Understanding the Air Quality Index (AQI)

Problem

You want to understand what the Air Quality Index (AQI) means, how it is calculated, or why AQI values from South Coast AQMD differ from other websites and apps.

What is AQI?

The Air Quality Index (AQI) reports air quality using a scale that communicates health concerns. AQI values are calculated from measured and modeled concentrations of five pollutants: fine particulate matter (PM2.5), coarse particulate matter (PM10), ozone, nitrogen dioxide, and carbon monoxide.

How does South Coast AQMD calculate AQI?

South Coast AQMD uses a method developed by its scientists and published in a peer-reviewed scientific journal. The method blends measurements from high-quality regulatory monitors, quality-controlled low-cost sensors (PurpleAir and Clarity Node-S), and an air quality model. The AQI map shows 1,218 grid cells, each 5 km by 5 km, providing neighborhood-level air quality data.

Why is my AQI different from other apps?

Differences between South Coast AQMD and private-sector apps occur for several reasons. Some apps only consider one pollutant at a location. Some use lower quality data without calibration. Some rely on global models poorly suited for the South Coast Air Basin's complex topography. South Coast AQMD uses EPA-consistent methods with multiple data sources tailored to this region.

Where to check current AQI

Visit www.aqmd.gov/home/air-quality/current-air-quality-data for the latest readings.

Sign up for Air Alerts at www.aqmd.gov/home/air-quality/air-alerts.

Contact

Meteorology Team: [e-mail]

Public Information Center: [phone]