In this May 3, 2017 photo, equipment operated by the San Joaquin Valley Air Pollution Control District checks air quality in Clovis, Calif., at one of several stations used by the district. The vast San Joaquin Valley in California, the country's most fertile farming region, is engulfed by some of the nation's dirtiest air. The poor air quality is the byproduct of booming farms, oil production near Bakersfield, two major highways, a web of rail lines and the valley's bowl-shaped geography. (AP Photo/Scott Smith)
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