"Sensitive individuals should avoid going out of doors," the Beijing Environmental Protection Bureau said on its website, announcing that air quality was at "hazardous" level five, the worst possible grade.
The main pollutant was suspended particulate matter, which is usually attributed to coal burning and vehicle exhausts.
But the bureau said experts this time blamed poor air quality on annual springtime sandstorms in Mongolia and China's region of Inner Mongolia that had blown thousands of tonnes of dust over the Chinese capital.
"This is the fourth storm of the season and air pollution levels were each time about the same, but this time the effect lasted longer," said an official at the bureau who asked not to be named.
Pollution was thickest during the early morning hours but had begun to dissipate after about midday, he said.
Beijing's air quality is routinely rated among the worst in the world by international agencies such as the United Nations and the World Bank, with the