But the actual act of hearing - the complex process that begins with sound waves entering our ears and vibrating through a channel of sensory hairs, fluids and structures to the brain - is something that we do without thought, until something interferes.

The major causes of hearing loss in women under 45 are: loud noise; degenerative diseases such as lupus, a chronic inflammatory disease that affects the skin, joints, kidneys, nervous system and mucous membranes; and otosclerosis, a condition in which bone grows over a tiny structure in the middle ear, said Dr. Douglas Mattox, chairman of the otolaryngology (ear, nose and throat) department at the University of Maryland in Baltimore. Infections, trauma, tumors and Meniere's disease can also cause hearing loss.

Some scientists say that our ears were made for a primitive world in which the loudest sound might be a twig snapping underfoot, so just the normal cacophony of the 21st century may pose a threat to our hearing.

Add the sound of pounding music in an aerobics class or from a portable tape player and you have a good chance of having significant hearing loss, says Dr. Edwin Monsell of the department of otolaryngology at Henry Ford Hospital in Detroit.

For the most part, though, how long it takes for these noises to damage your hearing probably depends on how your ears are built and how much noise they have been exposed to throughout your life. As a general guide line, if you listen to a sound at 85 decibels for eight hours, 88 decibels for four hours or 91 decibels for one hour, you are at risk for hearing loss, Mattox says.

Decibels are units that measure loudness of sound. Normal conversation is 50 decibels and busy traffic is 70. Standing next to a running truck engine is 84 decibels and standing 1,000 feet away from a revving propeller aircraft is 88 decibels. A 140-decibel gunshot can result in immediate and permanent hearing loss, Monsell said.

If you ever walked off a plane or out of a concert and felt a ringing in your ears as though your hearing were muffled, you probably have lost some of your ability to hear, at least temporarily.

Monsell noted that most noise-induced hearing loss is related to factory work, loud motors and firearm discharges and said it is unlikely that women (or men) would be at risk from typical noise volumes in the office or at home.

Prevention

Fortunately, there are several simple ways to prevent noise-induced hearing loss.

Keep noise levels below a shout. If you have to shout when you're standing four or five feet away from someone, the surrounding noise is loud enough to damage your hearing, Mattox says.

Wear protection. Wear ear protectors whenever you know you'll be exposed to loud noise. The decibel level inside a subway car is 95. Standing beside a lawn mower, it's 96. A live rock concert can reach 130 decibels.

Earplugs that fit into the outer ear canal will reduce your exposure to noise six to 35 decibels. Look for the noise reduction rating, which provides a rough estimate of effectiveness, on the package of any earplugs you buy to figure out which pair is best for you. Some earplugs are premolded, while others are soft enough to be molded to the shape of your outer ear canal.

Earmuffs that fit firmly over the entire ear to form a seal will reduce the sound level 14 to 29 decibels. They are available from industrial supply houses.

By Dr. Tarvez Tucker