It is so bad that over the new year period hundreds of flights were grounded and highways were closed because there was not enough clean air to breathe.
Why is the pollution so bad?
Burning coal is one of the main causes. Heavy traffic also contributes to the pollution. Decades of robust economic growth and expansion of industries have seriously damaged the quality of China's skies, soil and water.
What is the government doing about it?
Three years ago China declared "war on pollution" aimed at reversing the damage. It has created emergency response systems that restrict traffic and shut down factories and construction sites during periods of heavy smog.
Local officials and enterprises that flout the rules are punished.
In December, Chinese inspectors identified 21 enterprises that had violated regulations by failing to close operations on time.
Over the new year, China sent 10 more inspection teams to cities across the region to mitigate the pollution and punished more than 500 enterprises and construction sites as well as 10,000 drivers.
How do people cope with the pollution?
In 2016, the total number of "blue sky days" in China reached 198, up 12 from the previous year. The air quality is generally so poor that people often wear masks when they go out. Beijing's hazardous air quality has also affected people's health.
Hundreds of flights were cancelled and highways closed across northern China over the New Year because the air quality was not breathable.
On Tuesday, three major ports in northern China suspended the loading of ships as a result of poor visibility, a maritime safety agency said.