By Michael Bristow
BBC News, Beijing
Air pollution remains a very visible problem in Beijing
China has admitted it could introduce further emergency measures to cut air pollution during the Olympic Games.
One expert said that could mean taking 90% of Beijing's private cars off the streets at particularly bad times.
Figures show pollution levels have been relatively high over recent days - on some days thick smog is severely reducing visibility.
The BBC found one pollutant at the Olympic Village was three times higher than the recommended level on Monday.
China has already introduced a series of measures to curb air pollution, including taking half the city's cars off the roads.
Polluting factories surrounding Beijing have also been told to close.
If this new series of measures don't work, it's hard to think of what else this city can do - apart from pray for wind or rain
But an article in the state-run China Daily gave details of the further, stricter measures that could be introduced.
"More vehicles could go off the roads, and all construction sites and some more factories in Beijing and its neighbouring areas could be closed temporarily," a front-page article said.
This was confirmed by Professor Zhu Tong, of Peking University, who advises the Beijing government about air pollution.
He confirmed that 90% of the city's private cars could be taken off the roads under these stricter controls.
Any emergency measures would be introduced 48 hours in advance of very bad pollution, he said.
"There is a chance... that we cannot meet the air quality standards so stricter measures are needed," said Prof Zhu.
He maintained that the current measures had reduced pollution, but not by enough to guarantee good air quality every day.
China promised to clean up its air pollution for this summer's games, but figures show it still does not meet the toughest World Health Organization standards.
Small particles in the air - PM10 - are a particular worry. WHO guidelines say 50 micrograms per cubic metre is the standard to aim for, but Beijing rarely hits that target.
At the Olympic Village on Monday, the BBC found the PM10 level was at least 145, while at the BBC office it was 134.
Separately on Monday, Greenpeace published its assessment of China's efforts to clean up Beijing for the Olympic Games.
It says, that overall the attempt to get rid of pollution has created a "positive legacy" for the city and should be commended.
"Greenpeace found that Beijing achieved, and in some cases surpassed, original environmental goals," the report says.
But it said in other areas, including air quality, Beijing had not met targets, and has had to bring in short-term measures.
"Beijing could have adopted clean production measures more widely across the municipality to speed up the improvement of air quality," the report says.