United States Air Quality Index (AQI)
US air quality has improved ~42% since 2000 thanks to the Clean Air Act. But wildfires, ozone, and diesel exhaust still challenge 149 cities that exceed EPA ozone standards. The EPA tightened the annual PM2.5 standard to 9 μg/m³ in 2024.
52
National Avg AQI
2024 annual
8.4 μg/m³
PM2.5 Average
1.7× WHO limit
4,000+
Monitoring Stations
EPA AQS network
−42%
PM2.5 Reduction
vs 2000
Most Polluted Cities
Cleanest Cities
West Coast
Mountain West
South
Midwest
Northeast
Understanding US Air Pollution
The Clean Air Act legacy
The Clean Air Act of 1970 established the US EPA and set the framework for modern air quality regulation. Since 1990, aggregate emissions of the six main pollutants have dropped by 78% while GDP tripled — proof that economic growth and clean air are compatible. However, climate change now threatens this progress via wildfires and hotter ozone seasons.
Wildfires: the new normal
Wildfire smoke is now the largest source of PM2.5 in the western US, reversing decades of progress. The 2020 California wildfire season, 2023 Canadian smoke events (which reached New York), and the 2025 Los Angeles fires illustrate how climate-driven wildfires can push AQI above 300 in areas far from the fires themselves.
Ozone: the summer problem
Ground-level ozone forms when vehicle exhaust and industrial emissions cook in sunlight. The US updated its national ambient air quality standard (NAAQS) to 70 ppb in 2015, but 149+ cities still violate it. LA, Houston, and Denver are chronic non-attainment areas. Climate change is making ozone seasons longer.
EPA 2024 PM2.5 standard update
In February 2024, EPA tightened the annual PM2.5 standard from 12 to 9 μg/m³ — the first update since 2012. This will classify many more counties as non-attainment and require states to develop cleanup plans. The WHO guideline is even tighter at 5 μg/m³ annual average. Most US cities now meet the old standard but not the new one.