Every figure on PlainAir originates in a public EPA dataset. We download the annual AQI summary files published by the U.S. Environmental Protection Agency's Air Quality System (AQS), load them through a documented, version-controlled data pipeline, and render them into county, metro, and state profiles using shared templates. No profile is hand-written, and no number is typed in by an editor. Each value you see is read directly from the official source record.

Our editorial team is responsible for the parts a pipeline cannot decide on its own: which datasets to use, how each metric is defined and labeled, what the methodology says, which derived measures (such as a median-AQI ranking or a state aggregate) are computed and how, and what we will not publish. The pipeline then applies those decisions uniformly across the whole country, so the rule that governs one county's page governs all of them.

We publish only data that comes from official government sources, and we name the source on every page. Our datasets are:

• EPA Air Quality System (AQS) annual summaries: annual AQI by county and CBSA for 2020–2024 — good, moderate, and unhealthy day counts, median and maximum AQI, and primary-pollutant breakdowns, from the official AirData bulk download files.
• EPA Outdoor Air Quality Data: the underlying ambient monitoring program that feeds AQS.
• EPA AirNow: referenced for current conditions; PlainAir itself presents historical annual data, not the real-time AirNow feed.

We do not scrape third-party sites, we do not model or interpolate values for unmonitored areas, and we do not generate any air-quality readings ourselves. Where a figure is derived from the official data (for example, a state average or a median-AQI rank), the page links to our methodology, which sets out exactly how it is calculated.

Because the data is read straight from EPA files, the most significant limitation is coverage: the EPA only requires monitors where population density, emissions sources, or attainment status warrant them, so roughly 995 of the 3,144 U.S. counties host an active monitor and the rest are simply unmonitored — not necessarily clean, just unmeasured. State and metro figures are averages of the monitored counties within them, not of the whole geography. Our pipeline applies systematic checks before a value is published: it screens out implausible outliers, shows a value as unavailable rather than guessing when the source omits it, flags AQI readings above 500 as beyond the EPA's standard 0–500 scale (typically extreme wildfire-smoke events), and reconciles derived measures against the full population they are drawn from.

When we find that a displayed number is wrong, we fix the cause, not the symptom. We trace the value back to the data layer, correct the derivation or screening rule there, and regenerate the affected pages, so the same class of error is resolved everywhere at once rather than patched on a single page.

PlainAir does not accept payment, sponsorship, or promoted placement from government agencies, environmental organizations, manufacturers, or any covered entity. Our only revenue source is contextual display advertising served by Google AdSense. Advertisers have no influence over which areas or data we cover or how it is presented, and they receive no preferential placement. We publish no subjective ratings and take no editorial position on whether a given place is "safe" or "dangerous"; we present the official EPA data and the caveats that come with it, and leave the judgment to you and the guidance of your local air agency.

The EPA publishes its annual AQI summary files typically within 6–9 months of the reference year's end, and 2024 is the most recent complete year available. We refresh our database to incorporate each new annual release and re-stamp the affected pages so the published date reflects when the content genuinely changed — not the day you happen to load the page. The data vintage in effect is named on every data page, on our About page, and in our methodology.

If a figure looks wrong, please tell us. We treat data-error reports as a priority and follow the same process every time:

1. Report. Email hello@plainairdata.com with the page URL and the figure you are questioning.
2. Verify. We check the value against the official EPA AQS source record for that county, metro, or state.
3. Fix at the source. If the figure is wrong on our side, we correct the underlying data or derivation rule and regenerate every page it affects.
4. Note it. If the figure is correct but reflects a known characteristic of the source data (for example, sparse monitoring or a wildfire-driven reading above the standard AQI scale), we explain the caveat rather than silently changing it.

Some apparent errors trace back to the EPA source itself: monitoring coverage changes year to year as stations come online or are decommissioned, and annual summaries reflect only days a monitor actually reported. When that is the case, we will tell you so and point you to the official EPA AQS record so you can verify it directly.

Questions about our standards, methodology, or a specific figure are welcome at hello@plainairdata.com. For more on what the data covers and how it is processed, see our About page and methodology.