Seven of the 10 worst cities in America for air pollution are in California, per the American Lung Association. The two worst—Bakersfield and the Visalia-Porterville-Hanford metro area—are trending down. The two bear much of the attack that California’s drought is waging on the state’s air quality. Their location at the southern tip of the Central Valley, where the sun beats down unobstructed, creates an “inversion layer” of warm air, trapping industrial chemicals beneath it.