How to Find Historical Air Quality Data for Your City or County
Historical air quality records for U.S. locations come from the EPA's network of outdoor monitors, and the main public gateway to them is the EPA AirData portal. From there you can pull past AQI values, monitored concentrations of the criteria pollutants, and summary reports for a city, county, or monitoring site, going back years. AirNow covers current and very recent conditions; AirData is where the archive lives.
What's actually in the records
The historical data are monitor measurements of the six criteria pollutants — ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead — the same pollutants with legal limits under the EPA's criteria pollutant program. Records come in two flavors, and it helps to know which you want before you search:
- Concentrations — the raw measured values in ppm, ppb, or µg/m³, tied to a specific monitor and averaging period. These are what regulators compare against standards.
- AQI values — the same measurements translated onto the familiar 0-to-500 scale. The translation rules are defined in the EPA's technical assistance document for reporting the AQI. If you're unsure how a concentration becomes an AQI number, see how the AQI is calculated.
The main tools, and what each is for
The AirData portal offers several ways in, suited to different questions:
- Daily AQI lookups answer "what was the air like in my county on a given date?" — useful for checking a past smoke event or comparing this summer to last.
- Summary and trend reports answer "is my area's air getting better or worse?" by aggregating years of data.
- Raw data downloads serve anyone who wants the underlying measurements for a spreadsheet or a school project — selectable by pollutant, geography, and time range.
- Interactive maps show where the monitors themselves are located, which matters for the caveat below.
For context on how these monitors feed the daily reports you see in the news, we've covered where daily air quality reports come from.
Tips and caveats before you search
- Data is tied to specific monitoring sites. A county's record reflects where its monitors sit, and not every county has a monitor for every pollutant. If your county lacks a monitor, the nearest data may come from a neighboring county and may not represent your exact neighborhood.
- Know which pollutant you care about. A location can have a long ozone record and a short PM2.5 record, because monitors for different pollutants were installed at different times.
- Mind the averaging period. Daily AQI values, hourly concentrations, and annual summaries answer different questions; pick the one that matches yours.
- Recent days may be provisional. Fresh data can be updated after quality checks, so the newest entries are not always final. AirNow is the better tool for current conditions and how to interpret today's category.
- Old AQI values reflect the breakpoints of their era. The EPA occasionally revises AQI breakpoints when standards change, so comparing very old AQI numbers to new ones is less reliable than comparing the underlying concentrations.
What this data can't tell you
Monitored history describes outdoor, ambient air at monitor locations. It won't tell you about indoor air, about a specific street's conditions between monitors, or about pollutants outside the criteria list — air toxics are tracked under a separate system, as explained in criteria pollutants vs. hazardous air pollutants.
Within those limits, the archive is deep and genuinely public: for locations with monitors — which include most metropolitan areas — years of daily readings and long-term trends are a straightforward lookup away.