Beijing ? Beijing’s 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 Friday in the journal Environmental Science and Technology. The pollution often exceeded what the World Health Organization considers safe.

China poured some $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.