In this June 23, 2017 photo, trains are lined up near a section of Highway 99 in Fresno, Calif. California’s vast San Joaquin Valley, the country’s most productive farming region, is engulfed by some of the nation’s dirtiest skies, forcing the state’s largest air district to spend more than $40 billion in the past quarter-century to enforce hundreds of stringent pollution rules. The investment has steadily driven down the number of days with unhealthy air - but on hot, windless days, a brown haze still hangs overhead, sending wheezing people with tight chests to emergency rooms and hundreds each year to an early grave. (AP Photo/Gary Kazanjian)
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