Much of the Chinese capital shut down Tuesday after Beijing's city government issued its first red alert for pollution -- closing schools and construction sites and restricting the number of cars on the road.
Beijing's Municipal Bureau of Environmental Protection warned that severe pollution would cloak the Chinese capital for several days, starting Tuesday morning.
According to the U.S. Embassy in Beijing, the air quality index stood at 250 this morning, classed as "very unhealthy" and 10 times higher than World Health Organization recommended levels.
Visitors, some wearing masks to protect themselves from pollutants, share a light moment as they take a selfie at the Jingshan Park on a polluted day in Beijing on December 7, 2015, the day Beijing's city government issued its first red alert for pollution, the highest level of warning.
CNN reporters in Beijing said the air pollution didn't feel quite as severe as last week, when air quality, as measured by the U.S. Embassy, went above 500 or "beyond index" on Monday and Tuesday.
Some residents have questioned why the unprecedented red alert level wasn't issued then. But others are resigned to living with air pollution that's regularly 10 times worse than recommended levels.
According to state-run news agency Xinhua, a red smog alert is only issued if heavy pollution is expected to last longer than 72 hours.
China is the world's largest greenhouse gas emitter and aims to halve its emissions peak by 2030.
Most of the country's carbon emissions come from burning coal to heat homes and to fuel power plants, a practice that spikes during cold winter months.
Can China's most polluted city lead a green energy revolution?
CNN's Tim Hume, Bex Wright, Vivian Kam and Beijing intern Guanghong Hu contributed to this report