Fresno's air suffered a setback last year, with an increase in the particle pollutants found in smoke and haze like soot and sulfur dioxide, which can aggravate asthma and heart issues.
The Fresno metro area's population sharply increased 16% between 2000 and 2010, spurring the once compact city to sprawl outwards. Now, residents drive much further distances to get to work, leaving a trail of exhaust and emissions.
"Air pollution is proportional to vehicular traffic," said John Swanton, an air pollution specialist with the California Air Resources Board.
Like other cities located in California's valleys, layers of warm air trap those emissions like a lid on a cooking pot and pollution builds up to unhealthy levels.
Source: American Lung Association's State of the Air 2013 report 1. Ranking is based on year-round particle pollution. This is in the air we breathe and consists of such things as dust, metals, smoke, exhaust, pollen and acids, like nitrates and sulfates. 2. The data for cardiovascular disease includes all cases in a given city whether they were caused by air pollution or not. Air pollution puts those with cardiovascular disease at a higher risk of health issues. 3. Ozone is created when a chemical or fossil fuel, like coal or gasoline, is partially burned and the unburned hydrocarbons, when combined with ultraviolet light, form a gas that is very corrosive to lungs.
By Les Christie@CNNMoney - Last updated April 24 2013 12:33 PM ET