Bless the Yanks and their hourly air quality report from the Embassy in Chaoyang. I have their App on my iPhone.
from: http://www.wunderground.com/health/pm.asp
US EPA (http://www.epa.gov/pmdesignations/faq.htm):
Particulate matter, or PM, is the term for particles found in the air, including dust, dirt, soot, smoke, and liquid droplets. Particles can be suspended in the air for long periods of time. Some particles are large or dark enough to be seen as soot or smoke. Others are so small that individually they can only be detected with an electron microscope.
