By Gordon Farquhar
BBC Radio 5live's Olympic correspondent
Time is running out for Beijing to resolve its pollution problem
The World Health Organisation's concerns about the Beijing smog will add sharply to the discomfort already being felt by the city's authorities over air pollution.
The International Olympic Committee measures its comments about host cities very carefully, so for its president, Jacques Rogge, to suggest some endurance events might have to be rescheduled because of the pollution will be viewed as serious criticism.
The mission of the Beijing Organising Committee (Bocog), whether stated or not, is to make next summer's Olympics the model for Games of the future.
But on air pollution, it is a hostage to fortune.
My experience of the city earlier this year was typical of colleagues who have been since.
Landing in a yellowish brown fug, Beijing did not look its best.
And on bad days, there is a perceptible sourness in the air - it feels unhealthy.
Yet two days later, with a change of wind speed and direction, the distant mountains moved into view, the sky cleared, and sunburn was the only environmental hazard.
The city fathers will argue long and hard about improvements they have made and are making.
Some of the worst polluting factories and power stations have been moved out of the city limits, while the dirtiest scooters and lorries have been cleared off the streets.
Yet Beijing's burgeoning economy is mitigating against the impact of these changes.
One thousand new cars a week are registered for use on the city's streets - and there will be nearly three million of them by the time the Games begin next August.
Air pollution is not a new issue for the IOC. Previous Games - Los Angeles in 1984, Atlanta in 1996 and Athens in 2004 - all presented environmental challenges
I would guess that this week's voluntary experiment - to see if keeping a third of those cars off the streets would temporarily improve air quality - is likely to become compulsory during the 17 days of competition.
Air pollution is not a new issue for the IOC. Previous Games - Los Angeles in 1984, Atlanta in 1996 and Athens in 2004 - all presented environmental challenges.
I remember stifling, breathless, unpleasant conditions in Atlanta and at times oppressive temperatures in Athens.
For instance, I will never forget the overwhelming heat on the day of the women's marathon in the Greek capital.
Sunstroke was a real concern for me, as a reporter following Paula Radcliffe's ill-fated bid for Olympic glory.
A medic later told me that her distress was probably linked to the fact her internal organs were on the point of being cooked - evidence that environmental factors can contribute significantly to performance.
Of course, part of the Olympic ethos of higher, faster, stronger demands that bodies are pushed to their limits.
In endurance events, a huge part of the attraction is to witness the battle of wills over exhaustion.
Let's just hope the drama does not envelope those who will be in Beijing to experience such feats at first hand.