The 2011 study offers an intensive analysis of air pollution in more than 900 counties and more than 200 Metropolitan Statistical Areas (MSA) and Combined Statistical Areas (CSA), or cities and their surrounding suburbs.

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The American Lung Association's study ranks the air quality of 900 counties and more than 200 U.S. Census-defined Metropolitan Statistical Areas and Combined Statistical Areas--cities and their surrounding suburbs--where an air pollution monitor is present.

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Each area retains its designation as a metropolitan or micropolitan statistical area (or NECTA) within the larger combined statistical area (or combined NECTA) when the areas are combined.

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The study, which looked at 385 metropolitan statistical areas, said nearly two in five metropolitan areas had one insurance company with a combined market share of 50 percent or more among health maintenance organization (HMO), point-of-service (POS) and preferred provider organization (PPO) plans.

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