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The bad air that hangs over Bay Area cities in part comes from Asia and India, where the world's worst air hangs.
Bay Area residents may be tempted to breathe easy, with the nearest cities with bad air pollution in the Central Valley or an ocean away in Asia.
But that air also reaches lungs in the Bay Area, according to Slate, which analyzed the list of the world's worst cities for air quality.
The bad smog and air pollution in Beijing has made headlines in the Western world, where "most Americans and Europeans now enjoy cleaner air than they did for much of the last century," Slate reported. Meanwhile, "air pollution is worsening in Asia, claiming millions of lives every year."
Microparticles of particulate matter lodge in lungs and help bring about fatal illnesses like lung cancer, Slate reported. A measure of air pollution is the number of parts per million of microparticles smaller than 10 micrometers, Slate reported, a metric called PM10.
Ahwaz in Iran has a PM10 level of 372, the world's worst. The world average is 71, Slate reported.
Bakersfield, by contrast, has the United States's worst air with "only" a 38 PM10 count.
However, it's not all bright news and clear skies for Bay Area residents.
"Pollution travels east along jet streams from Asia to the North American West Coast," Slate reported. "Research indicates that nearly one-third of the soot in the San Francisco Bay Area blew over from Asia."
So forget the butterfly effect: when a coal plant fires up or a tuk-tuk sputters a world away, it causes coughs here indeed.