A sandstorm blown by gusting winds has enveloped a huge area of central and northern China in thick pollution that is a hazard to people going outside.
The Beijing Meteorological Observatory has advised people to minimise time spent outdoors and forecast that the murky conditions would linger into Friday morning.
Tens of millions of residents across eight provinces and autonomous regions and in municipal Beijing were told to use masks or scarves when outside to protect their eyes and lungs.
The low visibility prompted warnings for drivers to slow down and for airports to take precautions.
Beijing's air quality index topped 800 in some areas, on a scale ranking anything above 150 as unhealthy for the general population.
The sandstorms begin in areas such as Inner Mongolia where overgrazing and deforestation have increased desert areas.
Strong winds pick up loose dust and dirt, mixing them with industrial pollution.