Cities Where Air Quality Is Getting Worse

A city qualifies as "worsening" when its 10-year linear regression on annual median AQI shows a slope steeper than +0.1 AQI points per year — air is getting measurably dirtier year over year, not just fluctuating with weather. This bucks the broad national pattern of improvement, and the cities on this list are worth understanding: the deterioration usually traces back to a specific identifiable cause rather than general pollution trends.

244 of 1,020 monitored US cities currently show worsening air quality trends based on 10-year EPA data — 23.9% of all tracked areas.

The dominant cause of worsening U.S. air quality over the past decade is wildfire smoke, particularly across the western states. Climate change has lengthened fire seasons, dried out forests, and increased fire intensity — and smoke can now travel thousands of miles, affecting cities far from the actual fire. A second driver is rising local emissions from population and freight growth in cities that have boomed economically: more vehicles, more trucks, more construction dust, more port activity. A third is rising ozone driven by warming summer temperatures, which accelerate the photochemical reactions that produce ground-level ozone.

For residents of worsening cities, the practical priorities are indoor air quality and exposure planning. A HEPA air cleaner sized to the bedroom and main living area is the single highest-impact intervention available to a household. MERV-13 or better HVAC filtration helps when windows have to stay closed. On flagged days, sensitive groups should limit outdoor exertion and consider N95/KN95 respirators outdoors. For local civic action, the leverage points are pushing for transit and electric-vehicle infrastructure, opposing new high-emission industrial siting in residential areas, and supporting state-level wildfire-mitigation funding.

Trend is calculated using a linear regression of annual median AQI values over the most recent 10 years of data. Cities with a slope below -0.1 AQI/year are classified as Improving, above +0.1 as Worsening, and between as Stable. Using a 10-year window smooths out year-to-year volatility from weather events and isolates the underlying signal.