies on people are in progress.

For examples of sounds measured in dBs, The American Academy of Otolaryngology-Head and Neck Surgery provides an Interactive Loudness Scale.

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 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.

Properly fitted earplugs or muffs reduce noise by 15 to 30 dB of sound. The better earplugs and earmuffs are approximately equally effective in sound reduction. However, earplugs are better protection against low-frequency noise (such as noise from a jackhammer), and earmuffs are better protection against high-frequency noise (such as noise from an airplane taking off). For high-frequency sounds, think of the high-pitched treble keys of the piano, whereas for low-frequency sounds, think of the low- or deep-pitched bass keys of the piano.

Simultaneous use of earplugs and muffs usually adds 10 to 15 dB more protection than either used alone. Combined use should be considered when the noise exceeds 105 dB. It is important to understand that ordinary cotton balls or tissue paper wads stuffed into the ear canals are very poor protectors since they only reduce noise by approximately 7 dB.

Excessive noise exposure may occur at live rock concerts as well as in more intimate venues for music whenever amplification is utilized. The damage to hearing from music is every bit as permanent as that incurred by other means. Special high-fidelity earplugs have been developed specifically for such situations and are being utilized by musicians and professional sound engineers. These earplugs are specially designed to eliminate the so-called plugged (occluded) ear effect and to maintain an even reduction of sound across the frequency range. Otherwise, when the ear is plugged, the plugged ear effect makes one's voice sound more bass, or deeper, and louder. Try it by occluding your ear(s) (gently) with your finger, and speak. You'll hear the plugged ear effect.

Studies have shown that one-half of the workers wearing hearing protectors receive only one-half or less of the noise reduction potential of their protectors. This diminished protection occurs because these devices either are not worn continuously while exposed to noise or do not fit properly.

As previously mentioned, a hearing protector can give an average of 30 dB noise reduction if worn continuously during an 8-hour workday. If taken off for just one hour while exposed to noise, however, such a protector would provide only an average of 9 dB of protection during the 8 hours. This substantial reduction in protection occurs because with the logarithmic scale used to measure decibels, a 10-times increase in noise energy occurs for each 10 dB increase in sound. Thus, during the hour with unprotected ears, the worker is exposed to 1,000 times more sound energy than if earplugs or muffs had been worn. (For the 30 dB, 10 x 10 x 10 = 1000 times more noise.)

In addition, noise exposure is cumulative. So, the noise at home or play must be counted in the total exposure during any one day. A maximum allowable on-the-job exposure followed by further exposure at home to a noisy lawnmower or loud music will exceed the safe daily limit.

Even if earplugs and/or earmuffs are worn continuously while exposed to noise, they do little good if there is an incomplete air seal between the hearing protector and the skin. As mentioned above, when using ordinary hearing protectors, it is common to hear one's voice louder and deeper. This plugged ear effect can be taken as a useful sign that the hearing protectors are properly positioned.

The answer is no, at least for people with normal hearing. 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.

It has been argued that hearing protectors might reduce a worker's ability to hear the noises that signify an improperly functioning machine. Most workers, however, readily adjust to the quieter sounds and can still detect such problems.