Methodology & Data Sources

US Air Quality is committed to transparency in how we collect, process, and present air quality information. This page explains our data sources, methodology, and how to interpret the information on our site.

Primary Data Sources

All air quality data on US Air Quality comes from official U.S. government sources. We aggregate and present this data to make it more accessible, but we do not create or modify air quality measurements.

EPA AirNow

Our primary data source. AirNow is operated by the U.S. Environmental Protection Agency (EPA) in partnership with NOAA, NPS, NASA, CDC, tribal, state, and local air quality agencies. AirNow provides official Air Quality Index (AQI) values, pollutant measurements, and forecasts from over 2,000 monitoring stations across the United States.

Visit AirNow.gov

EPA Air Quality System (AQS)

Historical air quality data and long-term trends come from the EPA's official Air Quality System database. AQS contains ambient air pollution data collected by EPA, state, local, and tribal air pollution control agencies from thousands of monitors across the nation.

EPA Air Quality Data

State Air Quality Agencies

State environmental and air quality agencies operate monitoring networks and provide local expertise on air quality conditions. Their data is incorporated into the EPA's national systems and helps ensure comprehensive coverage across all regions.

How We Collect the Numbers You See

Every AQI value on this site is a real measurement published by EPA AirNow. Nothing is modelled, estimated or interpolated. Here is the pipeline, end to end:

1. We read AirNow's reporting-area feed
Each build pulls AirNow's public reporting-area file and keeps only records AirNow flags as observed for the current day — no forecast values are mixed into the current-conditions numbers.
2. Each city is matched to a reporting area
103 of our 149 cities match an AirNow reporting area directly. For 43 more, AirNow has no area of that exact name, so we use the nearest reporting area within 50 km and say so on the page — the city page names the substituted area and the distance, so a nearest-monitor reading is never presented as a reading from downtown.
3. The reporting agency is credited per city
AirNow identifies which air district or state agency reported each area — South Coast AQMD, TCEQ and so on. We print that agency alongside the reading so you can trace any number back to the body that measured it.
4. Gaps stay gaps
In the current snapshot 3 of 149 cities have no usable observation. Those pages say so plainly and point to nearby cities that do have data. We never fill a gap with a placeholder or an average.
5. The snapshot is frozen at publish time
This site is a set of static pages, so the observations are captured when we publish and do not move until we publish again — several times a month. That means a reading is usually a few hours old and can be a few days old. Rather than paper over it, every city page prints the observation time behind its number. The snapshot behind the pages you are reading was captured August 20, 2026 at 12:32 AM UTC. For minute-by-minute conditions, AirNow.gov is the live source.

How the Air Quality Index (AQI) is Calculated

The Air Quality Index (AQI) is a standardized system developed by the EPA to communicate daily air quality conditions. The AQI translates complex air quality data into a simple 0-500 scale that the public can easily understand.

AQI Calculation Steps:

Pollutant Measurement: Monitoring stations measure concentrations of five major pollutants: ground-level ozone (O3), particulate matter (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2).
Breakpoint Conversion: Each pollutant concentration is converted to an AQI value using EPA-established breakpoint tables that relate pollutant concentrations to health effects.
Primary Pollutant: The pollutant with the highest AQI value becomes the "primary pollutant," and its AQI value becomes the overall AQI for that location.

AQI Categories:

0-50: Good - Air quality is satisfactory

51-100: Moderate - Acceptable for most people

101-150: USG - Unhealthy for sensitive groups

151-200: Unhealthy - Everyone may experience effects

201-300: Very Unhealthy - Health alert

301-500: Hazardous - Emergency conditions

Data Update Frequency

Data Type	Update Frequency	Source
Observed AQI	Re-captured on every publish (several times a month)	EPA AirNow reporting-area feed
AQI Forecasts	Issued twice daily by EPA; captured on every publish	EPA AirNow
Historical Averages	Annually (current year: 2026)	EPA AQS Database
City Rankings	Monthly	Calculated from EPA data

Data Limitations

Important Considerations

Monitor Coverage: Air quality monitors are not evenly distributed. Rural areas and some neighborhoods may be far from the nearest monitoring station. Data represents regional conditions, not necessarily your exact location.
Localized Conditions: Air quality can vary significantly over short distances due to local emissions sources, terrain, and microclimates. Highway corridors, industrial areas, and valleys may have different conditions than reported city averages.
Data Age: Our pages are static, so each AQI value is frozen at the moment we captured it. A reading is typically a few hours old and can be a few days old between publishes. Every city page prints its observation time so you never have to guess.
Pollutant Coverage: Not all monitors measure all pollutants. Some areas may have data for PM2.5 but not ozone, or vice versa. We display available data and note when certain pollutants are not monitored.
Indoor Air Quality: AQI measures outdoor air quality only. Indoor air quality depends on ventilation, filtration, and indoor sources and may differ significantly from outdoor conditions.

How to Use This Data

For Daily Activity Planning

Check the current AQI and forecast before planning outdoor activities. When AQI is elevated, consider rescheduling strenuous outdoor exercise or moving activities indoors. Pay attention to the primary pollutant, as different pollutants affect people differently.

For Sensitive Groups

People with asthma, heart disease, respiratory conditions, the elderly, children, and those who exercise outdoors should take precautions at lower AQI levels than the general population. Consult the EPA's guidance for your specific health condition.

For Relocation Decisions

Use historical averages and annual statistics rather than single-day readings when comparing cities. Consider seasonal variations, as air quality can differ significantly between summer and winter or during wildfire season.

For Official Information

For live, minute-by-minute conditions, we recommend visiting AirNow.gov directly. Our site aggregates and presents EPA data but AirNow is the official source.

Health Information Disclaimer

This website provides general information about air quality based on EPA data and guidance. It is not intended as medical advice and should not replace consultation with healthcare providers.

If you have respiratory conditions, heart disease, or other health concerns that may be affected by air quality, please consult with a healthcare provider for personalized guidance. In case of health emergencies, contact emergency services or your doctor immediately.

Questions About Our Methodology?

Have questions about our data sources, methodology, or how to interpret air quality information? We welcome your feedback and inquiries.