Here’s an update on this story: U.S. Embassy Creates New Air Quality Description for Beijing: Crazy Bad
According to the U.S. Embassy’s air quality monitor in Beijing, Beijing’s Air Quality Index as of 1:00AM on November 18, 2010 is 464.
From: http://iphone.bjair.info/
This is the worst AQI I’ve seen.  Ironically, I was having lunch with a friend yesterday and we were both pretty happy about how good the air in Beijing had been the last few weeks, clear enough to see quite a few stars at night. I’m in Hong Kong now, glad to be out of the toxic air and the cold weather.
January 6, 2011 at 05:54
What is acceptable air quality index?
January 14, 2011 at 16:06
Please take a look at this explanation of the AQI standard: http://www.airnow.gov/index.cfm?action=aqibasics.aqi
If you have trouble viewing *.gov sites, here’s a basic summary: 0-50 is good, 50-100 is moderate, 101-150 is unhealthy for sensitive groups, 151-200 is unhealthy, 201-300 is very unhealthy, and 301-500 is hazardous.