China pollution: Beijing smog masks tallest building
25 February 2014 Last updated at 12:21 GMT
As smog once again blankets Beijing, the BBC's Martin Patience looks at the impact on China's capital and its residents.
Smog levels regularly top levels considered hazardous by the World Health Organization (WHO).
Severe pollution has hit much of northern China for the past six days, with some particle readings well over 10 times the internationally accepted safety limit.
The WHO's China representative admitted the thick smog had resulted in a "crisis".