IN AN extraordinary move, Australian athletes have been given permission to withdraw from any event at the Beijing Games if they feel their health could be affected by the city's pollution problems.

This comes as Chinese authorities consider an unprecedented ban on all cars, except for a small number of necessary vehicles, from Beijing roads in the 48 hours before the August 8 opening ceremony.

Yesterday evening thick smog hung over the Chinese capital.

Australia's deputy chef de mission, Peter Montgomery, said athletes were unlikely to pull out, given they had spent years preparing for the Games.

But he reassured Australia's 433 team members, who started to arrive in Beijing overnight, that there would be no repercussions if they withdrew because of the pollution.

Ethiopian marathon champion Haile Gebreselassie has already pulled out from that event in Beijing to concentrate on shorter races as he suffers from asthma.

"It is extremely unlikely an athlete will not compete - most athletes train for 10 years for this moment," Montgomery said. "For us, the athletes' attitude to the event is paramount. If they don't want to compete, that is fine. They will be under absolutely no pressure to compete if they feel uneasy or don't want to compete."

The thick smog continues to be a key issue for Games organisers desperate to present blue skies for global television. Officials have flagged other stringent measures to be introduced if the pollution, which has thickened in the past week, does not dissipate.

A Beijing environment bureau official, Li Xin, told the official China Daily newspaper that an emergency plan - believed to involve banning 90 per cent of all vehicles from Beijing's roads - would be implemented if the air quality deteriorated further.

Last week, officials introduced an odds-and-evens number plate system that took a million cars off the road, and shut down heavy industry surrounding Beijing.

But relatively still conditions have meant the sky is increasingly grey and visibility poor.

AOC secretary-general Craig Phillips said Australia was better prepared to deal with pollution issues than their rivals.

Australian teams had previously visited test events in Beijing and made plans to deal with the problem. All athletes had been screened for possible asthma indicators, and team members who were asthmatics had been under constant care.

"We always knew air quality was going to be a challenge here, we have done a lot at home to prepare for that," Phillips said. "I have seen [the pollution] worse here … it is probably on par with what I expected. I think they've done a lot [but] there is still some issues with the [air] quality."