Cities with the Best and Worst Air Quality in the US

Metro Areas with the Best Air Quality

These metros achieve the highest percentage of "Good" AQI days (AQI ≤ 50) among metros monitored 200+ days, based on EPA AQS 2025 annual data. Source: EPA Air Quality System (AQS) Annual Summary via plainairdata DB.

Metro Area	State	Good Days %	Median AQI
Santa Rosa, CA	CA	100.0%	9
Evanston, WY	WY	100.0%	9
Show Low, AZ	AZ	99.5%	13
Clearlake, CA	CA	99.2%	31
Vallejo-Fairfield, CA	CA	99.2%	33
Taos, NM	NM	98.9%	16

These metros share a common profile: relatively low population density, minimal heavy industry, and geography that promotes air circulation. Rural western metros benefit from open terrain and steady winds. Coastal Pacific Northwest locations like Aberdeen, WA receive clean marine air. Rankings are computed live from the EPA AQS database and match the cleanest metros ranking page.

Explore the complete rankings on our best air quality rankings page or browse all 504 metro areas.

Metro Areas with the Worst Air Quality

These metros have the lowest "Good" day percentages among metros monitored 200+ days, based on EPA AQS 2025 annual data. Source: EPA Air Quality System (AQS) Annual Summary via plainairdata DB.

Metro Area	State	Good Days %	Median AQI
Phoenix-Mesa-Scottsdale, AZ	AZ	6.6%	78
Riverside-San Bernardino-Ontario, CA	CA	7.4%	87
Houston-The Woodlands-Sugar Land, TX	TX	10.1%	63
Los Angeles-Long Beach-Anaheim, CA	CA	10.7%	74
Bakersfield, CA	CA	17.5%	70
San Diego-Carlsbad, CA	CA	17.5%	67

The worst metros span multiple regions: Riverside-San Bernardino and Los Angeles sit in California basins that trap pollution; Houston faces a combination of petrochemical industry emissions and stagnant gulf coast air; El Paso, Phoenix, and Las Cruces in the desert Southwest experience high ozone from intense sunlight and warm temperatures. Geography, not just population or industry, determines how effectively an area can flush its air.

See the full list on our worst air quality rankings page.

What Drives Urban Air Quality

Four factors explain most of the variation in metro air quality rankings:

Basin and valley geography: Cities surrounded by mountains trap pollution that would otherwise disperse. Los Angeles, Bakersfield, and Salt Lake City all sit in basins. Wind patterns and temperature inversions determine whether pollution escapes or accumulates.
Vehicle and industrial emissions: Dense highway networks generate ozone precursors (NOx and VOCs). Industrial corridors, ports, refineries, manufacturing plants, add PM2.5 and NOx. Diesel trucks are disproportionately impactful: a single heavy-duty diesel truck emits as much NOx as dozens of passenger cars.
Agricultural activity: In the Central Valley, crop dust, fertilizer application, and field burning contribute enormous quantities of PM10 and PM2.5. Dairies and feedlots add ammonia that reacts in the atmosphere to form fine particles. This agricultural pollution layer is a major reason valley cities rank worse than similarly-sized industrial cities elsewhere.
Wildfire smoke: Western metros that historically had clean air, Seattle, Portland, Boise, Denver, now experience Unhealthy or Very Unhealthy AQI days during August and September fire seasons. In 2020 and 2021, smoke from California and Oregon wildfires pushed some normally-clean Pacific Northwest metros into the top 20 worst for those years.

The Wildfire Effect on Rankings

One of the most important recent trends in metro air quality is the increasing impact of wildfire smoke on cities that are not themselves fire-prone. Between 2020 and 2024, western metros have seen their rankings deteriorate relative to eastern counterparts.

Portland, Oregon provides a clear example: in years without major wildfire events nearby, Portland ranks among the top 50 cleanest US metros. In 2020 and 2021, catastrophic Pacific Coast wildfires pushed Portland's air into the Hazardous category for multiple days, the worst air quality recorded in the city's monitoring history. The 5-year averages now reflect this new reality.

This dynamic means multi-year data tells a more complete story than any single year. A city that looks clean in 2022 but had terrible air in 2020 and 2021 faces a different long-term reality than one that is consistently clean year over year. Check the 5-year trend data on individual metro pages to see year-by-year variation.

How to Check Your City's Air Quality

PlainAirData provides EPA AQS annual summary data for all 501 US metro areas. Each metro page shows:

Annual Good, Moderate, and Unhealthy day breakdowns for 2020–2024
Median and maximum AQI by year
The primary pollutant driving air quality problems
How the metro compares to state and national averages

For real-time conditions, visit AirNow.gov, which updates hourly from EPA monitoring stations. PlainAirData's annual data is best for understanding your city's typical air quality profile rather than today's conditions.

Browse all metro areas →

Frequently Asked Questions

Which US city has the worst air quality?

Riverside-San Bernardino-Ontario, CA consistently ranks as the metro with the worst air quality in the US based on EPA AQS annual data (2025). The Inland Empire's combination of interstate freight traffic, industrial activity, and geography that traps pollution from the LA Basin gives it the lowest share of Good air days among major monitored metros. Houston and Los Angeles are close behind.

Which US cities have the best air quality?

Rural and coastal metros in the West and rural South consistently top clean-air rankings. Santa Rosa, CA; Show Low, AZ; and Bozeman, MT lead the 2025 EPA AQS rankings among metros monitored 200+ days. These metros share low traffic density, minimal heavy industry, and geography that promotes air circulation rather than trapping it.

Why is California's Central Valley so polluted?

The Central Valley is a natural bowl surrounded by the Sierra Nevada to the east and the Coast Ranges to the west. These mountains block air circulation, trapping pollutants from agriculture (field burning, dust, pesticides), oil and gas extraction, diesel trucking, and vehicle emissions. Hot, still summer days intensify ozone formation. Despite California having the strictest air regulations in the country, geography limits how much improvement is possible.

Do big cities always have worse air than small towns?

Not necessarily. Population size is one factor, but geography and local industry often matter more. Los Angeles is large and polluted, but so is Riverside, which has lower traffic volume yet worse air, because it receives LA Basin pollution AND has its own industrial sources. Meanwhile, some of the cleanest US metros are rural but large in area, with geography that disperses pollutants naturally.

How has air quality in US cities changed over time?

Overall, US urban air quality has improved dramatically since the 1970s Clean Air Act. Average PM2.5 and ozone levels are much lower than 40 years ago. However, wildfire smoke has reversed some of these gains in western metros since 2018. Cities like Seattle, Portland, and Boise that historically had clean air now frequently experience Unhealthy or Very Unhealthy days during late summer fire seasons.

Can I check my city's specific air quality data?

Yes. PlainAirData provides annual EPA AQS data for 501 US metro areas, including Good day percentages, median AQI, and 5-year trends. Search for your metro on the Metros page, or check the state page for a summary of metros within your state.

Sources

U.S. Environmental Protection Agency — Air Quality System (AQS) Annual Summary, 2020–2025
EPA — AirData: Air Quality Data Collected at Outdoor Monitors Across the US
American Lung Association — State of the Air 2024 Report

This content is for informational purposes only. Annual data reflects historical EPA monitoring records. For current air quality conditions, visit AirNow.gov or your local air quality management district.