Air Quality by State: How All 50 States Compare
Air quality varies enormously across the United States, not just between regions but between neighboring states. This analysis uses EPA Air Quality System data covering 2020-2025 to compare state-level air quality patterns, identify the cleanest and most challenged states, and explain what drives the differences. Data from the EPA Air Quality System (AQS) and AirNow, covering all 50 states; see our methodology.
Key Findings
- 1.Northern New England and upper Midwest states consistently report the highest percentage of Good AQI days, driven by low population density and favorable wind patterns.
- 2.California remains the most challenged state for air quality despite decades of aggressive regulation, the combination of geography (valleys that trap pollution), population, and wildfire risk creates persistent AQI problems.
- 3.Western states show the most year-to-year volatility, with wildfire seasons capable of doubling a state's annual unhealthy day count in a single bad year.
- 4.Several eastern states that improved steadily through the 2010s saw their progress stall or reverse during wildfire smoke transport events in 2023.
What Drives State-Level Variation
State-level air quality is shaped by four primary factors: emissions sources (industrial, vehicular, agricultural), geography (mountain valleys trap pollution, coastal areas benefit from onshore winds), population density (more people means more vehicle emissions), and wildfire exposure (increasingly the dominant factor in western states).
The interplay of these factors explains apparent paradoxes. Montana has extremely low population density but posts more unhealthy days than some eastern urban states because of wildfire smoke. New Hampshire has significant forest cover and moderate density, producing some of the nation's cleanest air. Texas has enormous industrial emissions but also enormous geographic area, so state averages can obscure extreme local variation.
Explore individual state profiles on PlainAirData's state pages to see county-level data that reveals the variation hidden within state averages. Or compare specific counties on the county pages for a more precise comparison.
Regional Patterns
The Northeast has benefited most from Clean Air Act regulations, with dramatic improvements in ozone and PM2.5 since the 1990s. The Southeast faces persistent ozone challenges during hot summer months. The Mountain West has the most volatile air quality, swinging between excellent and hazardous depending on fire activity. The Pacific Coast combines some of the cleanest days (ocean air) with some of the worst events (wildfire smoke inversion).
These patterns are visible in PlainAirData's rankings, which can be filtered by region and sorted by various AQI metrics. Understanding your region's dominant pollutant and seasonal pattern is key to interpreting the data correctly.
Explore the Data
- All state profiles, AQI summaries, trends, and county breakdowns for every state
- County-level data, Detailed AQI data for nearly 1,000 monitored counties
- Metro area comparisons, AQI data for 500+ metropolitan statistical areas
- Wildfire smoke guide, How fire season affects air quality data
- Relocation guide, Using air quality data to evaluate potential new locations
Data source: EPA, Air Quality System (AQS) (2020-2025).
Analysis published: April 2026
Download the EPA AQS annual county AQI extract cited on this page: annual-aqi-by-county.csv or the JSON distribution (public domain / CC0). Method notes live on /methodology and /data.