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Understanding the Air Quality Index: A Guide for Non-Technical Users

June 18, 2026 · The Clime Team
Understanding the Air Quality Index: A Guide for Non-Technical Users

The Air Quality Index (AQI) is a standardized system used to communicate daily air quality levels to the public. Developed by the U.S. Environmental Protection Agency (EPA), the AQI simplifies complex air pollution data into a single value, indicating how clean or polluted the air is and what associated health effects might be a concern. (airnow.gov)

What Does the AQI Measure?

The AQI focuses on five major pollutants regulated by the Clean Air Act:

  • Ground-level ozone: A key component of smog, formed when sunlight reacts with pollutants from vehicles and industrial emissions.
  • Particulate matter (PM₂.₅ and PM₁₀): Tiny particles or droplets in the air that can penetrate deep into the lungs.
  • Carbon monoxide: A colorless, odorless gas produced by burning fossil fuels.
  • Sulfur dioxide: A gas produced by burning fossil fuels and industrial processes.
  • Nitrogen dioxide: A gas produced by burning fuel, especially in vehicles and power plants.

How Is the AQI Calculated?

Each pollutant has its own AQI scale, and the highest AQI value among them determines the overall AQI for the day. The AQI scale ranges from 0 to 500, divided into six categories:

  • 0 to 50 (Good): Air quality is satisfactory, posing little or no risk.
  • 51 to 100 (Moderate): Air quality is acceptable; however, there may be some health concern for a small number of unusually sensitive individuals.
  • 101 to 150 (Unhealthy for Sensitive Groups): Sensitive individuals may experience health effects; the general public is less likely to be affected.
  • 151 to 200 (Unhealthy): Everyone may begin to experience health effects; sensitive groups may experience more serious effects.
  • 201 to 300 (Very Unhealthy): Health alert: everyone may experience more serious health effects.
  • 301 to 500 (Hazardous): Health warnings of emergency conditions; the entire population is more likely to be affected.

How to Use the AQI

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

  • Check the AQI regularly: Many local news outlets, weather apps, and websites provide daily AQI updates.
  • Plan outdoor activities accordingly: On days with higher AQI levels, consider limiting outdoor exertion, especially if you belong to sensitive groups such as children, the elderly, or individuals with respiratory conditions.
  • Stay informed during events: During events like wildfires or industrial accidents, AQI levels can spike. Stay updated through reliable sources.

Protecting Yourself During Poor Air Quality

When the AQI indicates unhealthy air quality, consider the following precautions:

  • Stay indoors: Limit outdoor activities, especially strenuous ones.
  • Keep windows closed: Prevent outdoor air from entering your home.
  • Use air purifiers: Devices with HEPA filters can help reduce indoor pollution levels.
  • Use air conditioning: If available, use air conditioning to filter indoor air.
  • Avoid using tobacco products: Smoking indoors can worsen indoor air quality.

Conclusion

Understanding the AQI empowers you to make informed decisions about your health and outdoor activities. By staying informed and taking appropriate precautions, you can minimize the health risks associated with poor air quality.

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Frequently Asked Questions