20 Most Polluted Cities in America

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5. Phoenix-Mesa-Scottsdale, AZ
> High ozone days per year:
43.2 days
> Number of days with unhealthy particle pollution: 5.3 days
> People with asthma: 447,405
> Population: 4,489,109

As is true for much of the Western United States, the Phoenix area is particularly vulnerable to large wildfires, which are enormous sources of air pollution. According to the National Interagency Fire Center, 1,543 fires in Arizona burned 205,199 acres of wildland in 2014.

4. Fresno-Madera, CA
> High ozone days per year:
103.8 days
> Number of days with unhealthy particle pollution: 44.7 days
> People with asthma: 89,916
> Population: 1,120,522

The Fresno-Madera metro area is one of only four cities suffering from more than 100 days of dangerously high ozone levels in a single year. Also, like only four other historically polluted cities reviewed by the ALA, this is up from the last period reviewed. In addition, particle pollution in the area spikes to unhealthy levels during 44.7 days out of the year, trailing only Bakersfield for the worst level of short-term PM pollution.

3. Visalia-Porterville-Hanford, CA
> High ozone days per year:
107.8 days
> Number of days with unhealthy particle pollution: 40.7 days
> People with asthma: 48,792
> Population: 608,467

While the Visalia area is nearly the most polluted city in the country by several measures of air pollution with 107.8 high ozone days a year on average, it is one of only 12 cities where the level of short-term spikes in particulate matter pollution improved since the last period reviewed by the ALA. Of the 608,467 area residents, 160,479 live in poverty. People living in low income communities are often exposed to greater levels of pollution.

2. Bakersfield, CA
> High ozone days per year:
113.3 days
> Number of days with unhealthy particle pollution: 48.7 days
> People with asthma: 70,085
> Population: 874,589

Bakersfield trails only LA in its level of ozone pollution. In terms of particle pollution, the area is by far the worst, with 48.7 days of dangerously high particle pollution. Also, both the number of days with unhealthful pollution levels and the city’s year-round average pollution level worsened from the last period reviewed by the ALA. Hot summers and extreme drought conditions in Bakersfield have contributed to the high levels of both ozone and particulate air pollution.

1. Los Angeles-Long Beach, CA
> High ozone days per year:
152.5 days
> Number of days with unhealthy particle pollution: 11.2 days
> People with asthma: 1,484,573
> Population: 18,550,288

The Los Angeles area has the worst ozone pollution of any U.S. city, as it did in nearly all of the 16 years the American Lung Association has measured it. The level of smog in LA reaches unhealthy levels during 152.5 days on average each year. During 2012 through 2014, there were 11.2 days of unhealthy particle pollution spikes. While this was the ninth highest level of short-term PM pollution, it was also the fewest number of such days ever recorded in LA.