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.

10. Anchorage, Alaska
> Long-term pollution score: 6.3 (tied for 9th)
> Population: 380,821
> Population density: 14.5 people per square mile

Mining, logging and construction, which account for 4.6% of the nation’s nonfarm employment, account for less than 2% of nonfarm jobs in Anchorage. Similarly, while 9% of national nonfarm jobs are manufacturing related, in Anchorage they account for slightly more than 1%. In addition, Anchorage has the second-lowest population density in the country among the largest metropolitan areas, a fact that has helped the region preserve some of the cleanest air in the country.

9. Boise City-Nampa, Idaho
> Long-term pollution score: 6.3 (tied for 9th)
> Population: 616,561
> Population density: 52 people per square mile

Boise City, which serves as the headquarters of the American Lung Association, is tied for the ninth-lowest score for long-term particle pollution in the U.S., an improvement from the previous year’s report’s 12th spot. Boise’s population density, according to the report, is just 52 people per square mile, the 25th lowest among the largest metropolitan regions in the country.

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8. Flagstaff, Ariz.
> Long-term pollution score: 6.1
> Population: 134,421
> Population density: 7.2 people per square mile

The Flagstaff metropolitan area has a population density of just 7.2 people per square mile — easily the most sparsely populated metropolitan region in the country. According to the Bureau of Labor Statistics, just 6.5% of the region’s nonfarm jobs are in manufacturing, compared to a national average of 9%. The share of leisure and hospitality jobs, on the other hand, is nearly double that of the national level. On top of scoring eighth best in the country for year-round particle pollution, Flagstaff was also one of the cities to report the having no days with unhealthy levels of particle pollution over three years.

7. Colorado Springs, Colo.
> Long-term pollution score: 6.0
> Population: 645,613
> Population density: 240.6 people per square mile

This city’s leisure and hospitality sector, as a portion of the area’s total nonfarm payroll, is significantly larger than the nation’s. In addition, only 5% of the metropolitan area’s jobs are in the manufacturing sector — just over half of the country’s rate of 9%. Colorado Springs is now the seventh cleanest in the country when it comes to year-round particle pollution. This is quite a feat considering that, in last year’s report, it did not even make the 25 cleanest cities.

6. Redding, Calif.
> Long-term pollution score: 5.9
> Population: 177,233
> Population density: 46.9 people per square mile

Redding, California, scored sixth best in the country for year-round particle pollution, an improvement from ninth best last year. However, the city did not make the list for either 24-hour particle pollution or ozone pollution. The metropolitan area is the 20th-most sparsely populated in the country at just 46.9 people per square mile.

5. Albuquerque, N.M.
> Long-term pollution score: 5.6
> Population: 887,077
> Population density: 95.6 people per square mile

The share of nonfarm employment in the manufacturing sector is just 4.8% in Albuquerque — significantly less than the national share of 9.0%. Albuquerque has among the cleanest air in the country as measured in both short-term and year-round particle pollution. Just one year ago, the metropolitan area was ranked much lower for year-round pollution and was not even ranked among the 25 cleanest for short-term pollution.

4. Tucson, Ariz.
> Long-term pollution score: 5.4
> Population: 980,263
> Population density: 106.7 people per square mile

Tucson’s long-term pollution score is the fourth best in the country, after being third best in last year’s report. Tucson also was one of the cities reporting no unhealthy days of pollution between 2007 and 2009. In March, 6.5% of the region’s employment was in manufacturing, compared to a national level of roughly 9%. The city also had a larger leisure and hospitality sector than the U.S. average.

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3. Prescott, Ariz.
> Long-term pollution score: 5.0
> Population: 211,033
> Population density: 26.0 people per square mile

Prescott, Arizona, has one of the lowest rates of population density in the country. It also has a smaller manufacturing sector relative to the size of its overall economy than the U.S. has. From 2007 to 2010, the metropolitan region has not recorded a single day of unhealthy levels of air pollution.

2. Cheyenne, Wyo.
> Long-term pollution score: 4.2
> Population: 91,738
> Population density: 34.1 people per square mile

Cheyenne, Wyoming, is easily the smallest city to make the Cleanest Cities in America list. It is also one of the smallest and most spread out of the largest metropolitan areas in the country. Cheyenne is one of the 79 cities that did not have a single day of unhealthy particle pollution levels between 2007 and 2009. According to the BLS, manufacturing accounts for just 3.3% of the city’s economy, barely more than a third of the national average of about 9%.

1. Santa Fe-Española, N.M.
> Long-term pollution score: 4.1
> Population: 184,416
> Population density: 75.5 people per square mile

The Santa Fe-Española metropolitan area ranks best for year-round particle pollution, moving up from second-place in last year’s report. From 2007 to 2010, not a single day with an unhealthy air pollution level was recorded. The metropolitan area is one of only two cities to rank either best or among the best for year-round particle pollution, short-term particle pollution and ozone air pollution. Santa Fe’s biggest industries, such as tourism and government, are hardly “dirty.” Additionally, New Mexico has particularly strict emissions standards.

-Michael B. Sauter, Charles B. Stockdale

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