Literally, the city of Beijing's air was so bad last night that it went off the chart. The US embassy has a sensor measuring PM 2.5 in Beijing. That's particulate matter of 2.5 micron size or less. Here's the screen shot of how bad things are over there.
All the grey bars are off the scale and the red ones are merely "hazardous" and you should stay indoors. Grey means stay indoors in a room with filtered air, and try to breathe as little as possible.