Some spectators attending the 2008 Olympic Games in Beijing face serious health problems due to air pollution, a leading health expert has warned.
Dr Michal Krzyzanowski of the World Health Organisation told the BBC that those with a history of cardiovascular problems should take particular care.
He also said the city's poor air quality could trigger asthma attacks.
The warning came as Beijing began a four-day test scheme to take 1.3m vehicles off the city's roads.
During the test period, cars with registration plates ending in odd and even numbers will each be banned from the roads for two days.
Any driver caught contravening the restrictions will be fined 100 yuan ($13, £6.50) by 6,500 police officers.
If the strategy works, it will be used next August to reduce air pollution and traffic during the Olympics.
Officials expect the ban to cut vehicle emissions by 40%, although correspondents said thick smog continued to hang over the city on Friday.
Beijing's residents, who are being told to take public transport rather than their cars during the test period, appear to be supporting the pilot project.
But despite the plans to cut emissions, Dr Krzyzanowski said the WHO still feared for the welfare of those planning to attend the games next year.
"All of the cities are pretty highly polluted by European standards, but even by the standards of Asia, Chinese cities are pretty highly polluted," he told BBC Sport.
"The main problem in Chinese cities is air pollution, small particles which are suspended in the air and penetrate deep into the lungs," he added.
"More importantly they penetrate other systems, like the cardio-vascular system and travel in the blood through the body."
Dr Krzyzanowski said people who were not in perfect health ought to think twice before travelling to the games, given the additional stress generated by the excitement of a sporting event, the heat and the poor quality air.
"For them, exposure to high pollution levels may be a trigger to serious problems if they already have, for instance, cardio-vascular disease," he said.
"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," he added.
"This might be more serious as it requires a much more specialised medical response."
Olympic officials have warned pollution could harm the Games
International Olympic Committee chief Jacques Rogge warned last week that events could be postponed if conditions were unhealthy, while some countries say their competitors will arrive in Beijing as late as possible to avoid exposure to pollution.
The air pollution expert also cast doubt on the effectiveness of the Beijing Organising Committee's experimental traffic ban, saying reducing pollution required long term planning rather than short term fixes.
"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, 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."
Beijing, home to about 16 million people, has just over 3 million registered vehicles, mostly comprising private cars, buses, taxis and government vehicles.