If you’re searching for the worst air quality in the US – or the best – look no further than west of the Mississippi. A Social Explorer analysis of 2024 EPA air quality data reveals a striking paradox: the western United States is home to both the nation’s cleanest and its most polluted air. From pristine high-desert counties in Arizona and Wyoming to the smog-choked valleys of Southern California, the regional air quality story is one of dramatic extremes.
Using EPA air quality index (AQI) data reported across 778 counties that logged more than 180 days of measurements in 2024, Social Explorer mapped where Americans breathe easiest – and where the air is most hazardous to their health.
Where the Air Is Cleanest: America’s Freshest Skies
Eight counties achieved a perfect record in 2024, recording good air quality on every single day measured. Leading the list is Navajo County, Arizona, which reported clean air on all 274 days for which data were collected. Sitting at roughly 5,700 feet elevation on the Colorado Plateau, the county’s dry, high-desert climate and sparse industrial activity help keep its skies exceptionally clear.
The other counties with consistently good air quality included:
- Apache County, Ariz. (270 days of good air quality)
- Pacific County, Wash. (194 days)
- Elko County, Nev. (182 days)
- Columbia County, Ore. (182 days)
- Uinta County, Wyo. (182 days)
- Carbon County, Wyo. (181 days)
- Taos County, N.M. (181 days)
Outside the West, the standout performer east of the Mississippi was Indian River County, Florida, which earned a good air quality rating on 234 of 235 measured days – a 99.6 percent clean-air rate. The county’s coastal location along Florida’s Treasure Coast, with steady Atlantic breezes helping disperse pollutants, likely contributes to its stellar record.
The lowest median AQI in the entire country – the best typical daily air quality score – was recorded in Hughes County, South Dakota, home to the state capital, Pierre. Good air quality was logged on 178 of 182 recorded days; the remaining four days were rated moderate.
The Worst Air Quality in the US: Southern California Dominates
When it comes to the worst air quality in the US, Southern California’s inland counties top the list by a significant margin. San Bernardino County, the largest county by area in the contiguous United States, reported good air quality on only 40 of 245 measured days – just 16.3 percent. More troublingly, the county logged bad or very bad air quality on 64 days, or 26.1 percent of the year. Its median AQI of 87 – classified as “moderate” by the EPA – was the worst of any county in the nation.
The nearby Inland Empire counties fared similarly. Riverside County, Calif., recorded good air on only 42 of 245 days (17.1 percent). Los Angeles County, the nation’s most populous, matched San Bernardino’s rate with good air on just 40 of 245 days (16.3 percent). The combination of traffic emissions, industrial activity, geography, and heat creates a persistent smog problem across this region.
Beyond California, other counties with notably poor air quality records included:
- Harney County, Ore.: good air on just 20 of 274 days (7.3 percent)
- Cameron County, Texas: good air on 12 of 182 days (6.6 percent)
The single worst air quality reading in the entire country in 2024 came from Inyo County, California – a vast, sparsely populated county bordering Death Valley and the Sierra Nevada. Its peak AQI hit 1,322, a score far into the “Hazardous” range. Despite its remote character, Inyo County recorded good air quality on only 93 of 272 days (34.2 percent). Wildfire smoke, windblown dust, and the county’s bowl-like topography that traps particulates are all contributing factors.
How the EPA Measures Air Quality
The EPA assigns an air quality index (AQI) score to each county daily, based on measurements of five key pollutants: particulate matter (PM2.5 and PM10), ozone, carbon monoxide, nitrogen dioxide, and sulfur dioxide. In 2024, recorded AQI values ranged from 1 to 1,322 across reporting counties.
The AQI scale breaks down as follows:
For more detail on how the AQI is calculated, the EPA’s AQI basics guide provides a thorough overview.
East vs. West: A Tale of Two Air Sheds
The geographic split in US air quality data is striking. Among counties east of the Mississippi, air quality challenges are far less severe. The highest median AQI east of the Mississippi was recorded in Susquehanna County, Pennsylvania, at just 26 – the 59th-highest nationally. That figure would rank among the healthiest in the country if compared to the national range.
The worst performers in the eastern US still compare favorably to the West’s most polluted counties. The lowest median AQI in the eastern US was shared by Shelby County, Tennessee (home to Memphis), and Cuyahoga County, Ohio (home to Cleveland), both posting a median AQI of 55. The highest share of bad or very bad air quality days east of the Mississippi was found in Miami-Dade County, Florida, at just 1.6 percent of days – a figure that would rank among the cleanest western counties.
Why the disparity? Several factors drive the West’s extremes. The region’s topography – mountain ranges, valleys, and basins – can trap pollution. Its drier climate increases dust and wildfire risk. And its massive, sprawling metro areas like Los Angeles generate enormous volumes of vehicle emissions with few natural mechanisms for dispersal.
Explore Air Quality Data for Your County with Social Explorer
The patterns described in this analysis are just the beginning. Air quality varies not just by county but by season, elevation, proximity to industrial sites, and dozens of other factors – all of which can be explored interactively with Social Explorer’s mapping and reporting tools.
Social Explorer makes it easy to visualize EPA Air Quality Index data alongside dozens of other demographic, economic, and environmental datasets. Whether you’re a researcher, journalist, public health professional, or simply a curious resident wondering about the air in your neighborhood, Social Explorer gives you the tools to find answers quickly and clearly.
Ready to clear the air about what’s happening in your community? Start your free trial of Social Explorer today and access our full suite of air quality maps, reports, and data tools – no experience required.