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Children in Fresno, California sometimes have to stay inside all day due to air pollution.
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These are the US cities with the worst air pollution

Zoë Schlanger
By Zoë Schlanger

Environment reporter

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Among the US cities where breathing the air is most dangerous to human health, California’s metro areas dominate the list.

The combination of the state’s topography and large population have kept many California cities on the most-polluted list throughout the 20-year history of the American Lung Association’s State of the Air report, and this year’s is no different.

It seems the US’s air pollution problem is worsening in several regions. The culprit for this rise is likely climate change: Excessive heat makes it easier for ozone pollution to form, for example, and wildfires, exacerbated by unusually hot and dry conditions, cause spikes in small particulate matter pollution (known as PM2.5) wherever soot from fires travels.

The 2019 State of the Air report, published on April 24, analyzed the 2015-2017 period, which is the most recent block of time for which nationwide air pollution data has been verified. Those three years were also notably the hottest years on Earth ever recorded.

An additional 7.2 million people were exposed to harmful spikes in pollution during that time than were in 2013-2015, the period analyzed by the previous State of the Air report. Eight cities on its list of most-polluted cities broke their own records for dangerous spikes in PM2.5. And overall, the US recorded more days when air pollution reached “emergency condition” levels than ever before.

All metropolitan areas in the US with at least one urban core of 50,000 people or more were included in the American Lung Association’s assessment.

Most-polluted metro areas by average year-round concentration of PM2.5

  1. Fresno-Madera-Hanford, CA
  2. Bakersfield, CA
  3. Fairbanks, AK
  4. Visalia, CA
  5. Los Angeles-Long Beach, CA
  6. San Jose-San Francisco-Oakland, CA
  7. Pittsburgh-New Castle-Weirton, PA-OH-WV
  8. El Centro, CA
  9. Cleveland-Akron-Canton, OH
  10. Medford-Grants Pass, OR

Notable on this list is Fairbanks, Alaska, which moved from 15th place on the last report’s list to third place this year, due to improvements in local air monitoring that revealed the city’s air-pollution problem is far worse than previously thought.

Most-polluted metro areas by dangerous “spikes” in PM2.5

  1. Bakersfield, CA
  2. Fresno-Madera-Hanford, CA
  3. Fairbanks, AK
  4. San Jose-San Francisco-Oakland, CA
  5. Missoula, MT
  6. Yakima, WA
  7. Los Angeles-Long Beach, CA
  8. Salt Lake City-Provo-Orem, UT
  9. Seattle-Tacoma, WA
  10. Pittsburgh-New Castle-Weirton, PA-OH-WV

California’s Bay Area—San Francisco, Oakland, and San Jose—doesn’t usually show up on this list, but this year it landed in fourth place, likely due to wildfires in the area.

Bakersfield, California remains the most polluted city in America in terms of spikes in particulate matter, with Fresno coming in second. A “spike” is defined as a day when the concentration of PM2.5 rises above the limit set by the US Environmental Protection Agency for acceptably healthy air. Both Bakersfield and Fresno are in California’s Central Valley, where the air is laden with PM2.5 because of high volume of oil and gas drilling and diesel engines in the area.

The Central Valley is also topographically cursed—it’s shaped like a bowl, so pollution is often trapped, unable to disperse. Los Angeles, similarly, is stuck in a topographic bowl, which helped to land it at number one on the list of the most ozone-polluted cities in America for the 19th time in 20 years (on that list, Visalia, a Central Valley city, came in second, and Bakersfield came in third).

Most-polluted cities by ozone pollution

  1. Los Angeles-Long Beach, CA
  2. Visalia, CA
  3. Bakersfield, CA
  4. Fresno-Madera-Hanford, CA
  5. Sacramento-Roseville, CA
  6. San Diego-Chula Vista-Carlsbad, CA
  7. Phoenix-Mesa, AZ
  8. San Jose-San Francisco-Oakland, CA
  9. Houston-The Woodlands, TX
  10. New York-Newark, NY-NJ-CT-PA

Both PM2.5 and ozone are linked to a long list of health problems, including asthma, lung cancer, premature death, and developmental delays in children. Exposure to PM2.5 has detrimental effects on the heart and lungs. It hits babies and the elderly hardest, and exposure in the womb has long been associated with an array of adverse outcomes that include preterm birth and low birth weight.

Plenty of studies have also found that children who live or attend school in places with high traffic-related air pollution perform worse on cognitive tests.

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