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Air Quality Index (AQI)

Christine Johnson

What Is the Air Quality Index?

The Air Quality Index, more commonly known as the AQI, is an index for reporting daily air quality. It is a way for governments to communicate the level of air pollution and the health hazards associated with it. Usually, every country has their own AQI values, standards and scales. The example below specifically refers to US AQI.

How Does the AQI Work?

AQI focuses on the air quality and related health concerns. Many countries have their own AQI standards and scales. For example, the US AQI index ranges from 0–500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. A low AQI score indicates good air quality with little potential to affect public health. As a general rule, AQI values of less than 100 indicate that the air quality poses decreased risk and is deemed to be safe.

The AQI Values & Air Pollution

AQI values tell you how clean or polluted your air is and what associated effects might be a concern for you. To help you better understand what local air quality means to your health, the AQI is divided into different categories based on the pollution concentration. Each category relates to a different level of health concern.

For example, the US AQI is divided into the following categories:

US AQI Categories

  1. Good: An AQI score of 0-50 poses low health risks. Everyone can continue normal outdoor activities.
  2. Moderate: An AQI score of 51-100 is acceptable, though there may be some concerns for people who are unusually sensitive to pollutants. Highly sensitive individuals should reduce outdoor activity.
  3. Unhealthy for Sensitive Groups: An AQI score of 101-150 indicates that there is light pollution and poses little impact for the general public, however, sensitive groups may experience slight irritations.
  4. Unhealthy: An AQI score of 151-200 qualifies as moderately polluted and poses considerable risk. Everyone is at risk of experiencing health effects.
  5. Very Unhealthy: An AQI score of 201-300 is considered to be heavily polluted and healthy people will commonly experience health effects. Everyone should reduce outdoor activity.
  6. Hazardous: An AQI score above 300 indicates that there is severe pollution and that the entire population will likely be affected. The general population should avoid the outdoors.

Calculating the AQI

Different countries have their own air quality indices, corresponding to different national air quality standards.

The Clean Air Act (a part of the US environmental laws) for example, calculates the AQI based on five major air pollutants:

Air pollution is a global health crisis. Read more about this issue here:

Air Pollution: What You Need to Know