Short answer
AQI is a number and color category for communicating outdoor air quality in the United States. PM2.5 is a type of fine particle pollution. PM2.5 concentration can be translated into a pollutant-specific AQI, but AQI can also reflect ozone and other regulated pollutants. 1 2
AQI is a reporting scale; PM2.5 is one pollutant that can contribute to an AQI value.
On this page
At a glance
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| Question | Air Quality Index (AQI) | PM2.5 |
|---|---|---|
| Kind of thing | Index value/category | Physical pollutant class |
| Unit | Index number | Often micrograms per cubic meter |
| Includes others? | Yes, multiple pollutants | No, specifically fine particles |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Air Quality Index (AQI). A US EPA reporting index that translates measured or forecast air pollution into health-concern categories. 1
PM2.5. Fine airborne particulate matter with diameters generally 2.5 micrometers or smaller. 2
Key differences
EPA AirNow’s AQI covers several regulated pollutants; EPA defines PM2.5 by particle size. The two figures cannot be compared as if they shared units. A reported overall AQI may be driven by PM2.5 on one day and ozone on another. 1 2
How to tell them apart
Check the label and unit. “AQI 120” is an index reading; “PM2.5 35 µg/m³” is a mass concentration. If comparing locations or days, also note the time averaging and which pollutant drives the index. 1 2
Where they overlap
The PM2.5 subindex is part of the AQI system, and particle pollution often drives air-quality alerts. Both help explain health-relevant outdoor air conditions. 1 2
Edge cases
Instant sensor readings, daily AQI and forecast AQI use different averaging or processing; a sensor concentration is not automatically an official AQI. Standards and breakpoints can be revised. 1 2
Why the distinction exists
A common index makes varied pollutants easier to communicate, while pollutant measurements retain the chemical and physical detail needed for analysis. 1 2
Common misconceptions
AQI is not the concentration of PM2.5. A low PM2.5 reading does not guarantee good overall AQI when another pollutant is high. 1 2
Examples
Wildfire smoke raises fine-particle concentration; AirNow may report a corresponding PM2.5-driven AQI category. 1 2
Sources
Sources checked October 3, 2026.
- EPA AirNow — AQI Basics. What is the U.S. AQI?.
- EPA — Particulate Matter Basics. What is PM?.