The U.S. Embassy monitors the air quality here in Beijing, posting updates every hour. These hourly updates are Tweeted, providing information for residents to use when assessing the benefits and/or risks of being outside.
The red bar in the middle of the screen reads: "Dangerous," and the description below the bar explains: "Health warnings of emergency conditions. The entire population is more likely to be affected." From my experience over the past seven months, this is not uncommon.
For some reason, the double negative "not uncommon" seems less ominous to me than the direct "common."

The picture to the right (blurry, my apologies) shows the Air Quality Index chart, which can be found at airnow.gov. Here is a direct link to the AQI for Beijing: Beijing Air Quality
Visibility is less than a block on this Monday morning. The temperature is expect to reach the mid-90sF or about 35C.
This is bad news for all Beijing residents, and especially for children, the elderly, and those with health conditions, as well as the hundreds of thousands of manual laborers toiling outside all day in these conditions. Privileged folks like me, with the luxury to stay inside air conditioned apartments, cars, and offices (all with central air cleaners and often individual room air cleaners as well), will stay inside as much as possible. We'll wait--for days perhaps, or maybe only for hours--for the winds to shift, blowing the city clean, or the rain to fall, pushing the polluting particles into the ground water and earth.