International Olympic Committee president Jacques Rogge has praised Beijing's "extraordinary" efforts to cut pollution before the Games, saying there would be no danger to the health of athletes.
He says there will be no danger to the health of athletes, despite continuing concerns about pollution levels.
A reading by the BBC suggests Beijing's air quality is far below World Health Organisation standards.
It put levels of particulate matter (PM10) at 191 micrograms per cubic metre. This far exceeds the World Health Organisation target of 50 micrograms/cubic metre, and also exceeds its target for developing countries of 150 micrograms/cubic metre.
But Mr Rogge praised Chinese authorities for having done "everything that is feasible and humanly possible to address this situation", adding: "What they have done is extraordinary."
He says there was "absolutely no danger" to the health of athletes taking part in events that last less than an hour. But if the pollution was bad, events lasting more than that could be shifted or postponed.
Mr Rogge has urged reporters to distinguish between fog and pollution - a point, correspondents say, often made by Chinese authorities.
No political statements
Separately, Mr Rogge said athletes would be prevented from making any political statement or protest in official venues - in accordance with Rule 51 of the Olympic charter, which forbids athletes from making political, religious, commercial or racial propaganda.
But he said they were free to do this in protest areas provided by Chinese authorities, and that "common sense" would be used to judge violations.
He spoke after more than 40 Olympic athletes signed an open letter to Chinese President Hu Jintao urging China to respect freedom of opinion and religion, particularly in Tibet.
The letter urged China "to protect freedom of expression, freedom of religion and freedom of opinion in your country, including Tibet".
