Olympic chief Jacques Rogge says air pollution could lead to some events at the 2008 Beijing Games being postponed.
China is celebrating the one-year countdown to Beijing 2008
Speaking a year to the day before the start of the 2008 Games, the president of the International Olympic Committee (IOC) said: "It is an option.
"Sports with short durations would not be a problem, but endurance sports like cycling are examples of competitions that might be postponed or delayed."
Billions have been spent in an attempt to reduce pollution without success.
A host of factories have been shut down, while many others have been moved out of town, but non-stop construction and booming car sales have made air quality even worse.
Beijing's filthy air and clogged traffic are known to have worried Beijing organizers and the IOC for some time, but this is Rogge's strongest statement on the subject.
BBC sports news correspondent Gordon Farquhar said: "All the talk has been about human rights but there has been a growing realisation that Beijing has a smog problem.
"People are beginning to come to terms with the fact it could be a major issue. Rogge said that some of the endurance events may be postponed and that would be disaster for the organisers."
BBC News' James Reynolds in Beijing added: "China is spending billions of pounds on new roads, new venues and on perfect celebratory shows but all that may come to nothing unless this city cleans up its air."
However, postponing events might not be an option because race schedules have already been decided, according to Wang Junyan, the director of cycling events for the games.
Balloons and pigeons are released over Tiananmen Square
She added: "Rogge's comment reminds us that we have to work harder to fix environmental problems."
Australian Olympic Committee president John Coates has already revealed that the country's athletes won't arrive in Beijing until just before the Games begin next August to avoid possible respiratory problems.
Otherwise, the IOC said the Chinese have laid "strong foundations" for the Games during Rogge's visit to mark one year until the event kicks off.
Sixty-three official events took place across China to mark the beginning of the countdown to 8 August 2008.
However, human rights groups are using the milestone to highlight concerns over China's record on arms sales to Sudan, its record in Tibet and child labour.
Even before Beijing was awarded the Games on 13 July 2001, fears were expressed about the compatibility of China's human rights record and the spirit of the Olympic movement.
I'm pleased to see that strong foundations are in place - the venues will be outstanding
Jacques Rogge (above)
Sudan and Tibet are two issues that have caused controversy, while last month a Chinese stationery firm accused of using child labour was stripped of its licence to produce official merchandise for the Games.
But Rogge is keen to stress the apolitical nature of the organisation and points to the Olympic education projects which he says have involved 400 million children in 50,000 schools in China.
"If anything, the Games can only be a catalyst for constructive dialogue in a complex and sensitive transformation that is taking place in compressed time," he said.
"We believe that in the past six years the Olympic Games have had a positive impact in a number of areas such as education, media regulations and on environmental standards.
"Whilst not denying that more remains to be done, we are encouraged by the progress in these areas.
Organisers say all the competition venues, with the exception of the main National Stadium, will be completed by the end of this year.
But while praising China's preparations so far, Rogge has emphasised that there is much still to be done, and that the test events that will take place in the next year will be crucial.
"The coming 12 months will be very busy," Rogge said, adding: "I am pleased to see that strong foundations are in place - the venues will be outstanding.
"I can already begin to imagine the exhilarating atmosphere the crowds will create as the athletes parade into the arena (the National Stadium) on 8 August."
The centrepiece of the festivities planned to mark the 8 August milestone was a huge party in Tiananmen Square, with the traditional fireworks and light show.
Chinese leaders and IOC officials joined a crowd of 10,000 in the televised event which aimed to invite more than 200 national Olympic committees to participate in next year's Games.