Noise-induced hearing loss (NIHL) is a product of modern life. It first surfaced during the industrial revolution, when workers were exposed to loud machinery for hours on end. Occupational exposure is still the most common cause of acoustic trauma, but recreational noise threatens to catch up.
UNDERSTANDING SOUND
The ear is divided into three parts. Sound waves first enter the outer ear, which is little more than a passive sound-collecting channel. Next, the waves strike the eardrum, the tympanic membrane, causing it to vibrate. The vibrations are transmitted through the Middle East along a short chain of three small bones, the hammer, anvil, and stirrup. Finally, in the inner ear these vibrations reach the cochlea, which is lined by tiny hair cells, the cilia. The vibrations caused by sound produce a shearing force on the cilia, which translate it into electrical impulses that are transmitted along the acoustic nerve to the brain.
Excessively loud sound produces excessive force, which can damage the hair cells. The cells can recover from mild damage, but severe damage will kill nerve cells, producing permanent hearing loss. Especially loud sounds will damage anyone's ears, but some people are more susceptible than others. In addition to genetic differences, environmental factors such as smoking and exposure to heavy metals and solvents can play a role. Still, in the last analysis, it is the sound itself that in time damages the fragile hearing apparatus.
HOW MUCH IS TOO MUCH
The intensity of sound is measured on the decibel (dB) scale; the higher the number of decibels, the louder the sound. A sound’s potential to damage the ear depends on the duration of exposure as well as the intensity of the sound. Sounds below 75 dB are safe, but eight hours at 85 dB can be harmful. Most often, noise-induced hearing loss begins with a subtle difficulty hearing high-frequency tones, and then slowly begins to encompass lower tones as it becomes more severe. Both ears are usually equally involved, but if one ear is closer to the offending sound, the impairment may be asymmetric.
THE LOSS
Once your hearing is lost, it can’t be restored; your only recourse is to wear a hearing aid, which amplifies whatever sound your acoustic nerve can still pick up. If your ears ring or buzz after being exposed to noise, it's loud enough to cause damage. And if noise exposure makes hearing painful, muffled, blurry, or distant for hours or days, your cilia are already in trouble. If you allow the noise exposure to continue, you’re likely to suffer permanent hearing loss.
KNOW YOUR RISK
Everyone who is exposed to loud noise is vulnerable. If background noise makes it necessary for you to shout to make yourself understood by someone just an arm’s length away, that noise is loud enough to be damaging. Occupational exposure is the most common cause of noise-induced hearing loss. Musicians pump operators, construction workers, factory workers, policemen, firefighters, military personnel’s, farmers, and truck drivers are especially at risk. Enthusiastic crowds at sporting events can also generate excessive sound, putting stadium workers and fans who attend many games at risk.