BEIJING — Beijing's dirty air showed dramatic improvement today, with a city environmental official saying curbs on cars and factories are having the desired effect in cleaning up the air for next week's Olympics.

The air pollution index today was 44, less than half of what it was a day earlier, and the lowest since July 20 when authorities began implementing a series of drastic measures to improve air quality ahead of the games.

The curbs included pulling half the city's 3.3 million vehicles off the roads, halting most construction and closing some factories in the capital and surrounding provinces.

"That's why we say the measures have been effective," Du Shaozhong, deputy director of Beijing's Environmental Protection Bureau, said in an interview with the Associated Press.

In the past 11 days, seven days have met the national standard for "good" air quality, while four did not, Du said. China considers any reading below 100 to signal "good" air quality, or a so-called "blue sky day."

The other days — from Thursday to Sunday — ranged from 110 to 118, levels classified as unhealthy for sensitive groups. On those days, high temperatures and a thick, grayish haze reduced Beijing's skyscrapers to ghostly outlines.

Du said a combination of heat, high humidity and little wind made it difficult to disperse airborne pollutants.

But a tropical storm that hit southeastern China earlier this week brought strong wind and some rain yesterday, lowering temperatures and helping clear up the skies.

The clearer skies highlighted how much weather conditions play a part in efforts to curb pollution. By this afternoon, much of the haze had returned.

Du said that if the air quality continues to be a problem in coming days, Beijing officials will consider contingency plans to expand the traffic and factory emission cutbacks already in place.

Additional measures could include taking more cars off the road and only allowing newer, cleaner vehicles.

Clean air for the games has been a sensitive point for Olympic organizers and the government.

The official Xinhua news agency in a commentary Wednesday downplayed fears about the pollution, and said talk by some athletes about bringing face masks to Beijing was the result of negative reporting by the western media.

The editorial also repeated official assertions that the recent string of hazy days were due to fog.

Some U.S. athletes are considering wearing masks. New Zealand's athletes have been issued face masks with team equipment. Team managers have advised athletes to wear masks around the Olympic village but not during competition.