Beijing was shrouded in orange dust over the weekend after a sandstorm that has spread over 313,000 square miles blew into the Chinese capital.
The city's weather bureau gave air quality a rare hazardous ranking.
Air quality is "very bad for the health," China's national weather bureau warned. It said people should cover their mouths when outside and keep doors and windows closed.
China's expanding deserts now cover one-third of the country because of overgrazing, deforestation, urban sprawl and drought. The shifting sands have led to a sharp increase in sandstorms — the grit from which can travel as far as the western United States.
The Chinese Academy of Sciences has estimated that the number of sandstorms has jumped six-fold in the past 50 years to two dozen a year.
The latest sandstorm also hit the regions of Xinjiang and Inner Mongolia and the provinces of Shanxi, Shaanxi and Hebei, affecting about 250 million people over an area of 312,000 square miles, the state-run Xinhua News Agency reported.
China's dust storms were at their worst in the 1950s and '60s after campaigns to raise farm and factory output following the 1949 communist revolution stripped the soil of vegetation.
Because of those campaigns, archaeologists have found that sandstorms are reducing some packed-earth sections of the Great Wall in western China to "mounds of dirt" that may disappear in 20 years.