A Chinese man walking on a street wears a mask amid heavy smog in Beijing, China on December 25, 2015. Hazardous smog blanketing China's north-east has sparked more red alerts, with authorities advising residents in 10 cities to stay indoors.

More than 220 flights at Beijing's Capital International Airport were canceled Friday as pollution caused poor visibility, China's official Xinhua news agency reported.



Beijing's pollution alert level was upgraded to yellow on Friday from Thursday's blue, Xinhua reported Friday, noting that the four color-coded levels, in order from most severe to least, are red, orange, yellow and blue. Beijing has issued two red alerts in December -- its only two ever. A red alert dramatically limits car use, advises schools to close, shuts down outdoor construction sites and closes certain industrial plants.

The air quality index readings, or the PM2.5 level, which measures airborne particles up to 2.5 micrometers, touched 500 around midday Friday, Xinhua said. These particles are considered particularly dangerous, because they can enter the lungs and make their way into the bloodstream.

Anything above 100 is considered unsafe for sensitive groups, such as children, the elderly, or people with respiratory problems. Anything above 150 is "unhealthy," above 200 is "very unhealthy," and above 300 is "hazardous."