In this April 3, 2017 photo, dairy cows stand in a pen in Fresno County, Calif. California's vast San Joaquin Valley, the country's most fertile farming region, is engulfed by some of the nation's dirtiest air. Its bad air is the byproduct of booming farms, oil production, two major highways, a web of rail lines - and the valley’s bowl-shaped geography. (AP Photo/Scott Smith)
Featured Photo Galleries
