How to Read the Air Quality Index (AQI)

What the AQI is

The Air Quality Index is the standardized scale the United States uses to report daily outdoor air quality. It runs from 0 to 500, and the official U.S. government explanation of the scale is maintained at AirNow's AQI Basics page. The core idea is simple: the higher the number, the more polluted the air, and the greater the level of concern. An AQI value of 100 is pegged to the level of the national ambient air quality standard for a pollutant, so values at or below 100 generally correspond to air that meets the federal standard, and values above 100 correspond to air that does not.

The index is divided into six color-coded categories, from green (good) through yellow, orange, red, and purple to maroon (hazardous). Each category spans a range of index values and carries a standard label. The colors are consistent everywhere the AQI is reported — weather apps, news broadcasts, and highway signs all use the same scheme — which is what makes the index useful at a glance.

What the number is actually measuring

A single AQI value is not a blend of everything in the air. The index is calculated separately for each of several major pollutants regulated under the Clean Air Act — including ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide — and the reported AQI for a location is the highest of those individual values at that time. EPA's overview of these criteria air pollutants explains what each one is and how the national standards for them are set.

This matters when you read a forecast. If your app reports an AQI of 130, one pollutant — often ozone in summer afternoons, or fine particles during wildfire smoke events — is driving that number. Many AQI displays name the "primary pollutant" for exactly this reason. Two days with the same index value can involve entirely different pollutants.

Reading the two pollutants that dominate the index

In practice, most elevated AQI days in the U.S. involve one of two pollutants:

• Ozone at ground level forms in sunlight from precursor emissions, which is why ozone-driven AQI values tend to peak on hot, sunny afternoons and fall in the evening.
• Particle pollution (PM2.5 and PM10) refers to solid and liquid particles small enough to stay suspended in air. Fine particles, PM2.5, are the ones most associated with smoke and haze, and research summarized by the National Institute of Environmental Health Sciences explains why particle size is central to how scientists study air pollution's effects.

What the AQI is not

The AQI describes outdoor ambient air measured at monitoring stations. It does not describe the air inside your home, a specific street canyon, or the immediate vicinity of a local source like a fire or an industrial release. It is also a communication tool, not a diagnosis: the health guidance attached to each category is written by EPA and its partners, and the authoritative version of that guidance is the AirNow site itself. If you have questions about what a given air quality level means for you personally, that conversation belongs with a medical professional — the index tells you about the air, not about your situation.

For global context, the World Health Organization publishes its own fact sheet on ambient air pollution and its worldwide burden, linked in the sources below; note that WHO guideline values are distinct from the U.S. standards that anchor the AQI, so a number that meets one benchmark may not meet the other.