Is the Air Getting
Better or Worse?

AirHistory is a public data project that turns a decade of EPA Air Quality System (AQS) monitor readings into something residents, journalists, and researchers can actually use. The federal government collects exhaustive air-quality data — every monitored county reports daily AQI by pollutant, going back to the 1980s — but the raw files are designed for atmospheric scientists, not for the parent who just wants to know whether the air around their kid's school is getting better or worse.

We aggregate every monitor in every county into annual averages for each of the six "criteria pollutants" the EPA tracks (ground-level ozone, PM2.5, PM10, carbon monoxide, nitrogen dioxide, and sulfur dioxide), then compute a single A-F Air Quality Grade for each city. The grade weights four signals: the 5-year median AQI (40%), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). A city that posts low AQI today but has been steadily worsening earns a lower grade than one with the same current AQI on a cleaner trajectory — and rightly so, because the trajectory is what residents experience over time.

The 10-year window matters more than today's number. Real-time AQI swings wildly with weather, wildfires, and seasonal cycles; a city's long-run grade tells you what living there actually means for your lungs over years and decades. Harvard's Six Cities Study and the long-running EPA cohort research consistently find that long-term PM2.5 exposure is associated with measurable reductions in life expectancy, with each 10-µg/m³ reduction worth roughly 0.5 to 1.0 added years on average. Day-to-day AQI alerts are still useful for sensitive groups — but the question "is this a healthy place to live?" is a multi-year question, and that is what these grades answer.

Browse by cleanest cities, worst air quality, fastest-improving, by AQI range, or by dominant pollutant. Every city page shows the full 10-year trend chart, year-by-year unhealthy day counts, the pollutant breakdown, and how the city compares to its state and national peers. All data comes directly from the EPA AQS — see the methodology for the full grade formula and known caveats.