Beijing's claim to have guaranteed safe air to Olympic athletes and spectators has been questioned by some foreign experts, and a testing station set up by the U.S. embassy has highlighted dangers from pollutants China does not yet measure.

But in the year since the Games the capital has enjoyed extended periods in which the skies have appeared their cleanest for years and officials say they are still pushing for further improvements even without the scrutiny brought by the Games.

"We can see that there are many areas we still have to work hard on," Du Shaozhong, deputy head of the Beijing Environmental Protection Bureau, told journalists on a tour of one of the city's air quality monitoring stations.

"One important area is not ceasing to strengthen laws and regulations; we must use the law to combat pollution. This is something for the local as well as central government," he added.

Du declined to comment directly on reports that the government was mulling tighter air pollution standards, but a colleague confirmed that changes might be on the cards.

"At the national level they have plans to do the revision, but because it is the national level it is not our work," said one environmental bureau official who declined to be named, adding that she did not know of any timeline for the change.

China has been criticised for not including two key pollutants in its air quality index -- ozone and fine particulate matter (PM 2.5) that scientists say damages the lungs and may also be able to seep into the bloodstream.

But Beijing monitoring centre chief Yu Jianhua hinted they might appear in new standards.

"When it comes to ozone and PM 2.5, from what I understand the country is currently positively researching pollution standards, and I hope this is the future trend of our control and monitoring activities," he said, when asked about revisions.

Beijing claimed victory over its chronic pollution problems during the Olympics, despite some days with smoggy skies, after a much-publicized drive to shut dozens of factories and keep hundreds of thousands of cars off the roads.

But U.S. scientists who came to Beijing to measure air quality during the Games said particulate pollution was about 30 percent higher than reported by Chinese environmental experts.

It was far worse than other recent Olympics and two to four times that of Los Angeles on an average day, according to research done before, during and after the Games by a team from Oregon State University.

Levels of coarse particulate matter were higher than World Health Organisation safety guidelines over 80 percent of the time, while that of the most dangerous smaller particles was unacceptable 100 percent of the time, they said.

"Considering the massive efforts by China to reduce air pollution in and around Beijing during the Olympics, this was the largest scale atmospheric pollution experiment ever conducted," said Staci Simonich, an associate professor at OSU.

"Despite all that it was some evening rains and favourable shifts in the winds that provided the most relief from the pollution. This demonstrates how difficult it is to solve environmental problems on a short-term, local basis," she added.

However, Simonich said there is no evidence of any health problems from short-term exposure of athletes or spectators during the fortnight the Olympics lasted.