A bit more info, here's the EPA's PM2.5 scale:

http://www.airnow.gov/index.cfm?action=pubs.aqguidepart

Above 100 is unhealthy for sensitive groups, above 150 is unhealthy for everyone, above 200 is very unhealthy, and above 300 is dangerous.

Here's China's current PM2.5 as relayed by the US Embassy (To prevent official tampering):

http://aqicn.org/city/beijing/us-embassy/m/

The average over the last 5 days is 150, the peak over the past 5 days was 201.