FRESNO, Calif. (KSEE/KGPE) – Of the top ten cities that EPA data shows the most worsened air quality from 2019 to 2020 – five of them are in the Central Valley, according to data published by Filterbuy.

Top of the worst of the Central Valley list is Fresno, followed by Madera, Bakersfield, and the combined Visalia-Porterville, followed by Hanford-Corcoran.

The Valley Air District says the local air quality was heavily impacted by a historically severe wildfire season – resulting in extremely high AQI in the San Joaquin Valley and throughout California. However, 2019 was a relatively less active wildfire season meaning that the contrast between the two would be greater.

The data was sourced from the EPA and organized by Filterbuy. The cities selected in the survey were metropolitan areas with a population of over 100,000 residents. The data compared the median AQI (air quality index) over 2019 and compared it to the media AQI over 2020, creating a percentage change between the two numbers.

Of the 334 cities listed, the city that experienced the worst change in air quality from 2019 to 2020 was the Phoenix-Mesa-Scottsdale area in Arizona with a 60.8% change in median AQI (air quality index). Fresno is at 331 with a 23.3% change, Madera at 330 with a 22% change, Bakersfield at 329 with a 20.9% change, Visalia-Porterville at 328 with a 19.7% change, and Hanford-Corcoran at 327 with a 19.6% change.

While the historically severe wildfire season did create more bad-air days in the Central Valley in 2020 compared to 2019, the Valley Air District says the number of good days reported in 2019 has swayed the data.

“The table in the report is sorted and ranked by the change in the median AQI from 2019 through 2020, and as the Valley’s metropolitan areas were heavily impacted by the 2020 wildfires, the change between these two years were higher than other areas of the state,” wrote the air district’s Heather Heinks in a statement to

However, Heinks added that reviewing the data showing the percentage of days with good AQI shows that the Central Valley’s metropolitan areas have more days with good AQI than metropolitan areas in southern California.