The best measure of air pollution in China is what is called PM 2.5. It measures the number of particles less than 2.5 µm in size per unit volume. No need to get into specifics, just know that BIG NUMBER = BAD.
These small particles can penetrate into the alveoli of your lungs, and they basically never go out once they go in. A WHO report said the maximum safe level of PM 2.5 was about 20. They have since revised that report to say that no level of PM 2.5 exposure is safe — any is bad, and more is worse.
The previous PM 2.5 record in Shanghai was around 300. Absolutely terrible, and 10X what Los Angeles experiences on its smoggiest of days. Today, Shanghai’s PM 2.5 is over 570. It’s hard to get an exact reading, as some monitoring stations are saying that their machines cannot handle that level of exposure, having never anticipated it could ever get so bad.
This is far from the worst that China has ever experienced. A few weeks ago, Harbin in NE China had PM 2.5 over 1,000 — so bad that a large portion of the city had to declare a state of emergency and shut down, and one of the city bus drivers lost their way in the impenetrable smog.