- What is Noise Induced Hearing Loss?
- What Causes Noise Induced Hearing Loss?
- How is Noise Induced Hearing Loss Diagnosed?
- How is Noise Induced Hearing Loss Treated?
- How Might Noise Induced Hearing Loss Affect My Life
- Research Studies on Noise Induced Hearing Loss
What is Noise Induced Hearing Loss?
Noise induced hearing loss is a permanent hearing impairment resulting from prolonged exposure to high levels of noise. One in 10 Americans has a hearing loss that affects his or her ability to understand normal speech. Excessive noise exposure is the most common cause of hearing loss. The National Institute of Health reports that about 15 percent of Americans aged 20 to 69 have high frequency hearing loss related to occupational or leisure activities. Because of occupational risk of noise induced hearing loss, there are government standards regulating allowable noise exposure. People working before the mid1960s may have been exposed to higher levels of noise where there were no laws mandating use of devices to protect hearing. Recent studies show an alarming increase in hearing loss in youngsters. Evidence suggests that loud rock music along with increased use of portable radios with earphones may be responsible for this phenomenon.
An example of a noise induced hearing loss is shown in Figure 1. There is a “notch” at 3000 Hz, with better hearing at both lower and higher frequencies. When noise is too loud, it begins to kill cells in the inner ear. As the exposure time to loud noise increases, more and more hair cells are destroyed. As the number of hair cells decreases, so does your hearing. Currently, there is no way to restore life to dead hair cells; the damage is permanent.
The damage caused by noise, called sensorineural hearing loss, can be caused by several factors other than noise, but noise-induced hearing loss is different in one important way – it can be reduced or prevented altogether.
Figure 1: Example audiogram showing noise induced notch.
Noise can also cause a reversible hearing loss, called a temporary threshold shift. This typically occurs in individuals who are exposed to gunfire or firecrackers, and hear ringing in their ears after the event (tinnitus).
What Causes Noise Induced Hearing Loss?
First, we have to define noise. Sound can be measured scientifically in two ways — intensity and pitch. Both of these affect the degree to which sound (noise) damages hearing.
Intensity of Sound
Intensity of sound is measured in decibels (dB). The scale runs from the faintest sound the human ear can detect, which is labeled 0 dB, to over 180 dB, the noise at a rocket pad during launch. Decibels are measured logarithmically, being 20 times the log of the ratio of a particular sound pressure to a reference sound pressure. This means that as decibel intensity increases by units of 20, each increase is 10 times the lower figure. Thus, 20 decibels is 10 times the intensity of 0 decibels, and 40 decibels is 100 times as intense as 20 decibels. Sound intensity may be given in two different units. Persons interested in the actual physical quantification of sound use units of sound pressure level (SPL). SPL is calibrated to a constant sound pressure level that does not vary with frequency. On audiograms, however, sound intensity is calibrated in hearing level (HL), meaning that the reference sound is one that that just barely heard by a normal population. Thus HL units are relative ones and do not generally correspond to SPL units. Higher intensity (db) of sound causes more damage. Many experts agree that continual exposure to more than 85 decibels may become dangerous.
The following table illustrates some common sounds and their intensity.
|Approximate Decibel Level||Examples|
|0 dB||the quietest sound you can hear.|
|30 dB||whisper, quiet library.|
|60 dB||normal conversation, sewing machine, typewriter.|
|90 dB||lawnmower, shop tools, truck traffic; 8 hours per day is the maximum exposure (protects 90% of people).|
|100 dB||chainsaw, pneumatic drill, snowmobile; 2 hours per day is the maximum exposure without protection.|
|115 dB||sandblasting, loud rock concert, auto horn; 15 minutes per day is the maximum exposure without protection.|
|140 dB||gun muzzle blast, jet engine; noise causes pain and even brief exposure injures unprotected ears; maximum allowed noise with hearing protector.|
Pitch is measured in frequency of sound vibrations per second, called Hertz (Hz). Frequency is measured in cycles per second, or Hertz (Hz). The higher the pitch of the sound, the higher the frequency. A low pitch such as a deep voice or a tuba makes fewer vibrations per second than a high voice or violin. Generally noise induce hearing loss occurs at a pitch of about 2000-4000 Hz. Frequency is measured in cycles per second, or Hertz (Hz). The higher the pitch of the sound, the higher the frequency. Young children, who generally have the best hearing, can often distinguish sounds from about 20 Hz, such as the lowest note on a large pipe organ, to 20,000 Hz, such as the high shrill of a dog whistle that many people are unable to hear.
