When people are exposed to harmful noise—sounds that are too loud or loud sounds that last a long time hair cells can be damaged, causing noise-induced hearing loss (NIHL). Once damaged, hair cells cannot grow back. 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.
Sound is measured in units called decibels. On the decibel scale, an increase of 10 means that a sound is 10 times more intense, or powerful. This sounds twice as loud. The humming of a refrigerator is 45 decibels, normal conversation is approximately 60 decibels, and the noise from heavy city traffic can reach 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.
Exposure to harmful sounds causes damage to the hair cells as well as the auditory, or hearing, nerve (see figure). 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.
Exposure to impulse and continuous noise may cause only a temporary hearing loss. 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. People can prevent NIHL from both impulse and continuous noise by regularly using hearing protectors such as earplugs or earmuffs.
Scientists believe that, depending on the type of noise, the pure force of vibrations from the noise can cause hearing loss. Recent studies also show that exposure to harmful noise levels triggers the formation of molecules inside the ear that damage hair cells and result in NIHL. These destructive molecules play an important role in hearing loss in children and adults who listen to loud noise for too long.
Information taken from www.nidcd.nih.gov