NIHL can be caused by a one-time exposure to a loud sound as well as by repeated exposure to sounds of various levels over an extended period of time. The loudness of sound is measured in units called decibels. For example, the humming of a refrigerator is 40 decibels, usual conversation is approximately 60 decibels, and city traffic noise can be 80 decibels. Examples of sources of loud noises that may cause NIHL are motorcycles, firecrackers, gunfire, or any sound at levels from 120 decibels to 140 decibels. Sounds of less than 75 decibels, even after long exposure, are unlikely to cause hearing loss.
Exposure to harmful sounds causes damage to the sensitive hair cells of the inner ear, also called the nerve of hearing. These structures can be injured by noise in two different ways: from an intense brief impulse, such as an explosion, or from continuous exposure to noise, such as that in a woodworking shop.
The effect from impulse sound can be instantaneous and can result in an immediate hearing loss that may be permanent. The structures of the inner ear may also be severely damaged. This kind of hearing loss may be accompanied by tinnitus, a ringing, buzzing or roaring sound in the ears, which may subside over time. Hearing loss and tinnitus may be experienced in one or both ears, and tinnitus may continue constantly or intermittently throughout a lifetime.
The damage that occurs slowly over years of continuous exposure to loud noise is accompanied by various changes in the structure of the hair cells. It also results in hearing loss and tinnitus. Exposure to impulse or continuous noise may cause only a temporary hearing loss; a temporary hearing loss is called a temporary threshold shift. Temporary threshold shifts tend to disappear within 16 hours after exposure to loud noise.
Both forms of NIHL can be prevented by the regular use of hearing protectors such as earplugs or earmuffs.
The symptoms of NIHL that occur over a period of continuous exposure tend to increase gradually with time. Sounds may become distorted or muffled, and it may be difficult for the person to understand normal speech. The individual may not be aware of the loss, but it can be detected with a hearing test.
Noise induced hearing loss can be prevented. Here are a few steps that you can take to protect yourself from sustaining an NIHL:
- Know which noises can cause damage (those above 75 decibels)
- Wear ear plugs or other hearing protective devices when exposed to loud sounds (earplugs and ear muffs may be found at most hardware stores, pharmacies, or discount stores)
- Be alert to hazardous noise occurring in the environment
- Protect children who are too young to protect themselves
- Make family, friends and colleagues aware of the hazards of noise.
- Have a hearing test by an audiologist, a professional trained to identify and measure hearing loss and to rehabilitate persons with hearing impairments. If indicated you may be referred for a medical examination by an otolaryngologist, a physician who specializes in diseases of the ears, nose, throat, head and neck.