As thousands of athletes prepare to breathe deeply in one of the world's most polluted cities, one group of Olympic competitors is contemplating a special challenge.

A startling number of elite athletes have some type of asthma – some studies suggest 20 percent or more, depending on the sport. In this Olympics, many of them will have to perform at their peak in potentially choking air, with airways already prone to spasm and narrow.

Athletes and doctors alike have been worrying about dirty air in Beijing and other Olympic sites since China was chosen for the 2008 Games.

European experts were concerned enough that they have launched a nine-nation study to assess breathing problems in their Olympic athletes. Well-known Ethiopian runner Haile Gebrselassie opted out of China's marathon, saying the pollution could hurt his health.

From ozone to vehicle and factory exhaust to choking desert dust, China's air quality is dismal, well outside international health guidelines.

In Beijing, from noon Sunday to noon Monday, the large-particle pollution that China usually tracks, called PM 10, measured 118 micrograms per cubic meter at the Olympic Sports Center Stadium.

That's even more throat-searing than the air Sacramentans breathed during this summer's wildfires. On four of the worst air days during the wildfires, preliminary PM 10 measures near Watt and El Camino avenues ranged from 83 to 92 micrograms per cubic meter.

Annually, the difference between Beijing and Sacramento air is even more stark. A 2004 World Bank summary put Beijing's annual average PM 10 at 89 micrograms per cubic meter. Sacramento is nearly four times lower, with an average annual PM 10 of 23 in 2004.

Pollution is one of the well-known triggers for asthma attacks, and in recent studies has been implicated as a cause for the underlying disease. Asthma narrows people's breathing passages, and attacks can range from mild to lethal.

For most people with asthma, symptoms get worse when they exercise. For some people with no other breathing problems, asthma-like symptoms appear only when they exercise.

Doctors have theories – but no firm answers – about why elite athletes – more than the rest of us – seem to have breathing problems.

The American Lung Association estimates that around 5 percent of the U.S. population has asthma. But rates can run as high as 50 percent among Olympic level swimmers.

Dr. Gina Lokna, a UC Davis sports medicine specialist who works extensively with athletes who have asthma, suspects that part of what's going on is that top-ranked athletes get "an extraordinary level of medical care."

They are poked, prodded, analyzed and tested. If lung function is a little off, somebody's going to notice.

If they want treatment, they'll probably need permission. The Olympics requires extensive testing before giving an athlete permission to take asthma medication, although it won't say how many approvals it has granted for 2008 until after the Games.

Evidence is building that something more is also involved.

People breathe 10 to 20 times more deeply when they're exercising intensely than when they're at rest, and they're more likely to breathe through their mouths, bypassing the nose's natural filtering system.

"They're going to get a heavier dose of anything that's out there in the air," said Barbara Weller, a pulmonary pathologist and toxicologist with the California Air Resources Board.

Endurance athletes, who train out-of-doors for long hours month after month, are especially vulnerable to asthma symptoms. That dovetails with studies showing children and adults living in polluted areas are likelier to get asthma.

More than half of Norway's Olympic cross-country skiers have some bronchial hyper-responsiveness – the airway narrowing that is characteristic of asthma, said Norwegian Dr. Kai-Hakon Carlsen, principal investigator in the nine-nation European Olympic asthma study.

Even more striking, the more years that someone skis competitively, the likelier he or she is to develop it, Carlsen said.

Two things seem to be responsible: that very deep, athletic breathing for long periods of time, coupled with an additional environmental irritant.

"For skiers, it's the cold air which gives increased risk," he said. "For swimmers, it's most probably the chlorine products in swimming pools. For marathon runners, it's pollution from cars – it's in big cities that most marathons take place. For cyclists it's the same."

In sports without that endurance and environment combination, the percentage of elite athletes who take asthma medication can be far lower, sometimes down to 2 or 3 percent, Carlsen said.

Whether you're a would-be Olympic medalist or a weekend jogger, much of the advice for how to perform at your peak with asthma is the same.

"Don't let your asthma control you," said Sacramento Monarchs forward DeMya Walker, who was diagnosed with asthma as a baby. "That was the biggest thing I learned as a kid. It's scary and it's not the easiest thing to live with, but you can still enjoy life and be active."

To stay active, you should work with your doctor to get your asthma under control, exploring which medications work best for you. Know what sets your asthma off, and avoid the triggers. And exercise wisely – indoors if you have to when the air is bad.

"People who exercise are healthier than people who don't, even in polluted areas," said Susan Gilbreath, an epidemiologist with the state Air Resources Board.

One piece of evidence: A recent study found longer life spans among people who exercise in polluted areas, compared with couch potatoes who live in the same locations.

Call The Bee's Carrie Peyton Dahlberg, (916) 321-1086.