3. Riverside/San Bernardino/Ontario, Calif.

Welcome to life in the smog belt. Riverside not only has to deal with its own pollution but also takes the leftover pollution from the L.A. basin as well as particulates that drift over from the port of Los Angeles — the busiest container port in the U.S. Last year, Riverside had 105 days of unhealthy ozone levels — pollution that can exacerbate asthma and other respiratory conditions.

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