How We Grade Air Quality: Scoring Methodology, AirHistory

AirHistory grades each of 1,020 U.S. cities on an A-F Air Quality Grade composite that weights 5-year median AQI (40 percent), 10-year trend direction (30 percent), unhealthy days per year (20 percent), and dominant pollutant severity (10 percent). All inputs are sourced from EPA Air Quality System monitoring data.

Data Sources

Our primary data source is the U.S. Environmental Protection Agency Air Quality System (AQS), the authoritative federal database of ambient air quality measurements collected by state, local, and tribal monitoring agencies. Specifically, AirHistory uses the Annual AQI by County CSV files published at aqs.epa.gov/aqsweb/airdata, covering ten years of data from 2014 through the most recent complete EPA reporting year. Real-time monitor readings come from the EPA AirNow network, although AirHistory itself focuses on the longer-run AQS aggregates rather than streaming real-time data.

The AQS dataset includes daily Air Quality Index (AQI) readings, pollutant breakdowns (PM2.5, ozone, CO, SO2, NO2, PM10), and the number of days at each AQI category — Good (0-50), Moderate (51-100), Unhealthy for Sensitive Groups (101-150), Unhealthy (151-200), Very Unhealthy (201-300), and Hazardous (301+). Where health context is needed, AirHistory references the World Health Organization Air Quality Guidelines, which publish health-protective concentration thresholds that are generally tighter than U.S. National Ambient Air Quality Standards.

How We Calculate the Air Quality Grade

Every city receives a proprietary Air Quality Grade on a 0-100 scale, mapped to letter grades A through F. The score is a weighted composite of four factors taken directly from the EPA AQS dataset:

5-year median AQI, 40 percent weight. The median (not mean) annual median AQI across the most recent five complete EPA reporting years. Median is used because AQI distributions are right-skewed and a handful of wildfire days can swing a mean unrepresentatively. Lower values produce a higher score, normalized so a city at the national city-level median scores roughly 50 on this dimension.
10-year trend direction, 30 percent weight. A linear regression slope fit across the full 10-year history of annual median AQI values. Negative slopes (improving air) score higher; positive slopes (worsening) are penalized. Trend bucket labels are: Improving (slope ≤ −0.5 AQI/year), Worsening (slope ≥ +0.5 AQI/year), Stable (between).
Unhealthy days per year, 20 percent weight. The average number of days per year rated "Unhealthy for Sensitive Groups" or worse across the past five years. Fewer unhealthy days produce a higher score. This is the count most relevant for residents with asthma, COPD, heart disease, pregnancy, young children, or older adults.
Dominant pollutant severity, 10 percent weight. The peak concentration of the city's dominant pollutant in the most recent year, normalized against the EPA National Ambient Air Quality Standard for that pollutant. Cities whose worst pollutant sits at or below the federal standard score higher.

Letter grades map as follows: A (80-100), B (65-79), C (50-64), D (35-49), F (0-34). Of the 1,020 cities currently in the dataset, the distribution skews B-to-D for typical metro areas, with A reserved for genuinely clean low-population regions and F reserved for cities with sustained high AQI plus worsening trends.

Data Collection and Cleaning

AirHistory downloads annual AQI CSV files from the EPA AQS bulk data portal, parses county-level measurements, and maps them to cities using U.S. Census place-to-county crosswalks. Monitoring stations with fewer than 200 valid measurement days in a year are dropped to avoid trend noise from poorly-instrumented sites. Where multiple monitoring stations exist within a single city, readings are population-weighted to better approximate per-capita exposure rather than averaging an industrial-corridor monitor against a residential-neighborhood monitor as if they were equivalent.

Cities without a dedicated monitor inherit county-level readings from the EPA AQS file. This is a known limitation: hyperlocal pollution near a specific freeway, port, or industrial site will not show up in the county-level summary. For monitor-by-monitor detail, the EPA AQS interactive map at aqs.epa.gov is the authoritative source, and the U.S. Department of Labor, the U.S. Department of Health and Human Services, and the EPA itself publish supplemental local-context data that AirHistory does not currently merge.

