The Cleanest and Most Polluted Cities in America: A 10-Year Analysis

The 10 Cleanest Air Cities in America

AirHistory ranks cities using the Air Quality Grade, a composite score based on 5-year average AQI, 10-year trend direction, unhealthy day count, and dominant pollutant. The cities with the highest grades consistently demonstrate low median AQI values, few unhealthy air days, and stable or improving trends over the full decade.

Several patterns emerge from the cleanest cities. Geography plays a dominant role — cities in Hawaii, northern New England, and the upper Midwest benefit from low population density, distance from major industrial centers, and weather patterns that disperse pollutants efficiently. Coastal cities with consistent onshore winds also tend to perform well, as ocean air is typically clean and replaces stagnant polluted air.

Notably, a city does not need to be small or remote to have clean air. Several mid-sized metropolitan areas rank in the top tier because of effective emission controls, favorable geography, and limited exposure to wildfire smoke. The common thread is not size but a combination of clean local emissions, favorable meteorology, and distance from external pollution sources.

The 10 Most Polluted Cities in America

The cities with the lowest Air Quality Grades share a different set of characteristics: proximity to heavy industry, geographic features that trap pollutants, high vehicle traffic, and increasingly, exposure to wildfire smoke.

California dominates the worst-air list, and the reasons are multifaceted. The Central Valley — home to Fresno, Bakersfield, and Visalia — sits in a natural bowl surrounded by mountains that trap both industrial emissions and agricultural dust. The region experiences frequent temperature inversions in winter and intense heat in summer that drives ozone formation. In recent years, wildfire smoke has compounded these existing challenges, turning what was already poor air quality into some of the worst in the developed world.

Outside California, cities that rank poorly tend to share at least two of three factors: heavy industrial activity or vehicle traffic, geography that limits air circulation (valleys, basins), and proximity to wildfire-prone areas or agricultural regions that generate dust and ammonia.

The Fastest-Improving Cities

Perhaps the most actionable data in AirHistory is the trend direction. Cities where air quality is improving represent places where environmental policy, economic shifts, or demographic changes are actively making the air cleaner. Currently, 346 of 1,020 tracked cities show an improving trend.

The drivers behind improvement vary. Some cities have benefited from the retirement of coal-fired power plants and the transition to natural gas and renewables. Others have seen declining industrial emissions as manufacturing has shifted or modernized. Tighter vehicle emission standards, the growth of electric vehicles, and cleaner fuels have contributed broadly. And some cities have improved simply because a particularly bad pollution source — a specific factory, refinery, or traffic pattern — has been addressed.

What Drives the Differences

Air quality is ultimately determined by three factors: emissions (what goes into the air), meteorology (how the atmosphere moves and disperses pollutants), and geography (terrain features that affect air circulation). Cities with clean air tend to have low local emissions, weather patterns that ventilate the area effectively, and terrain that does not trap pollutants. Cities with poor air quality typically face challenges in at least two of these three dimensions.

Climate change is increasingly disrupting these patterns. Rising temperatures worsen ozone formation. Longer, more intense wildfire seasons inject unprecedented amounts of PM2.5 into the atmosphere, affecting cities hundreds or thousands of miles from the fires. And shifting weather patterns may alter the frequency of inversions and stagnation events that trap pollution. The next decade of air quality data may look very different from the last.

How to Use This Data

If you are evaluating cities for relocation, look beyond the current AQI and focus on the Air Quality Grade and trend. A city with a B grade and an improving trend may be a better long-term choice than a city with an A grade and a worsening trend. Consider the dominant pollutant — if you have asthma, you may be more affected by ozone than PM2.5, or vice versa. Check the full cleanest air rankings and use the comparison tool to evaluate specific cities side by side.