Air Pollution Index (API)
Beijing is a bit notorious for its high air pollution levels which can sometimes get really bad. Sometimes you can see the entire city is covered in heavy smog. This is mainly due to burning of fossil fuels, heavy construction, and also in part to high car emission levels. The quality is measured by an Air Pollution Index (API) which ranges from anywhere from 0-500, the higher the number the more severe the pollution. "Good Air" is anywhere from 0-50, 150 is not healthy while 300 and above is hazardous. If the air quality is poor, like today, the kids in school are not allowed to go outside and play during recess or lunch. There is an announcement like this posted on doors around the school. Oh by the way, the API today was at 331. Very bad!