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Understanding AQI Categories and Their Workflow

June 18, 2026 · The Clime Team
Understanding AQI Categories and Their Workflow

The Air Quality Index (AQI) is a standardized system used to communicate the quality of air in a specific area and its potential health impacts. Developed by the U.S. Environmental Protection Agency (EPA), the AQI focuses on the concentrations of five major air pollutants: ozone (O₃), particulate matter (PM₂.₅ and PM₁₀), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Each pollutant has its own AQI scale, and the highest value among them determines the overall AQI for that location. (epa.gov)

AQI Categories and Their Implications

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

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

  • Moderate (51–100): Air quality is acceptable; however, there may be a moderate health concern for a very small number of people sensitive to air pollution.

  • Unhealthy for Sensitive Groups (101–150): Sensitive individuals, such as those with respiratory conditions, may experience health effects. The general public is less likely to be affected.

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

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

  • Hazardous (301 and above): Health warnings of emergency conditions. The entire population is more likely to be affected.

These categories are color-coded for easy identification:

  • Good: Green

  • Moderate: Yellow

  • Unhealthy for Sensitive Groups: Orange

  • Unhealthy: Red

  • Very Unhealthy: Purple

  • Hazardous: Maroon

Understanding these categories helps individuals make informed decisions about outdoor activities and health precautions. (epa.gov)

Calculating the AQI

The AQI is calculated by converting the concentration of each pollutant into an AQI value using specific formulas. The highest AQI value among all pollutants determines the overall AQI for that day. This method ensures that the most significant pollutant is highlighted, providing a clear picture of the day's air quality. (epa.gov)

Using AQI Information

The AQI is a valuable tool for public health communication. It informs the public about air quality conditions, identifies who may be affected, and provides steps to take to reduce exposure when pollution levels are unhealthy. By understanding the AQI categories and their implications, individuals can take appropriate actions to protect their health, such as limiting outdoor activities or staying indoors during high pollution periods. (epa.gov)

Incorporating AQI information into daily routines empowers individuals to make health-conscious decisions based on current air quality conditions.

Highlights:

  • Air Quality System (AQS) | US EPA, Published on Sunday, May 31
  • AirData Basic Information | US EPA, Published on Wednesday, April 01
  • Environments and Contaminants - Criteria Air Pollutants | US EPA, Published on Sunday, December 14

Frequently Asked Questions