Air quality has been a frequent problem in the Chinese capitol, stretching to before the Olympic Games in 2008 when many countries were concerned that the poor air quality would have health impacts to athletes.
The source of the concern is particulate matter smaller than 2.5 micrometers (0.00000025 meters). The particles (PM2.5) are about 1/30 the width of a human hair. The World Health Organization considers normal safe levels of PM2.5 to be about 25 micrograms per cubic meter.
In the last few days, levels exceeding 400 micrograms per cubic meter have been recorded by the Chinese government and more than 800 micrograms per cubic meter at the U.S. Embassy in Beijing. The latter publishes their readings on their twitter account every hour.
The Air Quality Index gives people an idea of how concerned they should be when it comes to outdoor exertion. The higher the number, the less it is recommended you exert yourself outside.
The Beijing monitoring center believes the air quality will start improving later this week.