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 in the inner ear that convert sound energy into electrical signals that travel to the brain. Once damaged, our hair cells cannot grow back.
Scientists once believed that only the pure force of vibrations from loud sounds caused the damage to hair cells. Instead, recent studies have shown that exposure to harmful noise may trigger the formation of molecules inside the ear that can damage or kill hair cells through oxidative stress. Another possible avenue is vasoconstriction-induced reduction in cochlear blood flow (CBF).
NIHL as Acquired Hearing Loss
NIHL is the primary cause of acquired hearing loss in the industrialized world. 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.
Exposure to harmful sounds causes damage to the hair cells as well as the auditory, or hearing, nerve. 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.
Understanding How The Ear Works
Hearing is a complex process. To put it simply, when something makes a noise, it sends sound waves through the air. These waves are funneled into the ear canal by the outer ear, and the sound waves strike the eardrum and cause it to vibrate.
At the end of the auditory canal lies the tympanic membrane (ear drum) and the middle ear. Within the middle ear cavity are the ossicles, the three small bones known by the layman as the hammer, the anvil, and the stirrup. When sound waves hit the eardrum, it vibrates and, in turn, moves the hammer. The hammer moves the anvil, which moves the stirrup, transmitting the vibrations into the inner ear. The middle ear functions to amplify sound, which is why significant hearing loss can result from any disruption in any of its parts.
The inner ear consists of the cochlea and the vestibular (balance) system. The cochlea converts sound waves into nerve impulses that travel to the brain via the movement of tiny hair cells. It is the brain that allows you to hear…as long as the message it is receiving is not distorted due to problems in the process just described.
When the tiny hair cells in the inner ear are exposed to loud sounds, they may be damaged and die. The damage may be immediate when sounds are too loud even for a brief time, or the damage may occur over time when sounds are loud and long-lasting. The louder the noise, the faster it can damage your hearing.
Who Is Affected By 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.
People of all ages, including children, teens, young adults, and older people, can develop NIHL. Approximately ten percent of Americans between ages 20 and 69—or 22 million Americans—already may have suffered permanent damage to their hearing from excessive noise exposure. Exposure occurs in the workplace, in recreational settings, and at home. Recreational activities that can put someone at risk for NIHL include target shooting and hunting, snowmobile riding, woodworking and other hobbies, playing in a band, and attending rock concerts. Harmful noises at home may come from lawnmowers, leaf blowers, and shop tools.
How Can Noise Damage Our Hearing?
The tops of the hair cells in our inner ear have very fine, wispy protrusions called stereocilia. These structures, similar to blades of grass, bend when exposed to loud sounds. They will become straight again after a recovery period. However, repeated or intense exposure will damage them beyond repair, leading to cell death. These cells do not regenerate in humans, leading to hearing loss.
Scientists once believed that only the pure force of vibrations from loud sounds caused the damage to hair cells. Instead, recent studies have shown that exposure to harmful noise may trigger the formation of molecules (free radicals) inside the ear that can damage or kill hair cells through oxidative stress. The free radicals react with the molecules that make up the cell wall, compromising the integrity of the cell and affecting cell function.
Another possible avenue of damage is noise-induced reduction in cochlear blood flow (CBF). Noise exposure induces the formation in the cochlea of a potent vasoconstrictor. The blood supply of the cochlea is supplied mainly by a single artery, making it particularly vulnerable to any circulatory constriction and increasing susceptibility to NIHL.
How Do I Know If Something Is Too Loud?
Any sound that you have to raise your voice to be heard over is potentially damaging and can lead to NIHL.
Noise Levels of Common/Everyday Sounds
The loudness of sound is measured in units called decibels. For example, the humming of a refrigerator is 40 decibels, normal conversation is approximately 60 decibels, and city traffic noise can be 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 and Signs of NIHL?
The most common complaint a hearing impaired person will say is “I hear you, but I don’t understand you.”
You may have hearing loss if…
- You hear people speaking but you have to strain to understand their words.
- You frequently ask people to repeat what they said.
- You don’t laugh at jokes because you miss too much of the story or the punch line.
- You frequently complain that people mumble.
- You need to ask others about the details of a meeting you just attended.
- You play the TV or radio louder than your friends, spouse and relatives.
- You cannot hear the doorbell or the telephone.
- You find that looking at people when they speak to you makes it easier to understand.
How Is Noise-induced Hearing Loss Diagnosed?
NIHL is diagnosed through an audiological evaluation. First, the ears are examined with an otoscope for any possible physical conditions such as excessive ear wax or infection. Next, the hearing sensitivity is determined using an audiometer which can produce sounds at different frequencies and intensities. The results of this pure tone test are plotted on an audiogram. NIHL is often associated with a high frequency loss of hearing, as the portions of the inner ear responsible for sensing high frequencies are more susceptible to noise damage due to their location in the cochlea.
Speech testing, including repetition of words and sentences in quiet and noise, is another important portion of the exam. Many consonant sounds, which help make speech clear, are softer and higher in pitch. Loss of high frequency hearing can lead to speech sounding muffled or unclear.
Can NIHL Be Prevented?
NIHL is 100 percent preventable. All individuals should understand the hazards of noise and how to practice good hearing health in everyday life. To protect your hearing:
- Know which noises can cause damage (those at or above 85 decibels).
- Wear earplugs or other hearing protective devices when involved in a loud activity (special earplugs and earmuffs are available from your audiologist).
- Be alert to hazardous noise in the environment.
- Protect the ears of children who are too young to protect their own.
- Make family, friends, and colleagues aware of the hazards of noise.
- If you suspect hearing loss, have a medical examination by an otolaryngologist (a physician who specializes in diseases of the ears, nose, throat, head, and neck) and a hearing test by an audiologist (a health professional trained to measure and help individuals deal with hearing loss).
Can Noise Induced Hearing Loss Be Reversed?
Exposure to impulse and continuous noise may cause only a temporary hearing loss. Recent research has found that this may be an adaptive process. As the noise level rises, cells in the cochlea release a hormone (ATP) which causes a temporary decline in hearing sensitivity. 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.
What Research Is Being Done On NIHL?
Numerous organizations are conducting research on the causes and prevention of NIHL, including identifying genes important for hair cell development and function. Researchers are also looking at the protective properties of supporting cells in the inner ear which appear to be capable of lessening the damage to sensory hair cells upon exposure to noise.
Current avenues of treatment research include antioxidants, neurotrophins, antagonists, inhibitors, vasodilators, vitamins, and steroid hormones.
Several products are available to help protect hearing from damaging noise levels: Expandable foam plugs are inexpensive and easy to use. The plugs are rolled into a thin tube and inserted into the ears, where they expand and conform to the shape of the ear. They can lower noise levels by 15 to 30 decibels.
Pre-molded plugs can be made from a variety of materials. They come in various sizes and are reusable. They are available in both off the shelf and custom versions, and generally provide greater noise reduction than foam plugs.
Earmuffs cover the outer ear, and some models also include electronic components to help with communication. They can lower noise levels by 15 to 30 decibels, and can be combined with plugs for greater noise reductions
Living With Noise-Induced Hearing Loss
The first step to living with NIHL is to acknowledge it. Being aware of the signs and symptoms of hearing loss and hazardous noise levels can help you prevent further hearing loss. Hearing aids, if warranted, can help you communicate with others and keep you involved in life. Studies have shown that untreated hearing loss is correlated with dementia, and properly fitted hearing aids may lead to a reduction of dementia risk. Hearing aid use may also lead to a reduction in the perception of tinnitus.