Update Frequency

The EPA publishes Annual AQI summary files each spring covering the prior calendar year. AirHistory refreshes the dataset within roughly two weeks of each EPA release, typically in March or April. Historical data is stable and does not change retroactively unless the EPA reissues a corrected dataset, in which case the affected city profiles are recomputed and a revision date is logged. The current dataset was last refreshed April 2026.

Known Limitations

Not all cities have a dedicated EPA monitor. Cities without one inherit county-level data, which can mask hyperlocal variation near freeways, ports, or industrial corridors.
Wildfire smoke days are not excluded from trend calculations. This faithfully represents real exposure but means Western U.S. cities show worsening trends driven primarily by smoke even when local emissions continue to fall.
Indoor air quality is not measured. AQS data reflects outdoor ambient conditions only, so a HEPA-filtered indoor environment can deviate sharply from the AQI on a given day.
The Air Quality Grade is AirHistory\'s own composite metric, not an official EPA designation.
Data is reported at the city level. For real-time hourly readings at your specific zip code, the EPA AirNow portal is the right tool.

How to Cite AirHistory Data

If you use data from AirHistory in research, journalism, or policy analysis, please cite both AirHistory (for the composite Grade and trend computations) and the EPA AQS (for the raw underlying readings):

AirHistory. "[City Name] Air Quality Data." airhistory.org, 2026. Accessed [date].
U.S. Environmental Protection Agency. "Air Quality System (AQS), Annual AQI by County." aqs.epa.gov/aqsweb/airdata. Accessed [date].

Underlying EPA data is in the public domain. AirHistory analyses are free to cite with attribution.

Frequently Asked Questions

Is the Air Quality Grade an official EPA designation?

No. The Air Quality Grade is AirHistory's own composite metric, calculated from EPA AQS data. The EPA itself does not publish A-F grades for cities — the agency reports raw AQI values, days in each AQI category, and annual summaries. The Grade is a transparent way to combine those signals into a single score for casual readers, and the methodology below documents exactly how the four inputs roll up.

Why do you use a 5-year median instead of a single-year average?

A single year can be skewed badly by a wildfire complex, an unusually hot ozone summer, or one extreme weather event that traps pollution. The 5-year median smooths out those single-year shocks while still being responsive to genuine trend changes. Median (rather than mean) is used because AQI distributions are right-skewed: a handful of bad days can drag the mean upward in a way that does not reflect the typical day a resident experiences.

How do you handle wildfire smoke in trend calculations?

Wildfire smoke is included as-is. The EPA does not exclude wildfire days from official AQI summaries, and AirHistory mirrors that choice because residents living downwind of fires really do breathe that air. Western U.S. cities therefore show worsening 10-year trends driven primarily by smoke even when local emissions continue to fall — that is a real exposure pattern, not a methodology artifact, and articles in the blog drill into it specifically.

What if a city has no EPA monitoring station?

Cities without a dedicated monitor inherit the readings from their county-level EPA monitor, since the EPA's Annual AQI by County dataset is the best available systematic source. This is a known limitation: hyperlocal pollution near a specific freeway, port, or industrial site will not show up in the county-level summary. Where this matters — for example, in environmental justice contexts — readers should consult the EPA AQS interactive map for monitor-by-monitor detail.

How do I cite AirHistory data in my own work?

Cite AirHistory for the composite Air Quality Grade and trend computations: "AirHistory, [city] Air Quality Data, airhistory.org, accessed [date]." Cite the EPA Air Quality System for the raw underlying readings: "U.S. Environmental Protection Agency, Air Quality System (AQS) Annual AQI by County, aqs.epa.gov/aqsweb/airdata, accessed [date]." All EPA data is U.S. government public domain and free to redistribute.

AirHistory Methodology