Published April 5, 2026 · Updated monthly
Cities Where Air Quality Is Getting Worse
While national air quality trends are generally positive, 244 US cities are moving in the wrong direction. Using 10 years of EPA Air Quality System data (2014-2023), we identified the cities with the steepest AQI increases — meaning their air quality is measurably declining year over year.
Cities With the Biggest AQI Increases
The following cities show the most significant upward trend in median AQI over the past decade. A positive trend slope means air quality is deteriorating. These are ranked by the rate of change, not absolute AQI level — some cities that started with good air are declining toward moderate, while others that were already moderate are pushing into unhealthy territory.
| Rank | City | State | Current Median AQI | Grade | Trend |
|---|---|---|---|---|---|
| 1 | Harrison, Ohio | OH | 33 | B | +6.3/yr |
| 2 | Delta, Colorado | CO | 44 | C | +4.1/yr |
| 3 | Johnson, Wyoming | WY | 40 | C | +4.0/yr |
| 4 | Lincoln, Wyoming | WY | 44 | C | +3.4/yr |
| 5 | Catano, Puerto Rico | PR | 52 | D | +3.3/yr |
| 6 | Archuleta, Colorado | CO | 35 | C | +3.0/yr |
| 7 | Sanders, Montana | MT | 37 | D | +3.0/yr |
| 8 | Maricopa, Arizona | AZ | 72 | F | +2.9/yr |
| 9 | Valley, Idaho | ID | 36 | D | +2.8/yr |
| 10 | SONORA, Country Of Mexico | MX | 53 | D | +2.3/yr |
| 11 | Cass, Minnesota | MN | 32 | C | +2.3/yr |
| 12 | Grant, Oklahoma | OK | 32 | C | +2.2/yr |
| 13 | Columbia, Washington | WA | 39 | C | +2.2/yr |
| 14 | Douglas, Oregon | OR | 53 | C | +2.1/yr |
| 15 | Winnebago, Illinois | IL | 52 | D | +1.9/yr |
| 16 | Codington, South Dakota | SD | 45 | D | +1.9/yr |
| 17 | Mono, California | CA | 24 | D | +1.9/yr |
| 18 | Rock Island, Illinois | IL | 51 | D | +1.9/yr |
| 19 | Neosho, Kansas | KS | 54 | D | +1.8/yr |
| 20 | Horry, South Carolina | SC | 43 | C | +1.8/yr |
Why Some Cities Are Getting Worse
The drivers of worsening air quality vary by region, but three factors dominate:
1. Wildfire Smoke
The single biggest factor driving AQI increases in western cities. Climate change has extended fire season and increased fire intensity. Cities in California, Oregon, Washington, and Colorado regularly see AQI spikes above 150 during fire season — sometimes for weeks at a time. Smoke from western fires now travels as far as the East Coast, affecting cities thousands of miles from the actual fires.
2. Ozone Formation
Rising temperatures increase ground-level ozone formation. Ozone is created when nitrogen oxides (from vehicles and industry) react with volatile organic compounds in the presence of sunlight. Hotter summers mean more ozone, and cities in sunny, high-traffic regions like the Sun Belt are particularly affected. The dominant pollutant for most worsening cities is either ozone or PM2.5.
3. Population Growth and Traffic
Fast-growing metros in Texas, Arizona, and the Southeast have seen vehicle emissions outpace gains from cleaner cars. More people means more cars, more construction dust, and more energy demand. Without aggressive transit investment, these cities struggle to maintain air quality as they grow.
What This Means for Residents
If you live in a city with a worsening trend, consider monitoring daily AQI readings through the EPA's AirNow app. On high-AQI days (above 100), limit outdoor exercise, keep windows closed, and consider using a HEPA air purifier indoors.
Long-term, these trends matter for property values, health outcomes, and quality of life. Our cleanest air cities ranking and city comparison tool can help you evaluate alternatives if air quality is a priority for you.
Frequently Asked Questions
No. Nationally, air quality has improved significantly since the Clean Air Act. However, 244 of 1,020 cities tracked show worsening trends, primarily due to wildfire smoke and increased ozone formation from rising temperatures.
California and western states are disproportionately affected by worsening trends due to wildfire smoke. However, fast-growing Sun Belt cities in Texas, Arizona, and the Southeast also show declining air quality from increased traffic and ozone formation.
Yes, directly. Higher temperatures increase ground-level ozone formation, extend wildfire season, and change weather patterns that disperse pollutants. Climate scientists project that air quality challenges will intensify in coming decades without significant emissions reductions.
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