Olympic chiefs believe Beijing's pollution will not harm athletes this summer but admit performances might be affected and some events postponed.
Beijing's stylish Bird's Nest stadium is hardly visible through the smog
An International Olympic Committee (IOC) study into air quality in Beijing last August found that athletes were not "largely impaired".
But the IOC said there will be some risk to athletes in endurance events.
"I believe the conditions will be good, although not necessarily ideal," the IOC's Arne Ljungqvist said on Monday.
"Athletes may breathe a lot of air that may be polluted. We may not see world records in unfavourable conditions."
Ljungqvist, the chairman of the IOC's medical commission, said there were no health problems reported at any of the test events in Beijing last summer or at track and field's world junior championships in August 2006.
They can say what they like but the reality is different
Marathon world record holder
But Ljungqvist's remarks about possible problems for endurance competitors will not reassure those already concerned by Beijing's pollution.
Last year, separate World Health Organisation and United Nations Environment Programme studies raised the alarm about the poor quality of the Chinese capital's air, particularly during the summer.
And last week, marathon world record holder Haile Gebrselassie said he was unlikely to race in the Olympics because he was worried the pollution would exacerbate his asthma.
In fact, the Ethiopian confirmed that he would opt for the 10,000m and not the marathon when it became clear on Monday that the IOC would not move the race out of the city centre.
Tennis star Justine Henin has similar concerns to the two-time Olympic 10,000m champion and had to pull out of the China Open in Beijing last September.
Ljungqvist dismissed these fears, saying: "People with asthma may suffer more than others. Gebrselassie's decision is a private one but I would not say his example should be the golden standard for others.
It may be that some events will not be conducted under optimal conditions and that we may not see records broken in Beijing
Professor Arne Ljungqvist
IOC medical chief
"Our experience and data do not support that this will become a problem for the vast majority of athletes in Beijing."
But the minority Ljungqvist is concerned about are those who must sustain "one hour continuous physical effort at high level". This includes road cyclists, mountain bikers, marathon runners, long-distance swimmers and triathletes.
That risk is deemed high enough for the IOC to begin working on "procedures which will allow a 'plan B' to be activated for such events if necessary".
Air quality and weather conditions will be monitored and events could be postponed. A change of venue was not mentioned as a possibility, much to Gebrselassie's disappointment.
"It may be that some events will not be conducted under optimal conditions - which is the reality of sports competitions - and that we may not see records broken in Beijing," added Ljungqvist.
"However, the Games are more about competing in the Olympic spirit than about breaking records.
"For a few sports where we do see a possible risk, we will monitor the situation daily during Games time, and take whatever decisions are needed at the time to ensure the athletes' health is protected.
"The IOC is confident that measures already put in place, plus those planned by Beijing organisers and city authorities, will continue to improve the city's air quality leading up to and during the Games."
Gebrselassie, however, will need a lot more convincing.
"They can say what they like but the reality is different," the 34-year-old told the Spanish news agency Efe.
"I'd love to go for (the marathon), but health is my first priority. The 10,000m presents a lot less problems. It's four times shorter, it's being run in the afternoon, it'll be less hot with less humidity."
China's rapid economic and industrial growth has seen a marked increase in pollution levels in recent years. An estimated 1,000 new cars are introduced to Beijing's streets every day.
Among the measures planned by officials to alleviate the problem this summer are factory shutdowns and temporary bans on people using their cars.