"The WHO said these tiny particles -- known as PM10 because they're 10 millimeters or less -- can penetrate the lungs, enter the bloodstream and cause heart disease, lung cancer, asthma and acute lower respiratory infections. The WHO recommends PM10 not exceed 20 micrograms per cubic meter as an annual average, but the data show it has reached 121 micrograms in Beijing and exceeded 200 in several cities in India and Pakistan. The global average was 71."
By contrast, the most polluted areas in the U.S. were well below the global average but above the WHO maximum of 20 micrograms.
According to the Environmental Protection Agency, "the overall mean was 18 micrograms per cubic meter in 2008 for the 375 locations that the WHO studied. It ranged from 7 in a few areas to 15 in Des Moines and San Francisco, 16 in San Antonio, Seattle and Boston, 19 in Philadelphia, 20 in Detroit, 24 in Phoenix, 25 in Los Angeles, Pittsburgh and San Diego, 35 in Fresno and 38 in Bakersfield, Calif."