PARADISE — As an uncontrollable wildfire turned the California town of Paradise to ash, air pollution researcher Keith Bein knew he had to act fast: Little is known about toxic chemicals released when a whole town burns, and the wind would soon blow away evidence.

He drove about 100 miles to Paradise, in the Sierra Nevada foothills, from his laboratory at the University of California at Davis, only to be refused entrance under rules that allow first responders and journalists — but not public health researchers — to cross police lines.

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