Worst air pollution in China and the U.S.

PM2.5 is a technical term for microscopic particles about one-thirtieth the width of a human hair. PM2.5 particulate pollution comes mostly from burning fossil fuels. It is extremely damaging to human health because the tiny particles have the ability to enter lung and blood tissue and can lead to asthma, cardiovascular disease and cancer. Read related story.

Sources: Chinese Ministry of Environmental Protection, American Lung Association and WHO | Simon Denyer and Richard Johnson/The Washington Post February 2, 2014
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