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Understanding the Air Quality Index: A Step-by-Step Guide

June 17, 2026 · The Clime Team
Understanding the Air Quality Index: A Step-by-Step Guide

The Air Quality Index (AQI) is a standardized system used to communicate the quality of air in a specific area, helping individuals make informed decisions to protect their health. Developed by the U.S. Environmental Protection Agency (EPA), the AQI evaluates concentrations of five major pollutants: ground-level ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. (airnow.gov)

How Is the AQI Calculated?

Calculating the AQI involves several key steps:

  1. Measure Pollutant Concentrations: Air quality monitoring stations collect data on pollutant levels, typically expressed in units like parts per million (ppm) for gases and micrograms per cubic meter (µg/m³) for particulate matter. (iere.org)

  2. Convert Concentrations to AQI Values: Each pollutant has a specific formula that converts its concentration into an AQI value. These formulas are based on established breakpoints defined by regulatory agencies like the EPA. For example, the formula for PM2.5 is:

AQI = [(IHi – ILo) / (BPHi – BPLo)] * (Cp – BPLo) + ILo

Where:

  • AQI is the index value for the pollutant

  • Cp is the pollutant concentration

  • BPHi and BPLo are the breakpoint concentrations above and below Cp

  • IHi and ILo are the corresponding AQI values for BPHi and BPLo

(iere.org)

  1. Determine the Final AQI: After calculating individual AQI values for each pollutant, the highest value is selected as the overall AQI for that location. This approach ensures that the most significant health risk is highlighted. (evs.institute)

Understanding AQI Categories

The AQI scale ranges from 0 to 500, with higher values indicating worse air quality and greater health risks. The EPA defines the following categories:

  • 0–50 (Good): Air quality is satisfactory, and air pollution poses little or no risk.

  • 51–100 (Moderate): Air quality is acceptable; however, some pollutants may be a concern for a small number of individuals sensitive to air pollution.

  • 101–150 (Unhealthy for Sensitive Groups): Members of sensitive groups may experience health effects; the general public is less likely to be affected.

  • 151–200 (Unhealthy): Everyone may begin to experience health effects; sensitive groups may experience more serious effects.

  • 201–300 (Very Unhealthy): Health alert: everyone may experience more serious health effects.

  • 301–500 (Hazardous): Health warning of emergency conditions; the entire population is more likely to be affected.

These categories help individuals understand the potential health impacts associated with current air quality levels. (airnow.gov)

Using the AQI to Protect Your Health

Monitoring the AQI can guide your daily activities, especially if you belong to sensitive groups such as children, the elderly, or individuals with respiratory conditions. When the AQI indicates higher pollution levels, consider limiting outdoor activities, especially strenuous ones, to reduce exposure to harmful pollutants. (airnow.gov)

Understanding the AQI empowers you to make informed decisions about your health and well-being in relation to air quality.

Highlights:

Frequently Asked Questions