Riders wearing masks wait during heavy traffic on a smoggy day in Beijing on Jan. 4, 2017.
Smog red alerts are triggered when levels of PM2.5 above 300 are forecast to last for more than 72 hours. Red is the highest level in the country's four-tier warning system.
People wearing masks dance at a square during a polluted day in Fuyang, Anhui province, on Jan. 3.
Red alerts triggers a series of regulations, including the closure of schools, factories and offices, and a blanket ban on up to half the vehicles in affected cities.
People wear face masks as they cross a street in Beijing, on Jan. 4.
People wear face masks as they cross a street in Beijing, on Jan 4.
A cyclist wears a face mask in Beijing on Jan. 3.
Most of the country's pollution is blamed on coal-fired power plants, along with vehicle emissions, building construction and factory work resulting from three decades of headlong economic expansion.
A woman wears a mask as she sits on a public bus amid heavy smog in downtown Shanghai on Jan. 3.
Although it is the world's biggest carbon emitter, China plans to reduce hazardous emissions from coal-fired power plants by 50 percent over the next five years, and says its overall emissions will peak by about 2030 before starting to decline.
The sun sets amid thick smog in Zhengzhou, Henan province, on Jan. 2.
Despite still depending on coal for more than 60 percent of its electricity, China is in the process of shifting to nuclear, solar and wind power.
A person rides a scooter during severe haze in Fuyang in Anhui province, on Jan. 3.
While Beijing's smog gets the most attention, the scourge strikes much of northern China on a regular basis, sometimes forcing the closure of highways because of poor visibility.
A woman walks past Chinese paramilitary police wearing protective masks on Tiananmen Square in Beijing on Jan. 4.
A girl walks among heavy smog on a bridge in Hefei, Anhui province, on Jan. 3.