BEIJING - BEIJING notoriously dirty air was cleaner during last summer's Olympic games, but pollution levels were still much worse than at recent Olympics, despite a massive Chinese cleanup campaign, a new report said.

Athletes in Beijing faced pollution levels that were up to 3.5 times higher than those in recent Olympic cities like Athens, Atlanta and Sydney, said the study published on Friday in the journal Environmental Science and Technology. The pollution often exceeded what the World Health Organisation considers safe.

The joint American-Chinese study - the first major one published on air pollution during the Olympics - also found that the weather, and not the Chinese government's strict controls imposed in the run-up to the games, played the largest role in clearing the air.

The government's plans to control air pollution for the event gave international researchers a unique opportunity to observe a large-scale experiment. Scientists from Oregon State University and Peking University looked at Beijing's worst air pollutant - tiny dust particles known as particulate matter - over an eight-week period before, during and after the games.

China poured some US$20 billion into 'greening' the city after it won the bid in 2001, including doubling the number of subway lines, retrofitting factories with cleaner technology and building urban parks.

Government officials also imposed drastic cleanup measures just before the games in mid-July, including pulling half the city's 3.3 million vehicles off the roads, halting most construction and shutting down dozens of factories.

The study - funded by the National Science Foundation in the US and the National Science Foundation in China - found that the level of particulate pollution in Beijing was twice as bad as in Athens, Greece; three times as bad as in Atlanta, Georgia; and 3.5 times as bad as in Sydney, Australia.

Researchers found that particulate air pollution did drop by about one-third during the two-week Olympic period. But coarser particulate matter, PM 10, exceeded levels the WHO considers safe about 81 percent of the time, while the smaller particulate pollution PM 2.5, which can cause more serious health consequences, exceeded WHO guidelines 100 percent of the time.

'It was a giant experiment and a noble effort. But in the end, the extra added measures didn't help reduce PM concentration as much as had been expected,' said Staci Simonich, an associate professor of chemistry and toxicology at Oregon State University who worked on the study.

There has been no evidence so far of any health problems linked to the short-term exposure of athletes or spectators during the Olympics, researchers noted. -- AP