"Athletes consume more air and this can end in cardiovascular problems. Particulates can get into the respiratory system and blood, creating an inflammatory response," said Wong Chitming of the University of Hong Kong's Department of Community Medicine.

"Blood viscosity goes up and this affects circulation and ... energy distribution. Muscles that need the energy may not get it. At worst, people can even land in hospital."

The Chinese capital is one of the most polluted cities in the world and its authorities last week removed 300,000 high-emission cars off its roads in a bid to clean the air and ease traffic.

And authorities in Tangshan and Tianjin, cities about 150 km (90 miles) and 115 km (70 miles) from Beijing, have ordered over 300 factories to shut down to improve air quality ahead of the Games, sources say.

In all, the country has spent 120 billion yuan (US$17.3 billion) in the past decade to clean up, but air quality remains a major headache.

Some athletes are worried, including twice champion Haile Gebrselassie, an asthma sufferer who has pulled out of the Olympic marathon, but he hopes to run in the 10,000-metre event.

ASTHMA ATTACKS

Recent studies say that the bad air could pose problems for competitors, especially those with asthma, said experts at the American Academy of Allergy, Asthma & Immunology (AAAAI).

The ozone, sulfur dioxide, nitric oxide and other pollutants in Beijing's hazy air are asthmagenic, meaning exposure can inflame the airways of sensitive people and even cause an asthma attack. Similar problems were witnessed in past Olympic cities of Atlanta, Athens and Seoul, they said.

"Not only will athletes have irritated eyes, but a good portion may have decreased potential to be competitive," said Timothy Craig, chair of the AAAAI Sports Medicine Committee.

"Exercise can enhance the adverse effects air pollutants have on health. Rigorous exercise combined with pollutants can sometimes stimulate an asthma attack."

The new research will be presented next month in the Journal of Allergy and Clinical Immunology (JACI).

Sandra Anderson from Australia's Royal Prince Alfred Hospital found recently that exercise-induced asthma results from injuries to the airway caused by breathing poorly conditioned air, particularly cold, dry air over long periods of time.

She and her colleagues concluded that cold-weather athletes and swimmers, who train in irritant environments, may be at risk of airway injury leading to increased airway sensitivity.

Exercise-induced asthma (EIA) affects an estimated 20 percent of top athletes and an estimated one in six of all Olympic athletes, according to the AAAAI. EIA frequently affects individuals who do not suffer from chronic asthma.

Typically, athletes with EIA experience difficulty breathing 5-10 minutes after exercise. Other symptoms include wheezing, chest tightness, coughing, chest pain and prolonged or unexpected shortness of breath.

Some asthma drugs can be used to control and treat EIA, but the experts warned athletes to seek official approval first because anti-doping regulations restrict the use of many asthma medications at the Olympics.

Spectators could also face problems from high pollution levels, especially those with a history of allergies or asthma.

Wong advised athletes to avoid crowded places and to keep to a simple diet with lots of vegetables and fruit.