The smog issue has hung low and heavy over the conscience of Beijing ever since the Games were awarded in 2001 and will continue to cause many sleepless nights over the next two weeks.
Despite the extreme measures put in place - from banning cars to closing factories and seeding the clouds - the haze filled with unsafe pollutants could very likely mar the grand spectacle . . . unless Mother Nature steps in. That is to say, the best guarantee of clear air would be wind or rain.
The worst case for the authorities - and China's international image - is the prospect of a pack of athletes running the marathon or in the road cycling race wearing face masks to protect their lungs. And the sight of athletes doing so in the Bird's Nest, say, would cause an outcry.
Several teams have mooted the idea of masks, and some of the top-10 nations refuse to rule out their use. Only last week, Japanese athletes said they might wear masks made for construction workers.
Competitors from the US are packing such protection, as are members of the British team, who have had a team of scientists work on special models just for Beijing.
The Australians have ruled out masks, but say they have brought the biggest team of specialists and equipment to combat respiratory problems in their Olympic history.
City joggers complain of loss of breath when the smog is at its worst, which it often is, despite a long and expensive clean-up campaign.
Young and old residents, plus those with breathing problems such as asthma, are often warned to stay indoors and close windows.
While it is the endurance events that the IOC admits face a real threat, most experts are satisfied other events do not pose a risk.
However, IOC president Jacques Rogge said he would postpone events if IOC doctors and those from the IAAF believe the air to be a health threat.
Controversy remains about the monitoring of damaging particles in Beijing's air, and the IOC has repeatedly said it would rely on Beijing's system, which has been accused of massaging figures. This week, the IOC said it was evaluating the air quality based on World Health Organisation standards and expressed confidence air pollution will not pose a 'major' risk to athletes and visitors.
'Those standards are fairly tough to meet, but in many respects the Beijing area does so,' said Arne Ljungqvist, chairman of the IOC medical commission. 'I'm sure, I'm confident the air quality will not prove to pose major problems to the athletes and to the visitors.'
Finding out who will call the shots when it comes to decisions about air quality and possible effects on athletes' performance has become a 'one-to-watch' as much as the 110m hurdles final.