America’s Cities with the Cleanest Air

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For the fifth year in a row, air pollution in the United States has declined, according to a new study by the American Lung Association. Despite the fact that the manufacturing industry and the population continue to grow, aggregate emissions in the U.S. have dropped by roughly 59% over the past 20 years. Of course, some regions continue to suffer from pollution levels that are considered too high. However, other metropolitan area have fairly clean air. 24/7 Wall St. examined the 10 cleanest cities to find the reasons for their best air quality.

Read: America’s Cities With the Cleanest Air

The 2012 State of the Air Report used data related to air pollution collected at Environmental Protection Agency monitoring stations in major metropolitan statistical areas all over the country. The data were collected in 2007, 2008, and 2009. Some cities, most of which were in California, reported dozens of days each year of unsafe levels ozone and particle pollution, while others reported not a single day of unsafe pollution. 24/7 Wall St. tried to find out the reasons behind these differences.

According to ALA’s vice president of National Policy and Advocacy, Janice Nolen, the lack of pollution in these cities is less a product of a policy effort, but rather the result of small, sparse populations and a lack of polluting industries. When it comes to policy, “In general, cities that are meeting the standard for particulate matter, especially these that are well below, probably aren’t even addressing it. They aren’t looking at it, because they don’t have to,” Nolen told 24/7 Wall St. in a phone interview. “These communities are so clean, relative to the standard, that this isn’t even on their worklist.”

Many of the cities with the cleanest air are also relatively sparsely populated. Six of the 10 fall within the bottom 10% of the largest metropolitan areas for population density. Low population density can affect many pollution-related factors, including automobile pollution and the size of local industry. According Nolen, good airflow is also a major contributor to clean air and is something that a spread out population makes much more likely. This is because buildings are farther apart, there are fewer tall buildings, less auto congestion and, first and foremost, because population centers are located farther away from polluting sources.

Most of the cleanest metropolitan areas also have smaller manufacturing sectors — in relation to their nonfarm economies — than the rest of the country. A relatively small manufacturing sector leads to less industrial pollution. The same is true for the mining, logging and construction industries in these regions.

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Additionally, the cities are frequently hotspots for the leisure and hospitality sector. Not only does this sector in and of itself not normally produce an exceptional amount of air pollution, but it encourages the regions to limit their pollution. Dirty air is not particularly attractive to tourists.

24/7 Wall St. based its list on the cleanest cities for year-round particle pollution, as reported in the American Lung Association’s State of the Air 2012 report. We also looked at reports from previous years to identify significant shifts in cities’ rankings. We used data from the Bureau of Labor Statistics regarding the size of relevant industries in each metropolitan area. Finally, we included population density data from the Census Bureau.