In this June 23, 2017 photo, cars and trucks roll along a section of Blackstone Ave. during rush hour in Fresno, Calif. California's vast San Joaquin Valley, the country's most fertile farming region, is engulfed by some of the nation's dirtiest air. On hot, windless days, a brown haze hangs overhead, sending wheezing people with tight chests to emergency rooms, and hundreds each year to an early grave. To combat the problem, California's biggest air district has spent more than $40 billion in the last quarter century to enforce hundreds of pollution rules and has driven down the number of unhealthy days by 80 percent, according to an Associated Press analysis. (AP Photo/Gary Kazanjian)
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