BEIJING — There was a rare color in the skies over Beijing today — blue.

One week before the start of the Summer Olympics, the usual dull, grayish hues gave way to swirling clouds and sunlit blue.

The host city's polluted air has been among the biggest worries for Olympic organizers, prompting drastic measures including pulling half the city's 3.3 million vehicles off the roads, halting most construction and closing scores of factories in the capital and surrounding provinces.

The Beijing Environmental Protection Bureau said the city's air pollution index fell to 28 today from 69 the previous day. It was the lowest reading since July 20 when pollution-fighting measures first came into effect.

A reading below 50 is considered good and between 51 to 100 is moderate. But even moderate levels are still above the World Health Organization's guidelines for healthy air.

It is not known if the blue skies are a result of the anti-pollution measures, wind and rain over the last several days, or a combination of both.

From the start, athletes participating in the Aug. 8-24 games have raised concerns about the impact of the city's pollution on their health and performances.

Some of the 10,500 Olympic athletes began arriving in large numbers this week, while others headed to train in neighboring South Korea, Japan and elsewhere to avoid Beijing's air for as long as possible.

The anti-pollution measures include closing some factories and construction sites in Beijing and surrounding areas, and banning odd/even license plates on alternate days.

But Chinese authorities have already said more factory closures and traffic restrictions could be imposed if the air quality worsens during the games.