Particulate air pollution as measured by PM2.5 (concentration of particles under 2.5 micrometers), PM10, and ground ozone has been associated with various health problems including increased chances of lung cancer and heart disease. In this article, I’ll rank counties with the worst air quality. More specifically, this ranking looks at the 10 counties whose surface air (i.e. excluding air in mines or caves) is the most harmful to human health. Harmfulness is modeled based on a time-averaged combination of AQI (air quality index), ozone level, and PM2.5 particulate pollution levels in each county. The result is that these are the 10 counties with the worst air pollution in the entire U.S.
1. Maricopa County, AZ
Maricopa has been rapidly climbing the ranks of air pollution, getting worse and worse over the past decade until it has now finally surpassed all of its California competitors. Median AQI last year was 122, and 10% of days had an AQI over 213 which the EPA deems not just “unhealthy” but “very unhealthy”.
2. San Bernardino County, CA
The EPA categorized air quality as unhealthy, very unhealthy, or hazardous for a total of 73 days last year. Median AQI was 87, and 10% of all days had an AQI over 172.
3. Riverside County, CA
Just outside LA, smog and smoke collect in Riverside County where 2.5 million residents are exposed to a median AQI of 88.
4. Kern County, CA
Median AQI in Kern is 84, and the county frequently fails to satisfy federal standards for both ozone and PM2.5 particle pollution levels, although some progress has been made in improving air quality.
5. Tulare County, CA
Tulare had a median air quality index of 80 in 2021, and 10% of days had an AQI of at least 153 which the EPA deems unhealthy for all members of the general public.
6. Los Angeles County, CA
The most surprising thing about LA is the fact that it only ranks number 6 on this list. The fact is that LA still has tons of smog, but the air is MUCH cleaner than it was 30 years ago. Median AQI last year was 77, and the worst 10% of days had an AQI of 143 or higher.
7. Plumas County, CA
Plumas only has a median AQI of 63, but its worst 10% of days are BAD with an AQI of 165 or higher. It also had 7 days with air quality above the “hazardous” AQI threshold of 300.
8. Fresno County, CA
Fresno County exposes its more than half a million residents to a median AQI of 67, and less than one third of days have what the EPA deems “good” air quality.
9. Pinal County, AZ
Being just outside Phoenix’s Maricopa County, it shouldn’t be too surprising that Pinal also has bad air. Median AQI was 69.
10. Kings County, CA
Kings County air quality is better than it was 2 decades ago, but it is still terrible. Median AQI is 61, and 10% of days had an AQI over 109.
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