Beijing is one of the most polluted cities in the world. Factories, litter, sewage, cigarette smoke, heavy traffic, and a widespread lack of environmental awareness seem to have created a not only unpleasant but also high dangerous and unhealthy pollution situation. (If this sounds bitter, it's because most of the white clothes I brought are now closer to cream, and my lungs are not so kindly requesting that I immediately return home.) If you want to know more, below is a link to and some quotes from a BBC article published last year.
Beijing pollution: Facts and figures
Air pollution in Beijing is a problem. Figures for particulate matter (PM10) - tiny airborne particles caused by the burning of fossil fuels like petrol in motor vehicles - are regularly several times higher than the WHO air quality guideline level of 50 micrograms/cubic metre.
In fact the WHO's usual target level...is even lower, at 20 micrograms/cubic metre.
Beijing's figures even exceed the WHO's interim target for developing countries of 150 micrograms/cubic metre. The interim or 'easy' target is intended to encourage developing countries to begin gradually cutting down emissions.