"I got out of bed to see my Mercedes entirely covered in dust," said a driver, who gave his surname as Yang. "There was just no way I could handle it myself, and I had to take it to a car wash."

The sandstorm engulfed Beijing despite intensive efforts to clean the air and prepare for the 2008 Olympics by planting broad belts of trees around the city.

The Beijing Environmental Protection Bureau warned the sandstorm, the worst to hit so far this year, could be harmful.

"Sandstorms can affect the human body, especially the respiratory system," said an official with the bureau, who declined to give her name. "We tell people who have problems breathing, especially the old, to stay at home."

Xie Jun, the owner of a car wash in the Chaoyang district, was one of the few Beijing residents who welcomed the sand.

"Normally I only get to wash a few dozen cars every day," he said. "So far this morning, I've washed 100."

The sandstorm originated on the border of China and Mongolia and started affecting Beijing on Sunday night, the environmental bureau said on its website.

The municipal government launched a pollution control scheme, dispatching sprinklers and ordering construction companies to halt earthworks.

It was the eighth sandstorm to sweep across Beijing since New Year, compared with a historical average of just six per year, according to state radio.

It conforms with other data suggesting that the air quality is getting worse in Beijing.

So far this year, the city has reported 56 "blue sky days," defined as days with excellent or fairly good air quality, 16 days fewer than the same period of 2005.

Northern China experiences sandstorms almost every spring but this year the situation has worsened because of high temperatures and a prolonged drought, according to Qiao Lin, an expert at the China Meteorological Administration.

China has 2.6 million square kilometers (1 million square miles) of desert, an area nearly 2.5 times the country's total farmland, according to government statistics.

More than a quarter of China's total area has been classified as desert, and its degradation is adversely affecting the lives of more than 400 million people, or 30 percent of its population, according to the statistics.

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