A concentration of manufacturing industries, Beijing geographical location and an enormous amount of cars on the roads quite fairly make Beijing infamous as one of the most polluted cities in the world.
An Air Quality Index or AQI is used to measure the level of the air pollution. AQI measures the content of harmful particles in the air, such as PM10, PM2.5 and ozone. One of the most important measurement is PM2.5, the smallest particle that is the most harmful for people breathing the polluted air.
To learn more about AQI measureents and rates visit www.airnow.gov
Currently there are two AQI monitoring systems in Beijing. One is provided by the US Embassy in Beijing, the other one is by the Beijing municipal government.
http://beijing.usembassy-china.org.cn/070109air.html - monitoring system of US Embassy
www.bjmemc.com.cn/ Beijing Municipal Environmental Monitoring Center / BMEMC (in Chinese only)
Very conveniently you can get data from both systems on the app Air Quality Index http://air.fresh-ideas.cc which you can freely download to your smartphone.
The AQI measurements are used by all the International Schools in Beijing to determine the safety of outdoor activities. For an AQI reaching 250, PE lessons are carried out indoors, and recess outside will exclude strenuous activities (sports tournaments are also cancelled). When the AQI exceeds 301, recess will be indoors, and all outside activities will be cancelled.