9. Stockton, Calif.

Forbes.com: America's Dirtiest Cities

Stockton summer heat exacerbates ozone levels that rank 23rd in the nation. Population is 650,000. The city has little means to fund environmental initiatives. It has sought to avert bankruptcy by laying off city employees, including a quarter of its police force.

Sperling Air Quality Index: 15
Sperling Water Quality Index: 35

(David Paul Morris/Bloomberg via Getty Images)

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Introduction These cities suffer from high levels of ozone, sulfur dioxide, air-borne particulates and tap water so polluted you can't (or shouldn't) drink it. This is the harsh, lung-searing, rash-inducing reality for millions of people in some of America's biggest cities. Is your town on the list? (Pictured is the Gowanus Canal Superfund site in Brooklyn, New York.)

Much of the data for our list of the 20 Dirtiest Cities In America is derived from rankings done by Bert Sperling on his Bestplaces.net website. Sperling is known for his books like “Best Places To Raise Your Family” and “Cities Ranked And Rated.” His website features tools that let you slice and dice cities by a whole host of categories, like cost of living and income, as well as by water and air quality.

Sperling’s air quality index is based on annual data from the EPA, which factors in ozone and pollutants like carbon monoxide, sulfur dioxide, lead and volatile organic chemicals.

(AP Photo/Mark Lennihan)

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10. Milwaukee, W... Decades of heavy industrial development along the Milwaukee River has contributed to significant amounts of PCBs and heavy metals polluting groundwater and draining towards Lake Michigan. The 1.5 million residents of the Milwaukee-Waukesha metro endure air that's ranked 20th for short-term particle pollution.

Sperling Air Quality Index: 26
Sperling Water Quality Index: 26

(AP Photo/Roger Schneider)

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9. Stockton, Cal... Stockton summer heat exacerbates ozone levels that rank 23rd in the nation. Population is 650,000. The city has little means to fund environmental initiatives. It has sought to avert bankruptcy by laying off city employees, including a quarter of its police force.

Sperling Air Quality Index: 15
Sperling Water Quality Index: 35

(David Paul Morris/Bloomberg via Getty Images)

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8. San Jose, Cal... You don't equate Silicon Valley with pollution, but the San Jose-Sunnyvale-Santa Clara metro area is home to more than a dozen Superfund sites where chipmakers like Fairchild and Intel leaked toxic solvents into the earth. Ozone pollution is also a problem.

Sperling Air Quality Index: 13
Sperling Water Quality Index: 30

(Terry Wilson/istockphoto)

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7. New Haven, Co... The 850,000 residents of the New Haven-Milford metro area may enjoy having a topflight school in Yale University, but due to their location at the intersection of I-95 and I-91, their lungs pay the price.

Sperling Air Quality Index: 6
Sperling Water Quality Index: 44

(AP Photo/Steven Senne)

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6. Riverside, Ca... The 4.2 million residents of the Riverside-San Bernardino metro area suffer from high levels of ozone. In 2009 the EPA placed on the Superfund list a site the for decades manufactured explosives, rocket motors and fireworks, and which leaked perchlorate and trichloroethylene, destroying drinking water supplies.

Sperling Air Quality Index: 1
Sperling Water Quality Index: 49

(David McNew/Getty Images)

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5. Modesto, Cali... Modesto is another polluted city in California's Central Valley. Its 500,000 people have a 15.5% unemployment rate, rank 5th in short-term particle pollution and 11th in ozone.

Sperling Air Quality Index: 6
Sperling Water Quality Index: 34

(Marty Bicek/ZUMA Press/Newscom)

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4. Bridgeport, C... Despite being in one of the nation's richest states, much of Bridgeport remains blighted. For decades the Raymark Industries site manufactured car parts and asbestos and filled in wetlands by dumping toxic waste on them. The EPA has been removing lead, asbestos, arsenic and dioxins for 20 years.

Sperling Air Quality Index: 8
Sperling Water Quality Index: 32

(Spencer Platt/Getty Images)

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3. Philadelphia,... The Philadelphia - Camden - Wilmington metro area sits on the Delaware River, which has been lined with refineries and chemical plants for decades. The region has 18.5 million pounds a year of toxic releases, according to the EPA, with 7.2 million pounds discharged into the water. Superfund sites include the Franklin Slag Pile and the Martin Aaron site.

Sperling Air Quality Index: 22
Sperling Water Quality Index: 12

(Spencer Platt/Getty Images)

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2. Bakersfield, ... Bakersfield is the oil capital of California, home to some of the oldest and biggest fields in the nation. Emissions from oil and gas processing contribute to Central Valley air pollution that is the worst in the nation. According to the Lung Association, the population of 800,000 is subject to the worst particle pollution in the country and third-worst ozone.

Sperling Air Quality Index: 1
Sperling Water Quality Index: 42

(Casey Christie/ZUMA Press/Newscom)

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1. Fresno, Calif... The Fresno-Madera metro area takes the prize for dirtiest city in America. The 500,000 people in this area suffer from being exposed to groundwater polluted by agriculture as well as having the 5th worst year-round particle pollution in the nation, according to the American Lung Association.

Sperling Air Quality Index: 1
Sperling Water Quality Index: 22

(AP Photo/Gary Kazanjian)

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More from Forbes... Where do New York City, Sacramento and Los Angeles fall on the list? Click here to find out!

(Spencer Platt/Getty Images)