10 most polluted cities

These 10 cities had the highest level of year-round pollution and some of the highest incidences of heart and lung issues as well, according to the American Lung Association's annual report.

6 of 10

Population: 519,000
Cardiovascular cases: 110,000
Rank among ozone-polluted cities: 13

Situated at the northern end of the San Joaquin Valley, Modesto shares many of the same pollution issues other valley cities face. One of the biggest culprits: Exhaust from California's Route 99, a major truck route for agricultural produce haulers.

But Modesto also gets emissions and industrial drift from the nearby San Francisco-Oakland metro area.

The American Lung Association's Bonnie Holmes-Gen said reducing particle pollution in Modesto has proven tough on a local level as well. Many area residents burn wood for heat in the winter and the area is prone to droughts. In July, August and September, virtually no rain falls in most years. Wildfires can break out and keep skies smoky for weeks.

NEXT: 7. Visalia, Calif.

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

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