20 Most Polluted Cities in America

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10. Fort Collins, CO
> High ozone days per year:
32.2 days
> Number of days with unhealthy particle pollution: 0.3 days
> People with asthma: 27,677
> Population: 324,122

Fort Collins residents are not as exposed to harmful particle pollution as most other highly polluted cities. Ozone levels, however, reach dangerously high levels during 32.2 days in a single year on average due to the hot summer months and the region’s wildfire season. In 2013 and 2014, some 2,000 wildfires burned over 200,000 acres of wildland. These large and heavily polluting fires are almost always caused by humans.

9. Las Vegas-Henderson, NV-AZ
> High ozone days per year:
34.5 days
> Number of days with unhealthy particle pollution: 2.7 days
> People with asthma: 183,612
> Population: 2,315,324

According to the ALA, much of the ozone pollution in Nevada, Arizona, and the Southwest in general is largely due to climate change and severe drought conditions. In Las Vegas, like in other urban areas, a great deal of the pollution is likely from vehicle emissions. An estimated 183,612 Las Vegas area residents, including 38,006 children, have asthma. Individuals with lung conditions such as asthma are more vulnerable to the effects of pollution.

8. Denver-Aurora, CO
> High ozone days per year:
35.7 days
> Number of days with unhealthy particle pollution: 2.2 days
> People with asthma: 287,005
> Population: 3,345,261

Like in most U.S. cities, ozone levels in the Denver-Aurora area have improved in recent years. Despite the improvement, there is still the equivalent of more than one month of unhealthy ozone days in a single year in the area. Three of the top ozone polluted cities are in Colorado. Largely due to climate change, the state has documented rising summer temperatures, which in addition to motor vehicle emissions and other pollutants, have contributed to the city’s high level of ozone pollution.

7. Modesto-Merced, CA
> High ozone days per year:
40.0 days
> Number of days with unhealthy particle pollution: 32.5 days
> People with asthma: 64,166
> Population: 798,350

With 40 high ozone days on average each year, the Modesto-Merced area is one of the smoggiest U.S. cities. And unlike most cities, the Modesto area is one of only four historically polluted cities where the number of high ozone days increased from the measurement documented in last year’s report.

6. Sacramento-Roseville, CA
> High ozone days per year:
41.3 days
> Number of days with unhealthy particle pollution: 5.8 days
> People with asthma: 202,417
> Population: 2,513,103

Like most cities on this list, Sacramento-Roseville has had some of the nation’s worst ozone pollution levels for many years. While the 41.3 days a year of unhealthy ozone days in the Sacramento area is once again nearly the worst, it is also one of only six polluted cities that reported this year the lowest level of ozone pollution in the history of the ALA’s measurement of the area. This is at least due in part to California’s efforts in recent years to control air pollution, according to Billings.