Hearing Healthcare Professionals | Physicians | Press

Prevention of Noise Induced Hearing Loss

Richard W. Danielson, Ph.D. National Space Biomedical Research Institute and Baylor School of Medicine, Houston, Texas

Introduction

Noise is one of the most common causes of hearing loss, and one of the most common occupational illnesses in the United States. A single shot from a shotgun, experienced at close range, may permanently damage your hearing in an instant. Repeated exposures to loud machinery may, over an extended period of time, present serious risks to human hearing. According to the National Institute on Deafness and Other Communication Disorders (NIDCD), 10 million Americans have already suffered irreversible hearing damage from noise, and 30 million more are exposed to dangerous noise levels each day.

Why has this problem become so widespread? Unfortunately, the effects of noise are often underestimated because the damage takes place so gradually, loud noises have become so common in our culture, and (although traumatizing to the parts of the body responsible for hearing) there are no externally-visible physical changes (like bleeding). As a result, people have traditionally not appreciated the serious impact of noise-related hearing loss on their daily living until they're frustrated by a permanent communication problem. Perhaps a bit too late, they then become passionate about "hearing conservation," in order to save the hearing they still possess. Doesn't it make more sense, though, to emphasize "HEARING LOSS PREVENTION," when you have good hearing sensitivity? This document will summarize how excessive noise can damage the hearing system, factors that influence this damage, and actions that you can take to prevent hearing loss.

Every day, we enjoy sounds. But, when an individual is exposed at work or at home to harmful sounds - those that are too loud or loud sounds over a long time - sensitive structures of the inner ear can be damaged, causing noise-induced hearing loss (NIHL). NIHL is characterized by a gradual, progressive loss of high frequency hearing sensitivity over time, as a result of exposure to excessive noise levels. Figure 1 illustrates this typical progression, in which the pattern of noise-induced hearing loss usually shows a "notch" that is usually seen at or near 4000 Hz. In later stages, the hearing loss may spread to frequencies that are more critical to understanding human speech (in the range of 500-3000 Hz). NIHL usually occurs bilaterally (in both ears). However, the hearing loss may not necessarily occur equally between the left and right ears when the exposure conditions favor one side of the head. It is even possible to see a unilateral hearing loss, as can happen in acoustic trauma, when a loud blast affects the ear nearest the explosion.

Progression of hearing loss following exposure to loud noise (95 dBA, averaged across
the work day. Data show hearing loss for white males at ages 20, 30, 40, 50 and 60 years
with 0 - 40 years of exposure, respectively). (ANSI 3.44-1996)

How do we hear?

While earlier we have explained the mechanisms of hearing, it would be useful to review these principles in terms of how noise can lead to permanent hearing loss damage. Our hearing system is designed to detect and process sounds over a wide range of levels, but probably not for the extremely loud noises that occur in our modern world. Acoustic signals enter through the outer ear, funneled by the pinna (the external part of the ear that we can see) and external ear canal. This funneling causes a "resonance" (like that created when you blow across the top of a glass bottle) which boosts energy in high frequencies, about 2000 Hertz (heard as a high pitch). The energy then reaches the middle ear where it causes the eardrum to vibrate three tiny bones, called the ossicles, in the middle ear. The eardrum and ossicles amplify the vibrations and carry them to the inner ear or cochlea, which is a fluid-filled chamber locked inside the inner ear. These vibrations can be dampened when loud sounds cause a contraction of two tiny muscles attached to the middle ear bones, but this action is not fast enough to offer protection from sudden bangs and cannot be sustained during long exposures. The vibrations move through the fluid in the cochlea, which contains about 20,000 sense cells of hearing, called the hair cells. Movements in the fluid stimulate the top portion of the hair cells, which causes chemical changes that produce nerve impulses. These nerve impulses are carried along the hearing nerve to the brain, where they are interpreted as sound. The brain uses the incoming nerve impulses in an elegant interpretation of which, when and how many hair cells are stimulated. The hearing sensitivity of young children, with no hearing damage, allows them to detect very soft sounds across a range of approximately 8-9 octaves.

