What ails Modesto is what ails Bakersfield (No. 1), Hanford (No. 3), Fresno (No. 4) and other small Central Valley cities (the Valley accounts for all the California cites on the list aside from Los Angeles): heavy doses of nitrogen oxide and hydrocarbons stemming from gasoline, chemical processing, paints and other industrial byproducts that harm the ozone and release particle pollution into the air.

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