BEIJING -- Beijing will start to suspend all earthwork construction projects on windy days from March 20, and the formal construction ban in the Olympics host city will begin on July 21, according to a cross-sector circular.

Four government departments including construction and environmental protection jointly issued the circular on Tuesday announcing that they will suspend construction in bad weather such as strong gales and sandstorms between March 20 and July 20 to prevent dust pollution, and a complete construction ban between July 21 and September 20.

The municipal meteorological observatory forecast earlier this month that the capital city is likely to see about 10 days of sandy weather this spring, close to the annual average but six more than last spring, citing the reason of warm and dry weather in sandy areas in the north China region.

"Particles and dust at construction sites, which cover about 100 million square meters in Beijing, are a major source of pollution," said Du Shaozhong, the Beijing Municipal Environment Protection Bureau deputy head, when he announced last month that major pollutants in Beijing were down for a ninth consecutive year in 2007.

Construction projects in Beijing include Olympic venues and commercial housing, among others. The Beijing municipal construction committee said that the construction of the National Stadium, the main venue for the Beijing Olympics known as the "Bird's Nest" will be completed in May.

The construction committee also banned advertisements posted on the fencing of construction sites, which should be no lower than 1.8 meters.

Beijing has invested 120 billion yuan (about 17 billion U.S. dollars) to improve its air quality since it won the bidding to host the 2008 Olympics in 2001. The number of "blue sky" days, or days with fairly good air quality, increased to 246 last year from 100 in 1998.