A sand storm struck the Chinese capital on Monday, covering homes, streets and cars in brown dust and leaving the skies a murky yellow as it suffers its worst pollution in years ahead of the 2008 Summer Olympics.

Desertification of the country's west and Mongolian steppes has made the spring sand storms worse in recent years, reaching as far away as South Korea and Japan.

Cold, windy weather, a glut of construction sites and poor plant cover around Beijing have also contributed.

So far in 2006, Beijing has notched up 13 days of the worst measure of pollution, more than last year's total and the highest in six years, state media said.

Only 53 "blue sky" days in Beijing had been recorded in 2006 by last Wednesday. The city sets a target of about 230 such clean days a year to help what it says will be a "Green Olympics."

Hospitals have also dealt with a sharp increase in patients with respiratory diseases, and on Monday local newspapers warned residents to wear masks outdoors.

"It took me quite a while to wipe off the sand on my car," said office clerk Chong Zi, who drove to work. "But there's no point in washing it -- who knows when the next storm will hit?"