BEIJING (AP) – There was a rare color in the skies over Beijing on Friday – blue.

The 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.

Much also depends on Mother Nature's cooperation. Without wind and rain to sweep away accumulated pollution, traffic and emission curbs will have a limited effect and may need to be ratcheted up.

The city's air pollution index fell to 28 on Friday from 69 the previous day, the lowest reading since July 20 when pollution-fighting measures first came into effect, according to the Beijing Environmental Protection Bureau. A reading below 50 is considered good and between 51 to 100 is moderate, though 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.

CONTINGENCY PLANS

Chinese authorities have outlined a series of contingency measures that would kick in if air quality does not improve once the games begin Aug. 8.

"If there are unfavorable weather conditions, and the air quality is forecast to not meet the standards in the following 48 hours, the command center would suggest the contingency plans be initiated," the measure stated.

The new emergency measures, drafted by China's Environmental Protection Ministry, include shutting down an additional 220 factories and further restricting vehicle usage across Beijing, along with the nearby city of Tianjin and surrounding Hebei province, according to a public notice posted Thursday on the government's Web site.

In Beijing, a total of 105 machinery, chemical and construction material factories would be shut down temporarily. In Tianjin, 80 miles to the southeast, 56 factories would be closed, while 61 factories would be shut in Hebei.

In addition, besides current restrictions banning odd and even license plates on alternate days, automobiles in Beijing whose last digit matches the last digit of the date would be banned. Tianjin and Hebei would begin implementing similar restrictions.

All construction across Beijing would also be halted.

AIR CONCERNS

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. Some Olympic delegations, including the U.S. Olympic Committee, are making protective masks available to their athletes.

Concerns were raised over air quality prior to the Summer Olympics in Los Angeles in 1984 and Athens, Greece, in 2004 and in both cases favorable wind conditions alleviated the problem.

On Thursday, the haze that normally cloaks the city lightened following heavy rains during the day. A typhoon that hit southeastern China earlier this week brought a cooling wind and some rain that helped sweep away pollutants and provide a respite from the heat and humidity.

Light winds were expected in the capital over the next three days.