How does noise affect our hearing?

When the hearing system is exposed to excessive noise, mechanical and metabolic changes can occur from this stress. Scientific research, based on studies of industrial workers, as well as lab studies of humans and animals, have investigated the effects of noise on hearing. This work has determined that, after excessive noise has stimulated cells in the inner ear, chemical processes occur that can exceed the cells' tolerance, damaging their function and structure. This damage results in sensorineural hearing loss (as opposed to a conductive hearing loss, where the outer or middle ear have been affected) and tinnitus (ringing of the ears). The sensory cells in the cochlea may recover from their damage (as you have possibly experienced after a loud concert or work with a loud machine). Usually, recovery from temporary threshold shift (or TTS) occurs quickly, largely disappearing in 16 to 48 hours after exposure to loud noise. However, if the hearing sensitivity does not recover within a few days, an irreversible permanent threshold shift has taken place.

Loud explosions (that peak for a few milliseconds at levels greater than 130-140 dB) may cause hearing loss. More often, however, hearing loss is caused by repeated exposure to noise above 85 dBA over long periods. The risk of noise-induced hearing loss depends on both the intensity and duration of the exposure. As intensity increases, the length of time for which the exposure is "safe" decreases. As a result, someone exposed to 85dBA (often produced by gas-engine lawn mowers) for 8 hours may be equally at risk for noise exposure after using a chain saw (producing 110dBA) for only a few minutes.

What are the symptoms of NIHL?

NIHL develops gradually so that people may lose a significant amount of hearing before becoming aware of its presence. During the early stages, sufferers often report having to turn up the volume on the TV or have difficulty understanding speech in groups or in the presence of background noise. As the hearing loss worsens, it becomes difficult to understand normal conversation even in quiet, one-on-one situations. The individual may not be aware of the high frequency hearing loss, but it can be detected with a hearing test. In fact, early identification is important in order to recognize the presence of NIHL and then take steps to prevent further hearing loss.

Some of the warning signs of the presence of or exposure to hazardous noise are as follows:

• You can't hear someone three feet away
• You have pain in your ears after leaving a noisy area
• You hear ringing or buzzing (tinnitus) in your ears immediately after exposure to noise
• You suddenly have difficulty understanding speech after exposure to noise; you can hear people talking, but you cannot understand them.

Can NIHL be prevented?

NIHL is preventable. Although hearing normally declines with age, the average, healthy, non-noise-exposed person can have essentially normal hearing at least up to age 60. Individuals vary in their susceptibility to hearing loss. While research has shown some trends, there currently is no reliable way to identify which particular individuals may be most susceptible to NIHL. To protect themselves when exposed to hazardous noise, everyone should take these precautions:

• Know which noises can cause damage (those above 85 decibels), including jet engines, lawn mowers, motorcycles, chainsaws, powerboats, and personal stereos. If you have to raise your voice to shout over the noise to be heard by someone within an arm's length away, the noise is probably in this range. More formal noise measurements can be made (and are required in most industries), to determine risks from noise exposures. The following figure (courtesy of Sight and Hearing Association www.sightandhearing.org) shows you the relative risk associated with certain noisy environments.

click on noise thermometer to enlarge

• If possible, try to reduce noise at the source. Sometimes, replacing mufflers, keeping equipment in good maintenance, or placing the machine inside an enclosure can shield a person from the risks of NIHL. When purchasing new tools and yard equipment, consider their noise outputs before buying units with ineffective mufflers. Personal listening devices (like CD players with earphones) can also present risks to hearing; avoid turning the volume up high.



• Wear hearing protective devices (HPDs) such as earplugs or earmuffs, when involved in loud activities (at work or when involved in noisy recreational activities). When properly selected and used, HPDs can be powerful tools for preventing NIHL. HPDs are required by law to be labeled with a Noise Reduction Rating (NRR) that is based on performance obtained under ideal laboratory conditions. Usually, people obtain far less protection than the labeled rating because they do not wear the devices correctly, or neglect to wear them during the entire period of the noise exposure. It must be emphasized that the best hearing protector is not the one with the highest NRR, but the one that people will consistently wear whenever exposed to loud noise. There is no single protector that will fit everyone, be universally comfortable, and be appropriate in every environment. This chart shows some typical HPD options, as well as some special-purpose options, that you may consider.

