When film noir detectives smelled something fishy, lives were saved and criminals were thrown behind bars, but sniffing the way to safety is not unique to fictional heroes. It is a universal self-preservation mechanism that doctors say many of us take for granted -- especially since about half of those above the age of 65 are losing the ability to do it.

If you forget to turn off the stove, the smell of burning food might prompt you to run to the kitchen and turn down the dial. If the milk in the fridge is a few days past expiration, most people pop the top and take a whiff to make sure it still smells edible.

"Loss of smell is dangerous and it's not appreciated by some," said Dr. Berrylin Ferguson, director of the UPMC Division of Sino-Nasal Disorder and an otolaryngologist, or ear, nose and throat doctor.

It can happen for a number of reasons -- among them: smoking, heart disease medications that cause nasal polyps, chemotherapy, zinc deficiencies and sometimes simply the process of getting older.

"If you're constantly in an environment where someone is smoking -- where there are environmental hazards in the air or if you're overusing nasal sprays -- it can affect how your body picks up and distinguishes odors," said Judy Dodd, an assistant professor of sports medicine and nutrition at the University of Pittsburgh.

But many unknowns remain, such as whether or not the loss of smell causes a loss of taste or vice versa.

"Most people misinterpret their loss of smell as a loss of taste," Dr. Ferguson said. "You can't tell chicken soup from pea soup if you can't smell."

The idea seems to fit -- holding your nose to block the nasty taste of medicine works, right? But most doctors say it is not so cut-and-dry; indeed, one of Dr. Ferguson's own patients is in the opposite situation.

Chas Metzger, 61, lost his senses of taste and smell after a severe sinus infection. Years later, his sense of taste has returned, but he cannot smell a thing.

"I find that that's probably been good in most situations. I think I've avoided more bad smells than I've missed good smells," the North Huntingdon resident said.

But he is still believes it is an important thing to cure.

"I've thought about such a thing as the presence of natural gas, and of course, I wouldn't know."

Even though women tend to have a more acute sense of smell than men, the two senses usually begin to diminish earlier in women (around age 40) than in men (around age 50).

By age 80, nearly three-quarters of both men and women discover they cannot taste or smell quite as well, according to the National Institute on Deafness and other Communication Disorders.

Much of the time, decreases in the two senses are intertwined.

As we get older, our taste buds become smaller and scarcer. Salty and sweet tastes are usually the first ones to dull, said Leslie Bonci, director of sports nutrition at UPMC.

And although olfactory neurons are some of the only regenerative cells in the body, they, too, begin to decrease with old age. And treatments are limited.

"Unfortunately, there is nothing that has come out in the past 15 or 20 years that allows you to bring it back," Dr. Ferguson said. "For those with nasal polyps, nasal steroid sprays and mouth steroids may bring it back."

But steroids such as prednisone can have harsh side effects, so its use is limited, she said.

There also are some other shaky options.

"People who have a true zinc deficiency tend to not smell foods appropriately. In those individuals, restoring zinc can help," Dr. Bonci said. "But should everyone run out and take zinc supplements? No, that won't help."

So is there any hope? Dr. Bonci said preparing food in a way that enhances flavors, such as adding spices, can help.

"We focus on changing the way that things are prepared to make them tasty and more appealing," she said.

The goal is to rouse the senses to make food enjoyable, otherwise some might experience a decrease in appetite.

"How many times have you walked down the street, smelled something, and absolutely wanted it? Smell stimulates appetite," Ms. Dodd said. "Think about living without that stimulation."