International Olympic Committee (IOC) officials are worried about poor air quality at the 2008 Games in Beijing.
Beijing hosts the 2008 Olympic Games
An IOC inspection team said the Chinese capital was making "considerable progress" after a visit on Thursday.
But it asked for more details on plans to improve air quality because of the smog that often hangs over the city.
"This is of the utmost importance to the athletes, who are the most important part of the Games," said IOC official Hein Verbruggen.
"(We have requested) contingency plans, if needed, for the test events this year and also for 2008... (but) it is not totally clear what the effects will be.
"We have asked that the effects of the contingency plans be calculated so that we know the air will allow the athletic performances we expect to happen here."
With only 477 days to go, there is still an awful lot of work to be done - that is normal
IOC official Hein Verbruggen
Verbruggen, the IOC's coordination commission, also said more work was needed to ease traffic flow on Beijing's notoriously congested streets.
"This kind of congestion, we will obviously have to avoid at Games time. But we have a lot of experience of this.
"Before the Athens Games it was a major worry ... but in the end it worked out," he said.
"We still need more details but we are confident it will be sorted."
The IOC team also told reporters that the Beijing Organising Committee for the Olympic Games (BOCOG) must make the most of the 26 test events planned for Beijing.
"Test events this summer are a crucial opportunity the organisers must use to test how operations, planned for so long, will actually work in practice."
Overall, the message from the eighth visit of the coordination commission was positive.
"We have had detailed presentations on many different issues and without exception considerable progress has been made," Verbruggen said.
"So we are able to say they are at a level of preparation, we would expect BOCOG to be. With only 477 days to go, there is still an awful lot of work to be done. That is normal."