The American Lung Association released its 27th annual "State of the Air" report this year, and the numbers behind it are worth more than a passing headline. This report has tracked ozone and particle pollution across the country since 2000, using three years of EPA monitoring data at a time, this edition covering 2022 through 2024. Below is a closer look at what the report actually measures, where the air is worst and cleanest in the country, how the grading works, and how the underlying monitoring data gets collected in the first place.
What Actually Gets Graded
The report doesn't produce a single air quality score. It grades every county with monitoring data on three separate measures, and each one reflects a different kind of pollution problem:
- Ozone. Ground-level ozone forms when nitrogen oxides and volatile organic compounds, largely from vehicle exhaust and industrial emissions, react in sunlight. It's worse on hot, sunny, still days, which is why ozone grades tend to track closely with heat and sun exposure in a region.
- Year-round particle pollution. This measures the annual average concentration of fine particles (PM2.5) in the air, reflecting chronic, ongoing exposure rather than any single bad day.
- Short-term particle pollution. This tracks how many days a county spikes into unhealthy PM2.5 levels, the kind of acute event typically driven by wildfire smoke, temperature inversions, or localized pollution sources.
A county can fail one, two, or all three. This year, about 10% of American children, roughly 7.3 million kids, live in counties that failed all three measures, and nearly 33 million people of all ages live somewhere that failed across the board.
Why Kids Carry the Heaviest Load
Children breathe more air relative to their body size than adults do. Their lungs are still developing, and they tend to spend more time outdoors. That combination is part of why the report weights pediatric exposure so heavily in its analysis, and why the pediatric numbers, 46% of all U.S. children living somewhere with at least one failing grade, get called out specifically each year.
How Ozone and Particle Pollution Affect the Body
The report frames these as public health measures for a reason. The two pollutants act differently once inhaled:
- Ozone irritates the lungs directly, causing inflammation that's often described as similar to a sunburn on lung tissue. Repeated exposure can worsen asthma, reduce lung function over time, and increase susceptibility to respiratory infection.
- Particle pollution is small enough (PM2.5 is roughly 1/30th the width of a human hair) to pass deep into the lungs and, in some cases, into the bloodstream. It's linked to increased risk of heart attack, stroke, lung cancer, and premature death, along with more emergency room visits for respiratory and cardiac events on high-particle days.
Where the Air Is Worst
There's no single "worst city" in the report. Three different metro areas topped the three separate pollution categories this year:
| Measure | Worst-Ranked Metro Area | Detail |
|---|---|---|
| Ozone | Los Angeles-Long Beach, CA | 159.2 unhealthy ozone days a year; worst in the nation in 26 of the last 27 reports |
| Year-round particle pollution | Bakersfield-Delano, CA | Worst nationally for a seventh consecutive year |
| Short-term particle spikes | Fairbanks, AK | Newly ranked worst in the country this edition |
California's Central Valley shows up repeatedly across these lists, six metro areas in the state land on all three Worst 25 lists, largely due to a combination of geography (mountains on three sides trap pollutants), heavy agricultural and vehicle traffic, and increasingly frequent wildfire smoke. Fairbanks' new ranking reflects a similar story: temperature inversions trap wood-smoke and vehicle emissions close to the ground during cold months.
Where the Air Is Cleanest
The report also tracks the other end of the spectrum. Fifty-five cities recorded zero days of unhealthy ozone, and 75 recorded zero short-term particle pollution spikes. Only one city, Bangor, Maine, made all three cleanest-city lists this year, earning an A grade for both ozone and short-term particle pollution while also ranking among the 25 cities with the lowest year-round particle levels.
State-by-State Progress Isn't Uniform
National totals can mask real regional differences. This year, 120 counties across 23 states received failing grades for short-term particle pollution, down from 154 counties across 27 states in the prior report, an improvement, though still the second-largest count in the last 17 editions. Five states, Colorado, Connecticut, New Hampshire, South Dakota, and Vermont, along with Washington, D.C., saw every one of their graded counties improve on fine particle pollution compared to the previous report. Nevada posted the single largest statewide average improvement in the country.
A Few Things Worth Knowing
Not every county agrees with its grade
Two long-term trends are pulling in opposite directions
The data comes from a mix of monitoring technologies
How the Underlying Monitoring Works
Every grade in this report ultimately traces back to a physical monitor sitting somewhere in the country, run by a federal, state, local, or Tribal agency. Air quality networks generally rely on two types of instruments, each suited to a different job.
| Factor | FRM (Filter-Based) | Continuous Monitoring |
|---|---|---|
| Measurement principle | Direct gravimetric, filter weighed before/after | Indirect, light-scatter or beta attenuation |
| Reporting frequency | Every 24 hours, per sampling run | Real time, typically every minute |
| Regulatory standing | 40 CFR 50 reference method for NAAQS compliance | Supplemental unless separately FEM certified |
| Best suited for | Regulatory compliance determinations | Real-time public alerts, trend tracking |
Reports like this one draw on both. Continuous sensors are what feed the real-time air quality index numbers most people check on their phones, while FRM data underpins the official compliance determinations that decide whether a county meets national air quality standards in the first place. Neither method alone tells the full story, which is part of why the national monitoring network is built from a combination of the two.
How the Grades Are Calculated
The Lung Association pulls three years of quality-assured monitoring data directly from the EPA, this year covering 2022 through 2024, and calculates a weighted average of unhealthy air days for each county and pollutant measure. Cities are then ranked by that weighted average, which is why a city can appear on one "Worst 25" list and not another: a place can have a severe ozone problem while having comparatively low year-round particle levels, or vice versa.
Key Takeaways
What this year's report actually shows
- 152.3 million Americans live in counties with at least one failing grade
- 46% of children live in a county that failed at least one measure; 10% live in counties failing all three
- Los Angeles, Bakersfield-Delano, and Fairbanks each top a different pollution category, geography and climate factor heavily into all three
- Only Bangor, Maine made all three cleanest-city lists this year
- Short-term particle pollution failures dropped from 154 to 120 counties year-over-year, a real improvement, though still historically high
- Clean Air Act progress and climate-driven wildfire smoke are pulling national air quality trends in opposite directions
- Some counties dispute their grades, an active and legitimate debate in the field
None of the above changes the report's core finding. A large share of Americans, and a disproportionate share of children, are living with unhealthy air. That part of the report isn't in question. Understanding how the grades are built, where the pollution is concentrated, and where reasonable people disagree just helps put any single number in context.