Skip to main content
AirHistory

Updated April 2026 · EPA AQS data

Air Quality Trend Reports

Across 1,020 U.S. cities tracked here, 346 are on a clear 10-year improving trend, 244 are worsening, and 430 hold roughly steady. These reports surface the biggest movers — the cities cleaning up fastest and the ones losing ground — using a decade of EPA Air Quality System monitoring data.

What's Driving U.S. Air Quality Trends

The dominant national pattern over the past decade is improvement: median AQI has fallen across most of the country as older coal plants retired, vehicle fleets turned over to cleaner standards, and EPA's Clean Air Act rules tightened on PM2.5 and ozone precursors. The exception is wildfire smoke — particulate readings spike in any year a major fire complex burns upwind of populated areas, and this signal increasingly dominates Western U.S. trend lines.

Trend reports focus on the cities at the extremes. The biggest improvers are typically Rust Belt and Sun Belt metros that retired coal generation in the 2010s. The worst-trending cities concentrate in California, Oregon, Washington, Nevada, Idaho, and Montana — all carrying multi-year wildfire-driven PM2.5 increases that swamp local emission gains.

Top 5 Improving Cities (10-yr trend)

Top 5 Declining Cities (10-yr trend)

Browse All Trend Reports

Improvement10 entities

Cities With Improving Air Quality

10-year trends show cleaner air in these areas

Decline10 entities

Cities With Worsening Air Quality

10-year trends show declining air quality in these areas

How Trends Are Calculated

For each city, we fit a simple linear regression to the past decade of annual median AQI values, with year as the independent variable and median AQI as the dependent variable. The slope of that line is the trend — negative means cleaner air, positive means worsening. We then bucket cities into Improving (slope ≤ −0.5 AQI/year), Worsening (slope ≥ +0.5), and Stable (between). The Air Quality Grade also weights this trend at 30% of the composite score. Read the full methodology.

Frequently Asked Questions

What is an air quality trend?

An air quality trend is the direction and slope of a city's median Air Quality Index (AQI) over multiple years of EPA monitoring data. AirHistory tracks 10-year trends across 1,020 U.S. cities — the slope is computed as the year-over-year change in median AQI. A negative slope means cleaner air; positive means worsening.

How many U.S. cities have improving air quality?

Of the 1,020 cities tracked here, 346 (34%) are on a clear improving trend, 244 (24%) are worsening, and 430 (42%) are roughly stable. The national pattern over the past decade is improvement, driven by cleaner vehicles, retirement of coal plants, and tighter EPA emission rules.

Why is air quality getting worse in some cities?

Most cities with worsening 10-year trends sit in the Western U.S. and reflect rising wildfire smoke exposure rather than new local emissions. A smaller number of metros see local upticks driven by population and freight growth, especially around port and warehouse corridors. Trend reports break down which factor dominates in each region.

How often are these reports updated?

Trend reports are recomputed each time EPA publishes new annual AQI data — typically once per year for the prior calendar year. The current dataset was last refreshed April 2026. Each underlying city profile includes year-by-year detail so you can see exactly when a trend turned.

Where does the underlying data come from?

Every reading comes from the EPA Air Quality System (AQS), which aggregates measurements from federally certified monitoring stations across all 50 states. Annual AQI by County CSVs are downloaded from aqs.epa.gov, processed into per-city profiles, and used to compute trends. All data is U.S. government public domain.

Source: U.S. Environmental Protection Agency, Air Quality System (AQS). Annual AQI by County data — public domain. See aqs.epa.gov/aqsweb/airdata.

Last updated 2026-04-06 · 1,020 cities tracked.