Beijing's plan to clear its polluted air in time for next month's Olympic Games launched Sunday, as the government enacted a driving ban on one-half of the city's 3.3 million cars, the Associated Press reports. Under the ban, drivers may only use cars with odd-numbered license plates one day, and even-numbered the next. The city's Olympic committee expects air pollution to drop 63 percent, and while Beijing's clogged streets have eased a little, foreign experts say that smog could still blow in from coal-burning plants outside the city.

"For about 20 years, they have gotten rid of coal-burning in the city, but in the surrounding provinces, coal-burning has increased, so there are some things outside of the purview of the blue sky plan," said Jennifer Turner, director of the China Environment Forum at the Woodrow Wilson Center.

This isn't the Chinese capital's first attempt at an auto ban; in August 2007 the city tried out the idea as a four-day trial run. Cars are a critical component of China's struggle with pollution as car ownership has risen 300 percent in 6 years, making it the world's second-largest auto market, according to a report by the China Environment Forum. The report also identifies other trends and environmental health impacts of China's car use, including booming growth in highways, drive-throughs and gas stations; its foray into biofuel use and production; and the health risks associated with car emissions.

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