Athletes will be giving their all, but spectators attending the Beijing Olympics are being advised to limit exercise because of China’s pollution levels.

Asthma sufferers in particular should take extra precautions against the smog that regularly envelopes Beijing, a China-based medical expert told the Business Travel Show in London yesterday.

“Most activities will be outdoors and visitors are likely to exert themselves more than usual, simply by travelling between the sporting venues and other tourist sites. Such increased physical activity, combined with exposure to pollutants, may present a challenge,” said Dr Ahmed Fahmy, a medical director for International SOS, which is advising governments, companies and national Olympic committees about managing staff during the Games in the August heat.

“All asthma sufferers should undergo a pre-trip evaluation by their physician, who will test lung function, assess day and night symptoms and look at activity limitations, including exercise,” he said.

The warning comes as British Olympic chiefs devise ways to combat the effects of pollution on athlete performance, particularly in endurance events.

A quarter of Britain’s top athletes, including Paula Radcliffe, the marathon runner, suffer from asthma, according to the English Institute of Sport, which will screen competitors selected for Beijing. “We can’t stop the effects of pollution but, if they are identified as asthmatics and medicated, they will at least have normal lung function,” said Ken van Someren, the British team’s chief national physiologist.

The level of particles in the air around Beijing — which sits in a basin that, depending on the wind direction, fills with smoke from factories or dust from the Mongolian plains — is of concern to competitors, coaches, sponsors and spectators as the day of the opening ceremony looms.

In December meteorologists measured record pollution levels 12 times the maximum recommended by the World Health Organisation. By closing nearby factories and taking a third of Beijing’s three million cars off the road, Chinese officials hope to reduce this substantially.

They point to improvements that saw more “blue sky” days last month than at any time in the past eight years. Yesterday Beijing, where 1,000 vehicles a day are registered, had a low reading on the Air Pollution Index. By the time of the Games — over 16 days and 31 venues — the city aims to have 70 per cent blue sky days.

More than three million visitors are expected in the Chinese capital amid unprecedented international scrutiny. Among them will be 20,000 to 30,000 Britons, including 6,000 corporate guests and 280 athletes.

The British Olympic Association, whose pre-Games training base is in Macau, is devising ways to counter the potential impact of pollution, including diets rich in fish oils and facemasks for endurance runners.

It is reluctant to reveal much detail for fear of ceding an advantage to rivals. “If the pollution is bad, we have made preparations. But we hope they won’t be necessary,” a spokesman said.

Many top athletes are leaving it to the last minute to arrive in China, choosing to train elsewhere, although scientific evidence suggests that they could “rebound” from the initial effects of ozone within ten days of arriving.

Haile Gebrselassie, 34, considered the world’s best distance runner, said that he might skip the Marathon because of pollution fears.

“To run more than two hours in these kind of conditions is really very, very difficult,” he said during a recent visit to Beijing. “Just to walk it’s really hard, too.”

Last month Dame Kelly Holmes gave warning that athletes participating in endurance events were particularly at risk. The double Olympic gold medallist said: “The Chinese have made a fantastic effort and these are going to be a great Olympics, but there is a problem with pollution.

“It will be a far bigger problem than in Athens. The problem is you can’t train for pollution.”

Pollution levels (micrograms per cubic metre)

Seoul (1988)

Fine particles 41
Sulphur dioxide 44
Nitrogen dioxide 60

Barcelona (1992)

Fine particles 35
Sulphur dioxide 11
Nitrogen dioxide 43

Sydney (2000)

Fine particles 20
Sulphur dioxide 28
Nitrogen dioxide 21

Athens (2004)

Fine particles 43
Sulphur dioxide 34
Nitrogen dioxide 64

Beijing (2008)

Fine particles 89
Sulphur dioxide 90
Nitrogen dioxide 122

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