Officials from the World Health Organisation (WHO) have warned that Beijing's heavy air pollution could cause serious health problems for spectators at the 2008 Olympic Games.
As a four-day test scheme to take 1.3 million vehicles off the city's roads began, the WHO said that those with asthma or cardiovascular problems would be particularly at risk.
Dr Michal Krzyzanowski told BBC Sport: "All of the cities are pretty highly polluted by European standards, but even by the standards of Asia, Chinese cities are pretty highly polluted.
"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."
His comments followed a warning last week from Jacques Rogge, head of the International Olympic Committee, who threatened event postponement if conditions were dangerous for the health of competitors and spectators.
