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Understanding AQI Categories: A Step-by-Step Guide

June 18, 2026 · The Clime Team
Understanding AQI Categories: A Step-by-Step Guide

The Air Quality Index (AQI) is a standardized system used to communicate the quality of outdoor air and its potential health impacts. Developed by the Environmental Protection Agency (EPA), the AQI simplifies complex air pollution data into a single number, making it easier for the public to understand and respond to air quality conditions. (airnow.gov)

What Are the AQI Categories?

The AQI is divided into six color-coded categories, each representing a different level of health concern:

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

  2. Moderate (51–100): Air quality is acceptable; however, for some pollutants, there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution. (lung.org)

  3. Unhealthy for Sensitive Groups (101–150): Members of sensitive groups may experience health effects. The general public is not likely to be affected. (lung.org)

  4. Unhealthy (151–200): Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects. (lung.org)

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

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

How Is the AQI Calculated?

The AQI is calculated based on measurements of key air pollutants, including particulate matter (PM₂.₅ and PM₁₀), ground-level ozone (O₃), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Each pollutant has its own sub-index, and the highest sub-index determines the overall AQI value. (iqair.com)

How to Use the AQI to Protect Yourself

Monitoring the AQI can help you make informed decisions about outdoor activities:

  • Good (0–50): It's a great day to be active outside.

  • Moderate (51–100): Unusually sensitive people should consider reducing prolonged or heavy exertion.

  • Unhealthy for Sensitive Groups (101–150): Sensitive groups should reduce prolonged or heavy exertion.

  • Unhealthy (151–200): Sensitive groups should avoid prolonged or heavy exertion; everyone else should reduce prolonged or heavy exertion.

  • Very Unhealthy (201–300): Sensitive groups should avoid all physical activity outdoors; everyone else should avoid prolonged or heavy exertion.

  • Hazardous (301–500): Everyone should avoid all physical activity outdoors. (lung.org)

Where to Find AQI Information

You can find current AQI information through various sources:

  • EPA's AirNow Website: Provides real-time AQI data and forecasts for most of the nation. (airnow.gov)

  • Local News Outlets: Many local news stations report AQI levels during weather forecasts.

  • Weather Apps: Some weather applications include AQI information.

Conclusion

Understanding the AQI and its categories empowers you to make informed decisions about your health and outdoor activities. By staying informed and taking appropriate precautions, you can protect yourself and your community from the adverse effects of air pollution.

Highlights:

Frequently Asked Questions