The 10 Most Polluted Cities in the US (2025): Ozone and PM2.5 Rankings

Air pollution is still a major public-health challenge in the United States. According to the American Lung Association’s “State of the Air 2025,” roughly 156 million people live in counties with unhealthy air quality. The report analyzes two core pollutants that drive most health risks: ozone (smog) and fine particulate matter (PM2.5, “particle pollution”). Behind the numbers are multiple causes — wildfire smoke, fossil fuel emissions from traffic and power plants, and geography that traps dirty air in valleys and basins — and the impacts are serious: more asthma attacks, cardiovascular events, premature death, and elevated risks of lung cancer.

Below you’ll find the latest top-10 rankings for the worst air quality in major US metropolitan areas, using the American Lung Association’s methodology and US EPA data. Because ozone and PM2.5 behave differently in the atmosphere, cities can rank very high on one list and not the other. The 2025 report covers EPA measurements from 2021–2023 and reflects the tighter annual PM2.5 health standard adopted in 2024 (9.0 µg/m³), which sharpens the focus on year-round particle pollution.

How these rankings work (Ozone vs. PM2.5)

Ozone forms in sunlight from precursors like nitrogen oxides and volatile organic compounds, leading to “worst air” days in warm seasons. PM2.5 (fine particulate matter) comes from combustion sources (diesel, power plants, industry) and from wildfire smoke; it harms health year-round and during short-term spikes. The American Lung Association ranks metro areas using weighted unhealthy days for ozone and short-term particle pollution, and design values for year-round PM2.5, based on US Environmental Protection Agency air quality data and the Clean Air Act framework. In practice, that means Central Valley metros (Bakersfield, Visalia, Fresno) and Los Angeles–Long Beach often appear near the top, with western cities affected by wildfire also climbing the list.

Top 10 Ozone (Smog): Most Ozone-Polluted Cities in the US

• 1. Los Angeles–Long Beach, CA
• 2. Visalia, CA
• 3. Bakersfield–Delano, CA
• 4. Phoenix–Mesa, AZ
• 5. Fresno–Hanford–Corcoran, CA
• 6. Denver–Aurora–Greeley, CO
• 7. Houston–Pasadena, TX
• 8. San Diego–Chula Vista–Carlsbad, CA
• 9. Salt Lake City–Provo–Orem, UT–ID
• 10. Dallas–Fort Worth, TX–OK

Ozone pollution correlates strongly with heat and sunlight. Western and Southwestern metros show up repeatedly due to hotter summers, strong sunlight, and heavy transportation emissions. Valleys surrounded by mountains can trap emissions and worsen ozone episodes.

Top 10 Year-Round PM2.5 (Fine Particle Pollution)

• 1. Bakersfield–Delano, CA
• 2. Visalia, CA
• 3. Fresno–Hanford–Corcoran, CA
• 4. Eugene–Springfield, OR
• 5. Los Angeles–Long Beach, CA
• 6. Detroit–Warren–Ann Arbor, MI
• 7. San Jose–San Francisco–Oakland, CA
• 8. Houston–Pasadena, TX
• 9. Cleveland–Akron–Canton, OH
• 10. Fairbanks–College, AK

These “particle pollution” rankings reflect chronic exposure to PM2.5. California’s Central Valley appears again because of geography and regional emission patterns, while Midwestern and Great Lakes metros rank due to industrial emissions and winter inversions. Fairbanks–College often sees extreme seasonal wood-smoke, and wildfire years (including 2023) push several Western communities higher, raising unhealthy levels and the overall pollution exposure profile.

Why these cities in America struggle with poor air quality

Air quality problems rarely come from a single source. In the West, wildfire smoke has become a dominant driver of short-term particle spikes, layering on top of traffic and industrial emissions. Metro areas that are surrounded by mountains or set in valleys (for example, the Central Valley in California or basins along the Rockies) experience temperature inversions that trap polluted air near the surface. Major freight corridors, marine ports, refineries, and power plants add diesel and other combustion emissions. Add hotter summers and stagnant high-pressure systems and you get more ozone formation and longer pollution episodes that lead to unhealthy air and higher pollution levels across entire metropolitan areas.

Health impacts and equity

Even at today’s concentrations, particle pollution and ozone are linked to asthma attacks, COPD flare-ups, cardiovascular disease, and elevated risks of premature death. Communities of color and low-income neighborhoods often live closer to highways, distribution hubs, and industrial sites — which means higher day-to-day exposure and more emergency room visits, especially during wildfire smoke events. Protecting public health requires reducing emissions at the source and improving air quality monitoring, while individuals can lower exposure by checking local air quality data, using clean air rooms or portable filters on high-PM2.5 days, and supporting policies that improve air quality.

Solutions that scale: from cleaner emissions to ecosystem restoration

Cutting tailpipe and power-plant emission is essential, but restoring forests also helps the wider climate system that influences ozone and particulate formation. Healthy forests reduce climate-warming greenhouse gas emissions over time, improve regional air quality, and strengthen biodiversity. If you run a company, pairing decarbonization with a credible tree planting service can make your sustainability roadmap more tangible today. Bloomy Earth provides a corporate reforestation platform with a tree-as-a-service API for automated tree planting for businesses, plus a sustainable impact dashboard that makes every contribution measurable and transparent.

Move from concern to action

If this topic is on your radar because you live in one of these metropolitan areas or you manage a US-based business with air quality commitments, you can act on both sides of the equation: reduce emissions and support restoration. Explore our reforestation projects, set up automated, per-transaction tree planting, or build a carbon-neutral website experience that reports impact in real time.

• Explore reforestation projects
• See individual and business plans
• Track impact on the Bloomy Earth dashboard

Sources (2024 and 2025)

• American Lung Association — “State of the Air” 2025 overview
• American Lung Association — Most Polluted Cities (2025)
• American Lung Association — “State of the Air 2025” full report (PDF)
• American Lung Association — Key findings: Ozone pollution
• American Lung Association — Key findings: Year-round particle pollution
• US EPA — PM2.5 trends
• US EPA — Air quality design values (2024/2025)
• US EPA — 2024 revised annual PM2.5 standard (NAAQS at 9.0 µg/m³)

Want to turn concern into measurable climate action? Plant trees where they matter, track every result, and make your operations part of the solution. Get started with Bloomy Earth.