AQI vs PM2.5: What Is the Difference During Wildfire Smoke?

Core distinction

PM2.5 is measured. AQI is interpreted.

The EPA defines PM2.5 as fine particulate matter with aerodynamic diameter generally less than or equal to 2.5 micrometres. AQI is the U.S. EPA index used to communicate outdoor air quality and health concern.

PM2.5 concentration

Reported in micrograms per cubic metre (µg/m³). It describes how much fine-particle mass is present in a volume of air.

AQI

A dimensionless index with color-coded health categories. Higher AQI means greater air-pollution concern.

Wildfire smoke

PM2.5 is the key pollutant shown on the Fire and Smoke Map.

AirNow identifies fine-particle pollution as the key pollutant in wildfire smoke and recommends its Fire and Smoke Map during smoke events. The map translates current particle measurements into AQI categories for easier interpretation.

A visible smoke plume does not automatically tell you ground-level PM2.5. Smoke can be aloft, dispersed unevenly or changing quickly; use current monitor or sensor information for local conditions.

AQI scale

Six categories communicate increasing concern.

0–50: Good
51–100: Moderate
101–150: Unhealthy for Sensitive Groups
151–200: Unhealthy
201–300: Very Unhealthy
301+: Hazardous

These are AQI index values, not PM2.5 concentrations in µg/m³.

Use the tools

Move from outdoor smoke to indoor control.

Wildfire Tools →Use the PM2.5 ⇄ AQI converter, indoor-smoke model and CADR calculator.Smoke CADR Calculator →Size clean-air delivery for a room using volume and target air changes.Air Purifier Buying Guide →Compare products after calculating the clean-air delivery your room needs.N95 & P100 Guide →Understand respiratory protection options for smoke-particle exposure.

Sources

EPA and AirNow.

AirNow — AQI Basics ↗

AirNow — Using AirNow During Wildfires ↗

EPA — Wildland Fires and Public Health Effects ↗

Published 21 September 2026.