Interview by Gordon Farquhar
BBC Radio 5live's Olympic correspondent
Athletes and spectators attending next year's Olympic Games in Beijing face major health risks, a leading air pollution expert has told BBC Sport.
The city is planning to cut emissions, but the World Health Organisation (WHO) still fears for people's welfare.
"Exposure to high pollution levels may trigger serious problems," said the WHO's Dr Michal Krzyzanowski.
"People who do have heart problems, they might wish to reconsider their willingness to travel."
Among Chinese cities, Beijing belongs to the most polluted... I'd be amazed if substantial progress is made in the next 12 months
Dr Michal Krzyzanowski,
World Health Organisation
Chinese cities are among the most polluted in the world - the World Bank says 16 of the planet's 20 most polluted cities are found in China.
"All of the cities are pretty highly polluted by European standards, but even by the standards of Asia, Chinese cities are pretty highly polluted," said Dr Krzyzanowski.
"Among Chinese cities, Beijing belongs to the most polluted."
International Olympic Committee chief Jacques Rogge has already said events could be postponed if conditions are unhealthy, while some countries say their competitors will arrive in Beijing as late as possible to avoid exposure to pollution.
Inhabitants of Beijing have worn masks to counteract pollution
Although it is long-term residents of the city who are more likely to be affected by the smog, Dr Krzyzanowski said even brief exposure could affect athletes and spectators.
"The main problem in Chinese cities is air pollution, small particles which are suspended in the air and penetrate deep into the lungs," he said.
"More importantly they penetrate other systems, like the cardio-vascular system and travel in the blood through the body. Years of living in such conditions leads to increased mortality.
"Athletes who visit Beijing will not be subject to this kind of risk.
"They are very healthy people probably without any sign of cardio-vascular disease, otherwise they wouldn't be at Olympics, so they do not belong to the susceptible group.
"On the other hand, they are exercising very vigorously, so their intake of air is very high."
But Dr Krzyzanowski's main concern is for spectators.
"There is a risk people with not perfect health may take trips to the Olympics," he said.
"For them, exposure to high pollution levels may be a trigger to serious problems if they already have, for instance, cardio-vascular disease.
"Those who come with asthma may suffer attacks.
"They usually know how to respond to it but I would be concerned for those who have some cardiac condition.
"This might be more serious as it requires much more specialised medical response."
The decision to carry out an experimental ban on cars in Beijing city centre for three days, starting today (Friday), may have some impact on air quality.
Beijing hopes an experimental ban on traffic leads to a cut in emissions
But that is just one of a number of measures the Beijing Organising Committee is implementing in a bid to cut emissions.
In total, they will spend £6bn making environmental improvements, such as relocating factories, improving water treatment centres and updating the transport infrastructure.
But Dr Krzyzanowski is sceptical about the impact such projects will have.
"I'd be amazed if substantial progress is made in the next 12 months," he said, pointing out that Beijing's problems are not just created locally.
"Particles have the ability of travelling thousands of kilometres in the air," he said.
"So it's possible the beneficial effect of cutting the traffic in the city will be compensated by the transport of pollution from other parts of China.
"But we have seen things which don't happen in other regions of the world happen in China, so maybe it is the case," he said.