As of 4:30 pm, Monday, April 2, 2007:
The “500” is for respirable particulates. The other numbers are concentrations of Sulfur Dioxide and Nitrogen Dioxide, respectively. Taken from the Shanghai Environmental Monitoring Center. I’m not sure if this is a computer error or not, but looking out the window at the entirely brown air seems to confirm it. Yuck.
Edit: To be perfectly clear, 500 is as high as the air pollution scale in China goes.