20 Most Polluted Cities in America

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15. Houston-The Woodlands, TX
> High ozone days per year:
22.2 days
> Number of days with unhealthy particle pollution: 0.3 days
> People with asthma: 448,924
> Population: 6,686,594

Houston is home to a number of oil and gas, manufacturing and other heavy industrial companies, notably Shell Oil Company, Kinder Morgan, Marathon Oil Corp., and Apache Corp. Billings noted that emissions from gasoline and byproducts of oil and gas extraction operations contribute to air pollution. Of the 6.7 million Greater Houston area residents, more than 1 million live in poverty. Poorer communities often face higher exposure to pollutants such as ozone.

14. New York-Newark, NY-NJ-CT-PA
> High ozone days per year:
24.3 days
> Number of days with unhealthy particle pollution: 4.3 days
> People with asthma: 2,285,782
> Population: 23,632,722

New York City is the nation’s most populated — and most crowded — city. Hundreds of thousands of people drive into New York every day, and the concentrated dose of motor vehicle exhaust is the major driver of smog in the area. Billings noted that nearby areas such as Fairfield County, Connecticut frequently have among the highest ozone levels simply because they are downwind from NYC.

13. San Diego-Carlsbad, CA
> High ozone days per year:
27.5 days
> Number of days with unhealthy particle pollution: 3.5 days
> People with asthma: 259,926
> Population: 3,263,431

The level of smog in the San Diego area worsened from the last period measured by the ALA. As in other parts of the Western United States, especially California, ozone levels increase in the summer. Southern California has endured extreme drought conditions over the past several years, which have likely contributed substantially to ozone pollution.

12. El Centro, CA
> High ozone days per year:
28.2 days
> Number of days with unhealthy particle pollution: 7.2 days
> People with asthma: 14,390
> Population: 179,091

El Centro is one of seven historically polluted cities where the number of days with unhealthy particle pollution levels — the short-term measurement of PM pollution — reached the worst level ever recorded in the city in 2012-2014. Each, particle pollution reaches unhealthy levels during 7.2 days on average, trailing just 12 other cities. El Centro also has among the most dangerously high ozone level days a year.

11. Dallas-Fort Worth, TX-OK
> High ozone days per year:
29.7 days
> Number of days with unhealthy particle pollution: 0.7 days
> People with asthma: 496,180
> Population: 7,352,286

The Dallas-Fort Worth metro area is one of six cities with historically high ozone pollution, but where the current level of ozone pollution is the lowest on record for the city. Still, with nearly 30 days of unhealthy levels of smog in a single year, the Dallas metro area is one of the smoggiest cities in the country.