On July 20, half of private cars in Beijing went off the road in a sweeping attempt to improve air quality in advance of the Olympic games. Commercial and governmental vehicles, taxis and buses are all running as usual, but traffic is moving noticeably faster. I’ve been watching the plates, and drivers really are obeying the regulations. Furthermore, the Chinese have suspended tourism and visas for anyone not attending the Olympics, which is improving traffic even more. But Beijing’s air pollution doesn’t appear to have changed. This should come as no surprise. Beijing’s air pollution varies a great deal from day to day. The fact that it seems worse is probably due to unfortunate weather conditions-high temperature and humidity, and low winds that would otherwise dissipate the smog.

Contrary to popular perception both inside and outside China, Beijing’s air pollution problem is not primarily due to increases in personal vehicle use. There have been dozens of press reports about the surging numbers of vehicles that Beijingers are buying. These reports are true, but also misleading. In contrast to the experience in the U.S., Beijing’s boom in vehicle ownership has not yet invaded its surrounding areas. It is mainly China’s wealthiest cities that are participating in the car boom; smaller cities and towns have seen more modest growth in vehicle ownership.

The real causes of Beijing’s air quality woes lie elsewhere. An article last year suggested the key component to Beijing’s ozone problem (the stuff that makes your eyes itch, causes shortness of breath and reduces visibility) is actually volatile organic compounds (VOCs) from small factories in neighboring Hebei province. A sniff test suggests that there are plenty of these factories operating in and around Beijing. Many are small (and possibly illegal), and they operate only at night to avoid the scrutiny of environmental inspectors. So the Beijing government has several problems: first, it must locate these hidden factories. Then they must shut them down—and convince neighboring provinces to do the same.

Beijing’s other major problem is particulates, which come from several sources. One is construction: Beijing is still rushing to complete non-Olympic buildings before the big day. Another is coal-fired power plants and factory boilers. China’s largest coal-mining province, Shanxi, is directly up wind of Beijing. Shanxi ships much coal to other provinces, but it also has mine-mouth power plants and coking plants that contribute to regional pollution.

A final source is trucks. As with the VOCs from small factories, emissions from heavy vehicles are worst during the night, since trucks are banned from the city during the day. Studies have shown that Beijing’s pollution levels are highest in the early morning. This would not be the case if most of the pollution came from passenger cars, which operate mainly during the day. But it is good evidence that the chief sources of pollution are the VOC-producing factories and trucks operating at night.