But healthy adults? Seldom do smog levels climb high enough to dictate a warning for them to seek cover from dirty air.

AT RISK: American Ambulance's Barry Neumann, left, and Michael Carey, help Dorothy Lutz, 74, onto a gurney outside her Fresno home. On this hot day in June, Lutz was taken to the hospital for difficulty breathing. Respiratory problems are common on bad-air days. (Mark Crosse / The Fresno Bee)

Are the very young, very old, physically weak the only ones at risk? Are the rest of us pollution proof – immune to ozone's corrosiveness and the sting of airborne particles?

Hardly, say researchers. We're all at the mercy of the air we breathe.

And healthy adults may be at greater risk from pollutants than they realize. Each day an adult inhales enough air to fill about 5,000 2-liter Pepsi bottles, and the amount of air breathed can increase as much as twentyfold with exercise or exertion.

A healthy adult is foolish to shrug off bad air warnings, says Jean Ospital, health effects officer at the South Coast Air Quality Management District in Los Angeles.

"If you're out in ozone for several hours, even at low levels, you can have respiratory effects," says Ospital. "And the longer you're out exercising in it, the effects can occur at lower and lower levels."

In the San Joaquin Valley, ozone levels tend to climb steadily, reaching peaks that remain elevated for hours.

During ozone season – between May and October in 2002 – the Valley exceeded the eight-hour health standard almost two-thirds of the time. Even before and after the season, ozone levels surpassed the standard nine times this year. By November, there were 124 health violations. (Violations of the federal eight-hour health standard begin at 85 parts per billion – the level at which lung damage is known to occur.)

The short-term effects of breathing ozone at unhealthy levels are well-documented: a tickle in the throat, uncomfortable burning in the lungs and a temporary shortness of breath. Even healthy adults rub stinging eyes and cough on smoggy days.

Ozone's damage to the lining of the lungs usually is temporary. Much like sunburned skin, lung cells shed and are replaced within a day or two with new, healthy cells.

Repeated exposures, however, can lead to scarring of the lungs. Air pollution researchers don't yet know if this scarring causes irreversible damage to the lungs of adults. Breathing the caustic gas can exacerbate illnesses such as emphysema or chronic bronchitis. And there is some speculation that breathing ozone can accelerate the loss of lung function that occurs naturally as part of the aging process. Lung function is the volume of air you can inhale and the speed at which you can exhale it.

So far, researchers have documented long-lasting lung damage in animals exposed to ozone. They speculate the same could be true for an adult man or woman who breathes smoggy air for a number of years.

There's little question children, the elderly and those with heart and lung problems can experience lingering health problems. That is why smog and soot alerts single them out for notification when pollution levels increase.

"I don't think the average healthy person needs to have great concern, day to day. But should people who may be compromised with heart or lung disease give consideration to what the air quality is? Yes," says Dr. Jonathan Samet, a pulmonary physician and epidemiologist from Johns Hopkins Bloomberg School of Public Health in Baltimore, Md.

Smoking a pack of cigarettes a day is far worse than breathing polluted air. A one-pack-a-day smoker is 22 times more likely to get cancer than a nonsmoker, according to the federal Centers for Disease Control and Prevention.

But another researcher is less convinced adults can breathe easy. He compares breathing polluted air over a period of years to an 18-year-old who starts smoking. There is no early impact, but the risk of heart and lung disease, as well as cancer, increases over time.

"This long-term exposure, day-after-day, month-after-month, year-after-year, is what matters the most," says C. Arden Pope III, a researcher at Brigham Young University in Utah who is studying the effects of particulate pollution. People can stop smoking and change other habits to improve their health, Pope says. Dirty air presents a unique wrinkle: You can't stop breathing. "Certainly, it's not as large a risk factor as poor diet, lack of exercise and smoking, but it's hard to find environmental risk factors that have as much impact over as large a part of our population," Pope says. "Exposure to air pollution is so ubiquitous."

Lucy Vasquez attributes her chronic respiratory problems that developed later in life to breathing polluted air in the San Joaquin Valley for 30 years.

A Fresno school community relations coordinator, Vasquez says she always has been athletic, spending hours outdoors on a tennis court or riding a bicycle.

She is convinced this repeated exposure to pollutants is what caused acute sinus infections that began about 10 years ago. Two years ago, she was diagnosed with asthma.

"It's got to be our air quality that is affecting me," she says.