Human speech, which ranges from 300 to 4,000 Hz, sounds louder to most people than noises at very high or very low frequencies. When hearing impairment begins, the high frequencies are often lost first, which is why people with hearing loss often have difficulty hearing the high-pitched voices of women and children.
Loss of high frequency hearing also can distort sound, so that speech is difficult to understand even though it can be heard. Hearing impaired people often have difficulty detecting differences between certain words that sound alike, especially words that contain S, F, SH, CH, H, or soft C, sounds, because the sound of these consonant is in a much higher frequency range than vowels and other consonants.
In addition, the duration (how long you are exposed to a noise) can affect the extent of noise induced hearing loss. The longer you are exposed to a loud noise, the more damaging it may be.
Every gunshot produces a noise that could damage the ears of anyone in close hearing range. Large bore guns and artillery are the worst because they are the loudest. But even cap guns and firecrackers can damage your hearing if the explosion is close to your ear. Anyone who uses firearms without some form of ear protection risks hearing loss.
Excessive noise is present in many situations. Some of the more common ones include occupational noise (machinery, etc.), loud music, and non-occupational noise (lawn mowers, snow blowers, etc.).
Habitual exposure to noise above 85 dB will cause a gradual hearing loss in a significant number of individuals, and louder noises will accelerate this damage. 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 per day at 90 dB, 4 hours per day at 95 dB, and 2 hours per day 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.
The Occupational Safety and Health Administration, in its Hearing Conservation Amendment of 1983, requires hearing conservation programs in noisy work places. This includes a yearly hearing test for the approximately five million workers exposed to an average of 85 dB or more of noise during an 8-hour work day.
Ideally, noisy machinery and work places should be engineered to be more quiet or the worker’s time in the noise should be reduced; however, the cost of these actions 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 tests reveal hearing loss of 10 dB or more in higher pitches in either ear, the worker must be informed and must wear hearing protectors when noise averages more than 85 dB for an 8-hour day.
Non-occupational noises are also regularly encountered during recreational activities and are a source of premature hearing reduction. Peak noise levels, in dB, are provided in the following table taken from Smith et al, 1999.
Musical instruments can generate considerable sound and thus can also cause hearing loss. The most damaging type of sound is in the high-frequencies. Violins and violas can be sufficiently loud to cause permanent hearing loss. This is typically worse in the left ear, which is nearer the instrument. Unlike other instruments, the ability to hear the high-frequency harmonics is crucial to these musicians. Mutes can be used while practicing to reduce long-term exposure. In addition, attending live concerts (where noise levels can exceed 120 dB) can damage hearing, as well as listening to loud music through headphones.
If you think you have grown used to a loud noise, it probably has damaged your ears, and there is currently no treatment – no medicine, no surgery, not even a hearing aid, that truly corrects your hearing once it is damaged by noise.
Anyone who is exposed to loud noise is at risk of hearing damage. Genetic factors may make some individuals more susceptible (Davis et al 2003, Konings et al 2009). Other things that have been linked with an increased risk of noise induced hearing loss include smoking (Palmer et al 2004, Wild et al 2005), male gender, race, poor diet, diabetes, cardiovascular disease (Daniel 2007) and concomitant exposure to carbon monoxide or hydrogen cyanide (Fechter 2004).
