Air quality guide
Best and Worst Air Quality in the US
Which counties breathe the cleanest air and which face the most pollution, and what drives the divide.
Key Takeaway
Air quality varies enormously across the US. The cleanest counties experience over 90% "Good" AQI days per year, while the worst, concentrated in California's Central Valley, may see more Unhealthy days than Good ones. Geography, industry, traffic, and wildfire exposure drive most of the difference.
Counties with the Best Air Quality
These counties have the highest percentage of "Good" AQI days, based on the most recent EPA annual data:
| County | State | Good Days % | Median AQI |
|---|---|---|---|
| Denali | Alaska | 100.0% | 38 |
| Sonoma | California | 100.0% | 9 |
| Beaverhead | Montana | 100.0% | 12 |
| Sanders | Montana | 100.0% | 15 |
| Martin | North Carolina | 100.0% | 28 |
The cleanest-air counties share common traits: low population density, minimal industrial activity, distance from major highways, and favorable wind patterns that disperse pollutants. Many are coastal, island, or mountainous areas where clean air masses dominate.
See the full list on our best air quality rankings page.
Counties with the Worst Air Quality
These counties have the lowest percentage of "Good" AQI days:
Most-polluted counties by median AQI
| County | State | Good Days % | Median AQI |
|---|---|---|---|
| Harris | Texas | 8.8% | 63 |
| Riverside | California | 10.6% | 87 |
| Los Angeles | California | 10.9% | 77 |
| San Bernardino | California | 12.4% | 90 |
| El Paso | Texas | 12.7% | 74 |
The worst-air counties are heavily concentrated in California's Central Valley and Southern California. The San Joaquin Valley, home to Kern, Fresno, and Tulare counties, is surrounded by the Sierra Nevada and Coast Ranges, which trap pollutants in a natural bowl. Heavy agriculture, oil production, and vehicle traffic add to the problem.
See the full list on our worst air quality rankings page.
What Drives the Differences?
Three factors explain most of the air quality variation across the US:
- Geography and topography: Valleys trap pollution (Los Angeles basin, Central Valley). Coastal areas benefit from ocean breezes. Flat, windy regions disperse pollutants efficiently.
- Emission sources: Dense urban traffic produces ozone precursors. Industrial corridors emit particulates and NOx. Agricultural operations generate PM10 dust. Oil and gas production contributes VOCs.
- Wildfire exposure: Western states face increasing wildfire smoke, which can dominate PM2.5 readings for weeks during fire season. This factor has worsened significantly since 2018.
Using Air Quality Data for Decisions
If you're choosing where to live or evaluating your current area's air quality, annual data is more useful than any single day's reading. Look at the Good day percentage over multiple years to understand the typical experience. A county with 70% Good days most years but a 50% year during a bad wildfire season tells a different story than one that's consistently at 50%.
PlainAir provides multi-year trend data for every county and metro area. Visit any county page to see how air quality has changed over the past five years.
Frequently Asked Questions
Why does California dominate the worst air quality lists?
California has a combination of factors that trap pollution: the Central Valley is surrounded by mountains that block air circulation, creating a natural "bowl" effect. Heavy agricultural activity produces PM10, while vehicle emissions in Los Angeles and the Inland Empire generate ozone precursors. Wildfire smoke during summer and fall adds PM2.5. Despite having some of the nation's strictest air quality regulations, geography makes California uniquely susceptible.
Which areas have the cleanest air in the US?
Rural counties in northern New England (Vermont, New Hampshire, Maine), the Pacific Northwest coast, and Hawaii consistently rank among the cleanest. These areas share common traits: low population density, minimal industrial activity, favorable wind patterns, and distance from major highways. Many experience over 90% "Good" AQI days per year.
Has air quality in the US improved over time?
Yes, significantly. Since the Clean Air Act was enacted in 1970, aggregate emissions of the six criteria pollutants have dropped approximately 79% while US GDP has grown more than 338%, per EPA’s "Our Nation’s Air" 2025 report (1970–2024). However, wildfire smoke has partially offset PM2.5 improvements in western states in recent years, and climate change is expected to worsen ozone pollution by increasing temperatures.
Do wildfires affect air quality hundreds of miles away?
Yes. Wildfire smoke can travel thousands of miles. In 2023, Canadian wildfire smoke blanketed the northeastern US, pushing AQI into the Hazardous range in New York City, over 1,000 miles from the nearest fire. Smoke events are becoming more frequent and severe as wildfire seasons lengthen due to climate change.
How does altitude affect air quality?
Higher-altitude areas generally have cleaner air because pollution sources are concentrated at lower elevations and atmospheric pressure decreases with altitude. However, high-altitude valleys (like Denver at 5,280 feet) can trap pollution through temperature inversions. Mountain communities may also be more directly affected by wildfire smoke.
Sources
- U.S. Environmental Protection Agency, Air Quality System (AQS), 2020-2024
- EPA, Our Nation's Air: Trends Through 2023
This content is for informational purposes only. Always follow official guidance from local authorities. For current air quality conditions, visit AirNow.gov.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.