Updated April 2026 · EPA AQS data
Air Quality Guides
In-depth, data-driven guides to understanding air quality, protecting your health from particulate and ozone pollution, and finding the U.S. metros where clean air is the rule rather than the exception. Every guide is grounded in 10 years of EPA Air Quality System monitoring data from 1,020 cities.
Why These Guides Exist
The EPA's AirNow program publishes the daily air quality numbers, but the underlying questions people actually have — "is the air I'm breathing today actually safe?", "is my city getting better or worse?", "should I buy an air purifier?" — usually need context that a single AQI reading doesn't provide. These guides translate raw EPA data into practical, sourced answers.
Topics span health implications of long-term PM2.5 exposure, what each AQI category means in daily-life terms, how to read a city's 10-year trend, and which metros consistently rank among the cleanest. All numbers in the guides are computed directly from EPA AQS Annual AQI by County data — no estimates, no proxies. Every page links back to the source so you can verify.
The library currently spans 3 guides across 3 categories: Analysis, Education, Health.
Browse All Guides
Understanding Air Quality: What the AQI Really Means
A comprehensive guide to the Air Quality Index, how it works, what each level means for your health, and why 10-year trends matter more than daily readings.
April 10, 2026
HealthHow Air Pollution Affects Your Health and What to Do About It
What happens when you breathe polluted air, who is most at risk, and evidence-based steps to reduce your exposure based on AQI levels.
April 10, 2026
AnalysisThe Cleanest and Most Polluted Cities in America: A 10-Year Analysis
A data-driven look at which US cities have the best and worst air quality, what drives the differences, and which cities are improving fastest.
April 10, 2026
Frequently Asked Questions
What does AQI mean?
The Air Quality Index (AQI) is the EPA's standardized score for daily air quality, running 0 to 500. 0–50 is "Good," 51–100 is "Moderate," 101–150 is "Unhealthy for Sensitive Groups," 151–200 is "Unhealthy," 201–300 is "Very Unhealthy," and 301+ is "Hazardous." The AQI is calculated separately for each major pollutant (PM2.5, PM10, ozone, NO2, CO, SO2) and the daily reading is whichever pollutant scores worst.
Which pollutants matter most for health?
Fine particulate matter (PM2.5) is the pollutant most strongly linked to cardiovascular disease, respiratory illness, and premature death — it penetrates deep into the lungs and bloodstream. Ground-level ozone is next, especially for people with asthma, and is the dominant summer pollutant in sunny inland metros. PM10, NO2, CO, and SO2 round out the criteria pollutants the EPA tracks; concentrations of NO2 and CO have fallen sharply since the 1990s thanks to vehicle and industrial emission controls.
How can I protect myself on bad-air days?
On flagged days, sensitive groups should limit outdoor exertion, keep windows closed, and run a HEPA air cleaner sized to a bedroom or family room (a properly sized unit can cut indoor PM2.5 by 80%+ during a smoke event). N95 or KN95 masks block PM2.5; surgical masks and cloth masks do not. Sign up for AirNow.gov alerts at your specific zip code rather than relying on city-level averages.
Where do AirHistory's guides get their data?
Health and behavioral guidance follows the EPA AirNow program (https://www.airnow.gov/) and the WHO global air quality guidelines. City rankings and trend numbers come from the EPA Air Quality System (AQS), which aggregates measurements from federally certified monitors. Across the dataset, the cleanest 5-year median AQI on record is in Alexandria City, Virginia (median 6), and the worst is Maricopa, Arizona (median 90).
Are the cleanest U.S. cities also the safest to live in?
For air quality, yes — sustained low PM2.5 and ozone exposure correlates with measurably lower rates of asthma hospitalization, cardiovascular events, and pollution-attributable mortality. That said, "best place to live" is multidimensional: cities with cleaner air sometimes carry tradeoffs in cost of living, climate risk, or job access. The right read is to use AQI as one input alongside the rest of your decision factors.
Methodology in Brief
City-level numbers come from EPA AQS Annual AQI by County CSVs (2014 onward), aggregated to a 5-year median AQI per city. Trends are computed by linear regression on the annual medians; grades use a four-factor weighted score. Health guidance is sourced from EPA AirNow and the World Health Organization's global air quality guidelines. Read the full methodology.
Source: U.S. EPA Air Quality System; EPA AirNow; WHO Global Air Quality Guidelines.
Last updated 2026-04-06 · 1,020 cities · 3 guides published.