Some woodworkers use a car key, but most guys rely on a little finger to dig out the excess earwax. Normally, wax protects the eardrum from dirt and dust, but it can build to excess when trying to provide a barrier to workshop sound. Woodworkers deal with the low-frequency hum of dust collectors and the high whine of routers on a daily basis, and very few of us take adequate precautions against noise. The result is NIHL (noise-induced hearing loss), which has literally millions of us asking people to repeat things, turn up the TV, and be quiet back there.

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About 10 percent of Americans spend time in an environment that includes hazardous noise levels. NIHL shows no favoritism; it attacks all age groups, from teenagers listening to loud music to aircraft workers, target shooters, snowmobilers and, of course, those of us who work (or play) at woodworking. Best estimates are that more than 10 million Americans have hearing loss that can be directly attributed to harmful noise levels.

Harmful sounds that are either too loud or too long, or both, can damage hair cells (small nerve endings) in the inner ear. These hair cells are directly responsible for changing sounds into electrical impulses that are sent to the brain via the auditory nerve and they do not repair themselves. The damage can be done in as little as one exposure (for example, miners who routinely use dynamite), or more commonly by extended exposures such as those created by using a router or a thickness planer.

The nature of noise

Most woodworkers are familiar with decibels, but few of us understand them. A decibel is a relative measurement, rather than a finite one. It expresses sounds in terms of other sounds (usually the base unit is the lowest noise that an average human can hear). We have an extraordinarily wide range of hearing, from somewhere around 0 (the base level) to something close to 120 decibels. However, along that scale we don't hear all sounds equally. So the scale is geometric in nature. As noise climbs the scale, it becomes more intense, more rapidly. Every 10 decibels, noise sounds twice as loud to our ears. The sound is doubled and then doubled again by the next 10-decibel level.

Interestingly, the first use of decibels in science was to measure the relative loss of audio levels in telephone lines. The word has its origin in the "Bel," a scientific unit that was actually named in honor of Alexander Graham Bell, the telephone's inventor.

A two-person conversation in an environment without lots of background audio runs at about 60 decibels. A semi going by on the street is generally somewhere between 80 and 90 decibels at its peak. A two-stroke motorcycle or a Harley that has had its pipes doctored by a length of rebar can emit up to 150 decibels, depending on the engine. Sounds above 85 decibels cause NIHL and these include many table saws and planers that can average about 100 decibels, and even the seemingly safe band saw, which can emit as many as 95. Small-shop dust collectors are right on the edge, usually running about 80 to 90 decibels. According to the Health and Safety Executive, a division of the British Department of Health, circular saws range from 97 to 102 decibels; molders clock in at 95 to 100; and a hand-held electric plane delivers 104.

The more decibels, the shorter the time before damage is done. Noise that is less than 75 decibels, even with repeated and very frequent exposure, is very unlikely to cause NIHL. Distance from the source of a noise is critical. Even a few feet can dramatically reduce damaging levels. If a dust collector can be housed in an adjacent unused room or in a special closet (with dust filters in the door to accommodate makeup air), this can often eliminate the problem by reducing the noise from 90 decibels to 65 or so.

The nature of damage

Hair cells are not the only susceptible parts of the ear, although they are the least tolerant of damaging noise levels. Injury can also occur to the auditory nerve. A loud, violent noise that creates a lot of pressure can sometimes cause instant hearing loss - usually partial but sometimes total. Partial loss due to such a concussive noise often results in tinnitus (ringing in the ears), and this may either go away after awhile or stay with us for life. Sometimes it comes and goes, and can take the form of a buzzing (like a bee), or roaring (like the sound made by waves on the seashore).

Woodworkers operate a number of machines that are right on the cusp (in the 80- to 90-decibel range), and may do so for long or frequent periods. This somewhat low-level noise will gradually damage hair cells. Continuous sound creates vibrations in the air that trigger the formation of new molecules in the inner ear, which in turn damage the delicate cells.

Symptoms of NIHL are a distortion or muffling of sounds, which is why television is such a good indicator. For the most part, the audio on TV shows stays within a relatively small range (except when some local stations turn up the volume during commercials). Your ears and brain tune in to this range. With hair cell damage, volume needs to be increased before the listener can hear distinct tonal variations and different sound shapes (as in broad and narrow vowels). Many people who suffer from NIHL are unaware of the damage until they sit with unaffected people in an audience and notice that nobody else is having difficulty understanding dialog.

Best lines of defense

Airport maintenance crews have long understood that noise doesn't just travel down the ear canal opening: it also penetrates the canal from the side by entering behind the ear. Members of ground crews are required to wear both plugs and earmuffs to counter this. In the woodshop, the same basic procedure works. Inexpensive, disposable plugs that conform to the shape of the outer ear canal can block much of the detrimental din. However, earmuffs complete the defense by reducing occasional peaks (for example, a minute of router noise) to safe levels. Better muffs such as Pelter's H10A have a noise reduction rating (NRR) of 30 decibels when used as directed on the package.

Theoretically, NIHL is 100 percent preventable. Unfortunately, many woodworkers already suffer some symptoms, but further damage can be avoided by protecting ears from sounds that measure 85 decibels or above. For woodworkers who suspect they have already damaged their ears' hair cells or auditory nerve, an exam by an audiologist can measure the extent of any hearing loss. An ear, nose and throat doctor (an otolaryngologist) can suggest treatment or technologies that can restore some or much of the lost ability.

John English is a freelance writer and runs the Black Hills School of Woodworking in Belle Fourche, S.D.

This article originally appeared in the May 2010 issue.