How is Noise Induced Hearing Loss Diagnosed?
Hearing loss usually develops over a period of several years. Since it is painless and gradual, you might not notice it. What you might notice is a ringing or other sound in your ear (tinnitus), which could be the result of long-term exposure to noise that has damaged hearing. Or, you may have trouble understanding what people say; they may seem to be mumbling, especially when you are in a noisy place such as in a crowd or at a party. This could be the beginning of high-frequency hearing loss; a hearing test will detect it. If you have any of these symptoms, you may have nothing more serious than impacted wax or an ear infection, which might be simply corrected. However, it might be hearing loss from noise. In any case, take no chances with noise – the hearing loss it causes is permanent.
If you suspect a hearing loss, consult a physician with special training in ear care and hearing disorders (called an otolaryngologist or otologist). This doctor can diagnose your hearing problem and recommend the best way to manage it.
People differ in their sensitivity to noise. As a general rule, noise may damage your hearing if you have to shout over background noise to make yourself heard, the noise hurts your ears, it makes your ears ring, or you are slightly deaf for several hours after exposure to the noise.
How is Noise Induced Hearing Loss Treated?
If you think you have grown used to a loud noise, it probably has damaged your ears, and there is no treatment – no medicine, no surgery, not even a hearing aid, that truly corrects your hearing once it is damaged by noise.
The only thing you can do at this point is to protect what remaining hearing you have. There are a number of things you can do.
While it may seem silly and obvious to point this out, usually the best way to prevent future injury from noise is to avoid exposure to noise! If you have control over the noise (i.e. if you are exposing yourself to the noise), stop doing it ! It is also generally prudent to avoid things that might contribute to ear damage — try to avoid ototoxic drugs like aspirin, and avoid whenever possible exposing yourself to situations where your ear might be damaged (for example, Scuba diving).
If cannot avoid excessive noise you should wear protectors. Examples of situations where you should wear them are when you are using power tools, noisy yard equipment, or firearms.
Hearing protection devices decrease the intensity of sound that reaches the eardrum. They come in two forms: earplugs and earmuffs.
Earplugs are small inserts that fit into the outer ear canal. To be effective they must totally block the ear canal with an airtight 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 ear, they can be fitted to a headband. Simple foam ear-plugs are available at very low cost from the drugstore. Custom made earplugs can be obtained from audiologists. Earplugs must be snugly sealed so the entire circumference of the ear canal is blocked. An improperly fitted, dirty or worn-out plug may not seal and can irritate the ear canal. Ordinary cotton balls or tissue paper wads stuffed into the ear canals are very poor protectors; they reduce noise only by approximately 7 dB.
Earmuffs fit over the entire outer ear to form an air seal so the entire circumference of the ear canal is blocked, and they are held in place by an adjustable band. Earmuffs will not seal around eyeglasses or long hair, and the adjustable headband tension must he sufficient to hold earmuffs firmly around the ear.
Properly fitted earplugs or muffs reduce noise 15 to 30 dB. The better earplugs and muffs are approximately equal in sound reduction, although earplugs are better for low frequency noise and earmuffs for high frequency noise. Simultaneous use of earplugs and muffs usually adds 10 to 15 dB more protection than either used alone. Combined use should be considered when noise exceeds 105 dB. Note that for such situations, it may be that there is no type of hearing protection that will stop a very loud noise from affecting you.
The mechanism of noise induced damage is proposed to include reactive oxygen species (Henderson, 2006; Le Prell, 2003), which can cause cell death. Reactive oxygen species are removed by antioxidants. Antioxidants that have been studies include N-acetylcyteine, magnesium, salicylate, vitamin E and ebselen (Coleman et al 2007, Le Prell et al 2007, Lynch & Kil 2005, Sendowski et al 2006, Suckfuell et al 2007). Two recent studies have demonstrated a degradation of cell-cell junctions (important for structural and function purposes) and genetic transcriptional changes in animal models. This means that noise-induced damage can modify “genetic maintenance” of the cochlea (Cai et al 2012, Zheng & Hu 2012).
