Alternative Names noise-induced hearing loss, acoustic trauma
Definition Occupational hearing loss is the term for hearing loss caused by noise trauma.
What is going on in the body? The inner ear has components for both balance and hearing. The portion for hearing is called the cochlea. It's shaped like a snail shell. Sounds cause the eardrum to vibrate. Small bones transmit the vibration to the inner ear, which is filled with fluid. The stapes, a bone in the inner ear, moves, and creates a fluid wave. Hair cells in the inner ear detect these waves and convert them into nerve signals, which are transmitted to the auditory nerve. The auditory nerve sends the signal into the brain, where it is interpreted as sound.
The snail-shell shape of the inner ear is arranged by pitch. At the top of the snail shell, low-frequency sounds are picked up. In the bottom turn of the snail shell, high pitches are detected. There are about 25,000 rows of hair cells, and each row responds to a particular pitch.
The hair cells can be injured by acute high-intensity sounds or long-term exposure to loud sounds. Acute noise injury can cause both temporary and permanent damage. Over time, loud sounds can cause permanent injury because hair cells are lost. As hair cells are lost, the person becomes unable to hear sounds at those frequencies. The high-frequency first turn of the cochlea is the most easily injured area of the inner ear.
What are the signs and symptoms of the condition? Acute noise injury is also called acute threshold shift. This form of hearing loss is caused by very loud sounds. These can be consistently loud, like at rock concerts, or around jet aircraft. Or the loud sounds can be very brief, such as the noise from an explosion.
In an acute threshold shift, the hair cells in the lower part of the cochlea are injured. These hair cells are the ones that pick up high-frequency sound. The person will have a feeling of fullness in the ears, and sounds will seem muffled. He or she cannot hear very well, especially at high frequencies. There is usually a high-frequency ringing. Most people recover their hearing completely within 24 to 48 hours.
Even when the hearing comes back, the hair cells are permanently damaged. Some sounds are so loud that high-frequency hearing is immediately and permanently lost. Such sounds include explosions, artillery fire, fireworks, and gunshots. Some people seem to be more prone to injury from noise exposure. Some people have no problem with the same noise that causes hearing loss in others. Hearing that does not return after an acute noise injury is called a permanent threshold shift.
People who are exposed to noise repeatedly over a long time will have those noise injuries build up. The result is a hearing loss at high frequencies that slowly gets worse and worse. These people may not even be aware that anything is wrong with their ears or hearing. The Occupational Safety and Health Administration (OSHA) sets limits on occupational noise exposure. People exposed to 85 decibels or more per hour are required to wear some form of hearing protection.
What can be done to prevent the condition? People should wear hearing protection if they work around noise that's than 85 decibels per hour or more. Others exposed to loud noise, such as construction workers and heavy equipment operators, should also use hearing protection. Ear protection should be worn whenever lawn mowers, string trimmers, or power tools are used. Power woodworking tools should be avoided especially in an enclosed space. If they must be used, hearing protection should be worn.
People who abuse their own hearing, such as listening to music that is too loud, are risking permanent high-frequency hearing loss. The average age of people who get hearing aids is dropping. This means that people are being exposed to too much noise earlier in life.
People who work around loud noise should have regular hearing tests. If people do have hearing loss, they should be very careful to protect their remaining hearing. There are a number of devices that help to protect the ear from noise. Silicone putty and foam earplugs reduce noise by 20 to 25 percent. The best hearing protection is the combination of acoustic ear muffs and earplugs. It can reduce noise by 30 to 35 decibels.
How is the condition diagnosed? The diagnosis is usually made with standard hearing tests. Newer tests, such as otacoustic emissions, can measure how well the hair cells are working. Sometimes hair cell loss shows up before a standard hearing test can detect a hearing problem. An x-ray or CT scan of the head may be used to rule out other causes of hearing problems.
What are the long-term effects of the condition? The main long-term effect of noise exposure is high-frequency hearing loss. The loss keeps getting worse and cannot be reversed. People with this loss often have ringing of the ears, called tinnitus. They may also have trouble hearing conversation over background noise.
What are the risks to others? Occupational hearing loss poses no risks to others.
What are the treatments for the condition? The best treatment for occupational noise exposure is prevention. Those whose hearing is getting worse can use hearing aids.
What are the side effects of the treatments? There are no side effects from treatments.
What happens after treatment for the condition? Many people with hearing aids have a hard time communicating when there is a lot of background noise. Avoiding loud sounds and careful ear protection may prevent further damage.
How is the condition monitored? People who work around a lot of noise need to have regular hearing tests.
Author: Mark Loury, MD Reviewer: HealthAnswers Australia Medical Review Panel Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia Last Updated: 1/10/2001 Contributors Potential conflict of interest information for reviewers available on request
