The 25 Most Polluted Cities in the US

© yhelfman / Getty Images

Salt Lake City, UT

  • High ozone days per year: 30
  • No. of days with unhealthy particle pollution: 9
  • People with asthma: 97,871
  • Population: 1,203,105

Though particulate levels have been reduced by 50% since the ’80s, ozone levels remain high in the Salt Lake Valley.

Because it is surrounded by mountains, smog tends to settle in the city in the winter after snowstorms and sometimes does not clear for weeks in a phenomenon known as thermal inversion.

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