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With less than a month to go until the start of the Beijing Olympics, the air in the Chinese capital remains gray and smoggy. While the International Olympic Committee has generally praised the city's preparation for the Games, it says that pollution remains an outstanding concern. And so as the countdown clock in Tiananmen Square winds down to zero, worries grow that the $17 billion spent on environmental cleanup won't keep the Games from being clouded by a choking haze.

But experts familiar with the city's plans for short-term pollution controls say Beijing's air should vastly improve in the final run-up to the Games. That will be good news for the country's reputation and a successful event. However, the solution is only a quick fix; once the controls are lifted, Beijing will most likely return to its smoggy norm.

At the core of the solution are government-decreed industrial and traffic crackdowns. Beijing has announced work stoppages on July 20 for construction sites, mines and chemical plants. A group of polluting factories in Beijing will be required to cut emissions by 30%. In the neighboring cities of Tianjin and Tangshan, more than 300 factories will be shuttered during the Olympics.

The biggest element of the short-term cleanup efforts will be a restriction on car traffic that begins July 20. On that day government-vehicle traffic will be ordered to cut back by 70%, and private vehicles will be permitted to drive only on alternating days. Although police cars, emergency vehicles and taxis will be exempt, the government estimates that up to 50% of Beijing's 3.3 million vehicles will be cleared from the streets. "By July 20, not only the traffic control will be at ... full scale, [but] all the other controlling measures should in place," says Zhu Tong, an environmental-science professor at Peking University. "With all these controlling measures working at the same time, I am confident that the air quality in Beijing should be significantly improved."

The optimistic projections of an air turnaround for the Olympics are based in part on the results of previous exercises in barring cars from the streets of the capital. During a Sino-African summit in 2006, vehicle restrictions were enacted that removed about 800,000 of Beijing's then 2.8 million cars. The restrictions, which were put in place for three days, were "remarkably successful" and led to a 40% drop in nitrogen oxides, according to a study conducted by researchers from Harvard University and the Royal Netherlands Meteorological Institute.

Another study by Chinese and German researchers found that the 2006 test also helped cut airborne particles. "There was a significant decrease," says Jost Heintzenberg, director of the Leibniz Institute for Tropospheric Research. Under this year's plan, with added restrictions to keep trucks and cars that don't meet inspection out of the city, Heintzenberg says, "I make an educated guess on a 50% visibility improvement that they can manage."

The benefits of the auto restrictions come from not just taking polluting cars off the streets. Beijing's notorious traffic jams mean that even newer, efficient cars pollute more than if they were traveling in free-flowing traffic, says Hao Jiming, a professor of environmental science at Tsinghua University. "Driver speeds will increase, especially in urban areas. The high speed makes emissions lower," Hao says. He estimates that simply removing cars will cut pollutants by 40%, and the higher speeds of the remaining vehicles will mean an additional 10% reduction in pollutants. Beijing has added subway lines and increased its number of subway cars to handle the crush on public transit during the car-restriction period. Offices and stores have also been ordered to stagger working hours, and buses to and from Olympic venues have been organized. But if the previous tests are any example, getting around town could still be tough.

One unknown will be to what extent weather helps. When the wind blows strong out of the north, Beijing's skies can clear quickly. But when there is no breeze, the city's northern and western hills can easily trap pollution. Last August a four-day car-restriction test resulted in only modest improvements, which the Beijing Municipal Environmental Protection Bureau blamed on the lack of a breeze. But unlike the 2006 and 2007 tests, which ran for just three and four days, respectively, this year's limitations will have been in place for nearly three weeks when the Olympics kick off. Hao says he expects to see clearer skies within one week. "After a few days, the air quality will be improved," he says. "How many days? That depends on the weather."

But while cleaner skies will be a welcome sight for the Olympic hosts, Beijing's residents won't have long to enjoy it. The restrictions will be kept in place for the Sept. 6-17 Paralympics, then end on Sept. 20. "The measures are only a short-term fix," says Wen Bo, China director for the NGO Pacific Environment. "I think the current Beijing government couldn't have the time and energy to think of long-term solutions for fixing air pollution."

• Austin Ramzy / Beijing