Device	Description	Care
	Disposable plugs are placed inside the ear canal to block out noise. They are commonly made of expandable foam. One size fits most everyone. They roll up into a thin cylinder for insertion. Once they're inside your ear canal, they expand to form a good seal.	Keep the plugs as clean as possible by inserting them with clean hands. Always inspect them before reinsertion. If they are damaged or dirty, throw them away.
	Reusable plugs are preformed to fit the ear. They are usually made of a flexible rubber of silicon. They may be flanged or cone-shaped and are often joined by a cord so that they're not easily lost.	Reusable plugs can be worn safely for months, depending on the type. They should be replaced as soon as they become hard, torn, or deformed. Inspect and clean them often with warm soapy water. Rinse well. Store them in the case supplied by the manufacturer.
	Earmuffs resemble stereo headphones. The soft plastic cushions, filled with foam or liquid, should form a good seal against noise. If you wear glasses with wide temples, you may want to choose another type of protector. If you're exposed to very loud noise, you can wear earmuffs and plugs together.	Wipe the cushions clean with a damp rag when they become soiled. Check the cushions often, and replace them if they're stiff, worn, cut, or torn. Do not modify your muffs in any way.
	When communication is required with hearing protection, special-purpose earmuffs may help you understand speech from co-workers or those transmitting signals to you by radio. Advances in active noise reduction may be effective in reducing low frequency noises that can interfere with speech.	Care of special-purpose earmuffs also requires that the internal electronics are maintained.
	Musicians may prefer to use a type of earplug that is designed to match the ear's natural response, making sound quieter but not distorted. Filters in these Musicians Earplugs use a diaphragm that reduces noise levels relatively equally across all frequencies. These filters can be placed in pre-formed or custom-made earplugs.	While these earplugs may be washed with water and mild soap, the filter should never be exposed to water. Remove the filter before such washing. Molds should be replaced when discolored, cracked, or obviously hardened.

• If you're exposed to hazardous noise on the job, your employer may already be providing annual hearing tests to identify any change in hearing that might indicate under-protection from the noise. Occupations particularly at risk for hearing loss due to exposure to noise are as follows:
• Firefighters
• Police officers
• Factory workers
• Farmers
• Construction workers
• Military personnel
• Heavy industry workers
• Musicians
• Entertainment industry professionals


• If you are aware of some of the symptoms of NIHL (like ringing ears or muffled speech), seek a hearing test from a qualified source. Although noise exposures are hazardous, other medical causes for hearing loss should be ruled out by a qualified healthcare provider, using data from your hearing test and your history.



• Be alert to hazardous noise. Since prevention is so critical, make sure that your family (especially children), friends, and colleagues are aware of the hazards of noise. Although animal research with drug therapies and the physiology of the hearing system may eventually lead to the development of treatment strategies to reduce NIHL, the most fundamental recommendation is the best. One-third of hearing loss is preventable with proper hearing protection.

PROTECT THE HEARING THAT YOU HAVE NOW!

References

ANSI (1996). American National Standard: Determination of occupational noise exposure and estimation of noise-induced hearing impairment. New York: American National Standards Institute, Inc., ANSI S3.44-1996.

National Institutes of Health (1990). Noise and Hearing Loss. NIH Consensus Development Conference Consensus Statement 1990, Jan 22-24; 8 (1).

National Institute for Occupational Safety and Health (1998). Revised Criteria for a recommended standard - Occupational noise exposure, U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication 98-126.

National Institute on Deafness and Other Communication Disorders (1999). Noise-Induced Hearing Loss. NIH Pub. No. 97-4233.

Occupational Safety and Health Administration (1983). Occupational Noise Exposure Standard. 29 CFR Chapter XVII, Part 1910.95.