Glucorticoids, such as cortisol, may modulate hearing sensitivity (Canlon et al 2007) and also shows some protective effects (Le Prell et al 2003, Oishi & Schacht 2011). Increasing interest is developing with regard to oral antioxidant treatment/prevention of NIHL. D-methionine (D-met), salicylate, ebselen, N-acetylcysteine (NAC), ACE Mg, and sodium thiosulfate are all in or near clinical trials. See the work of Kathleen Campbell for a thorough discussion of pharmacologic otoprotective agents .
Gene therapy and stem cell therapy are also under investigation for the treatment of sensorineural hearing loss(Sun et al 2011).
What Are the Common Problems of Hearing Protectors?
Studies have shown that one half of the workers wearing hearing protectors receive one half or less of the noise reduction potential of their protectors because these devices are not worn continuously while in noise or because they do not fit properly.
A hearing protector that gives an average of 30 dB of noise reduction, if worn continuously during an 8-hour work day, becomes equivalent to only 9 dB of protection if taken off for one hour in the noise. This is because decibels are measured on a logarithmic scale, and there is a 10-fold increase in noise energy for each 10 dB increase.
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. In addition, noise exposure is cumulative. So the noise at home or at play must be counted in the total exposure during any one day. A maximum allowable while on-the-job followed by exposure to a noisy lawnmower or loud music will definitely exceed the safe daily limit.
Even if earplugs and/or muffs are worn continuously while in noise, they do little good if there is an incomplete air seal between the hearing protector and the skin. When using hearing protectors, you will hear your own voice as louder and deeper. This is a useful sign that the hearing protectors are properly positioned.
Can I Hear Other People and Machine Problems If I Wear Hearing Protectors?
Hearing protectors reduce hearing and therefore do reduce one’s ability to understand normal conversation. A normal hearing person wearing hearing protectors should be able to understand a regular conversation. A person who has impaired hearing already, perhaps due to noise, might be unable to understand coworkers when they are wearing hearing protectors.
There are a number of strategies that can be used to reduce the change of noise injury from other instrumentalists. Musicians ear plugs are generally “flat” so that bass and treble notes are not relatively favored, thus distorting perception. Nevertheless, a “vented” ear plug can be used to tune the ear cavity to low frequencies, which are less damaging. Drummers should use musicians ear plugs, such as the ER-25. Guitarists and vocalists can use the less attenuating ER-15. Too much ear protection can result in overplaying and not enough protection can result in hearing loss.
Plexiglas baffles can be used to reduce the noise from other instruments. These are particularly relevant for drummer’s high-hat cymbals. Drums and brass can be particularly a problem. Ear monitors are small in-the-ear devices that look like hearing aids that can be used to electronically protect hearing, while allowing the musicians to hear themselves. Acoustic monitors are stethoscope like devices that block sound from other in the group, but allow the instrumentalist to hear their own instrument.
Loudspeakers produce both high- and low-frequency sounds. High frequencies tend to emanate in almost a straight line, while low frequencies are present in nearly all directions. Thus, standing besides a high-frequency source may provide some protection. Humming just prior to, and through a loud noise such as a cymbal crash or rim shot may provide some protection. Small protective muscles in the ear contract naturally when we sing or hum, and thus humming may protect from other noises.
Hearing aids can be set to limit noise and thus can be set up so that they do not contribute to additional hearing loss. For someone whose life is affected by noise induced hearing loss, a hearing aid may be a very reasonable thing to consider.
How Might Noise Induced Hearing Loss Affect My Life?
