Roughly four million vehicles are on its arterial roads, seven times the total 15 years ago. As a result, Beijing now has some of the worst air pollution on earth. But even as the country has earned its dirty reputation, Beijing is trying to clean up its air in conjunction with China’s policies of green energy and pollution abatement. Through September, the government counted 221 days when its zero-500 pollution index was below 101, and the city says it had just two days with dangerously high air pollution this year, the lowest in a decade. Outside experts caution that the city’s measurements are potentially misleading as they do not measure especially dangerous microscopic particulates. The 2008 average concentration of particulates in city air was six times the standard recommended by the World Health Organisation. Authorities are beginning to move against air pollution problems with growing tenacity. Since 1999 the city’s new-car emissions standards have matched those of the EU. Twenty percent of its buses use clean-burning compressed or liquefied natural gas and it bans heavy trucks from entering Beijing during the day.