A clear sky can still hold pollution, and a hazy skyline can hide several different pollutants at once. AQI exists because raw concentrations are hard to compare quickly. Ozone, PM2.5, PM10, carbon monoxide, sulfur dioxide, and nitrogen dioxide use different units and different averaging periods.
AQI is a shared outdoor air-quality language
The U.S. Air Quality Index is EPA's index for reporting outdoor air quality. It translates pollutant concentrations into a scale with six color-coded categories: Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous.
The key idea is simple: as the AQI value rises, pollution and health concern rise as well. An AQI at or below 50 represents the Good category, while values above 100 move into levels considered unhealthy for at least some groups.
Why AQI categories matter
AQI categories make a number readable at a glance. A value of 42 and a value of 92 are both below 100, but they do not communicate the same air-quality context. The category color and label help separate a low-pollution day from a day that may call for more caution, especially for people who are unusually sensitive to air pollution.
The EPA technical guidance also describes pollutant-specific breakpoints and sensitive groups. PM2.5, PM10, ozone, carbon monoxide, nitrogen dioxide, and sulfur dioxide do not share the same concentration ranges, so each pollutant needs its own conversion before an AQI value can be reported.
How a measurement becomes an index
Air-quality systems often begin with physical measurements. OpenAQ describes its platform as sharing near real-time and historical data in physical units rather than as an air quality index. That distinction matters: a PM2.5 reading can be a concentration, while an AQI score is an interpretation of that concentration against a defined scale.
Daily AQI reporting commonly identifies the critical pollutant, meaning the pollutant with the highest AQI value for the reporting period. If PM2.5 produces the highest index value, PM2.5 drives the AQI category even when other pollutants are also present.
Why AQI is useful before outdoor movement
AQI does not measure personal exposure along a route. It does, however, help turn changing outdoor air into planning context. A walking or jogging decision usually depends on time, place, distance, freshness of data, and route conditions. AQI supplies one part of that context.
PM2.5 research remains important because fine particles are linked with cardiovascular and respiratory concerns. Current studies continue to examine associations even at relatively low ambient PM2.5 concentrations, which is one reason AQI context can remain useful outside obvious smoke or haze events.
Sources
- AirNow AQI Basics: AQI definition, categories, color scale, and five major pollutants.
- EPA Technical Assistance Document for AQI reporting: pollutant breakpoints, critical pollutant reporting, and category mechanics.
- OpenAQ API overview: physical-unit measurements, near real-time and historical data, and metadata context.
- Environment International UK Biobank PM2.5 study: long-term PM2.5 exposure and cardiovascular admissions research context.
Plan with AQI in context
Air Quality Router brings estimated AQI, route distance, timing, and freshness labels into one planning surface for walking and jogging. Download Air Quality Router on the Apple App Store to compare cleaner route options before an outdoor routine.
Download on the Apple App StoreAir Quality Router is an informational planning tool, not medical advice. Personal health decisions belong with qualified care guidance.