Hearing loss can impact ones life in many ways. You may be less able to understand conversation or appreciate music. A ringing in the ears, called tinnitus, commonly occurs after noise exposure, and it often becomes permanent. Some people react to loud noise with anxiety and irritability, an increase in pulse rate and blood pressure, or an increase in stomach acid. Very loud noise can reduce efficiency in performing difficult tasks by diverting attention from the job.
Research Studies on Noise Induced Hearing Loss
As of 8/2012, a visit to the National Library of Medicine’s search engine, Pubmed, revealed 6260 research articles concerning noise induced hearing loss published since 1951. At the American Hearing Research Foundation (AHRF), we have funded basic research on noise induced hearing loss in the past, and are interested in funding sound research in the future. Learn more about donating to American Hearing Research Foundation (AHRF) to diagnose and treat noise induced hearing loss.
Graphics are courtesy of Northwestern University.
- Cai Q, Patel M, Coling D, Hu BH. 2012. Transcriptional changes in adhesion-related genes are site-specific during noise-induced cochlear pathogenesis. Neurobiology of disease 45: 723-32
- Canlon B, Meltser I, Johansson P, Tahera Y. 2007. Glucocorticoid receptors modulate auditory sensitivity to acoustic trauma. Hearing research 226: 61-9
- Coleman JK, Kopke RD, Liu J, Ge X, Harper EA, et al. 2007. Pharmacological rescue of noise induced hearing loss using N-acetylcysteine and acetyl-L-carnitine. Hearing research 226: 104-13
- Daniel E. 2007. Noise and hearing loss: a review. The Journal of school health 77: 225-31
- Davis RR, Kozel P, Erway LC. 2003. Genetic influences in individual susceptibility to noise: a review. Noise & health 5: 19-28
- Fechter LD. 2004. Promotion of noise-induced hearing loss by chemical contaminants. Journal of toxicology and environmental health. Part A 67: 727-40
- Konings A, Van Laer L, Van Camp G. 2009. Genetic studies on noise-induced hearing loss: a review. Ear and hearing 30: 151-9
- Le Prell CG, Dolan DF, Schacht J, Miller JM, Lomax MI, Altschuler RA. 2003. Pathways for protection from noise induced hearing loss. Noise & health 5: 1-17
- Le Prell CG, Yamashita D, Minami SB, Yamasoba T, Miller JM. 2007. Mechanisms of noise-induced hearing loss indicate multiple methods of prevention. Hearing research 226: 22-43
- Lynch ED, Kil J. 2005. Compounds for the prevention and treatment of noise-induced hearing loss. Drug discovery today 10: 1291-8
- Oishi N, Schacht J. 2011. Emerging treatments for noise-induced hearing loss. Expert opinion on emerging drugs 16: 235-45
- Palmer KT, Griffin MJ, Syddall HE, Coggon D. 2004. Cigarette smoking, occupational exposure to noise, and self reported hearing difficulties. Occupational and environmental medicine 61: 340-4
- Sendowski I, Raffin F, Braillon-Cros A. 2006. Therapeutic efficacy of magnesium after acoustic trauma caused by gunshot noise in guinea pigs. Acta oto-laryngologica 126: 122-9
- Suckfuell M, Canis M, Strieth S, Scherer H, Haisch A. 2007. Intratympanic treatment of acute acoustic trauma with a cell-permeable JNK ligand: a prospective randomized phase I/II study. Acta oto-laryngologica 127: 938-42
- Sun H, Huang A, Cao S. 2011. Current status and prospects of gene therapy for the inner ear. Human gene therapy 22: 1311-22
- Wild DC, Brewster MJ, Banerjee AR. 2005. Noise-induced hearing loss is exacerbated by long-term smoking. Clinical otolaryngology : official journal of ENT-UK ; official journal of Netherlands Society for Oto-Rhino-Laryngology & Cervico-Facial Surgery 30: 517-20
- Zheng G, Hu BH. 2012. Cell-cell junctions: a target of acoustic overstimulation in the sensory epithelium of the cochlea. BMC neuroscience 13: 71