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.govEPA 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 DataState 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 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:
Data Update Frequency
| Data Type | Update Frequency | Source |
|---|---|---|
| Real-time AQI | Hourly | EPA AirNow API |
| AQI Forecasts | Daily (morning and afternoon) | 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.
- Reporting Delays: While we display hourly data, there may be occasional delays in data transmission from monitoring stations to the EPA system and to our site.
- 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 the most authoritative and real-time information, 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.
Contact Us