Air Quality Index

By Tabitha Mishra
Last updated: December 26, 2023

What Does Air Quality Index Mean?

The Air Quality Index (AQI) is a color-coded system that measures and forecasts outdoor air quality on a daily basis. 

The AQI was developed by the Environmental Protection Agency (EPA) and is used by government agencies to communicate air quality risks to the public. Doing so provides everyone with an opportunity to take appropriate precautions when the risks are elevated, such as minimizing time spent outdoors or wearing an N95 mask or a similar respirator. 

The AQI quantifies air quality on a scale from 0 to 500, based on the concentration of pollutants in the air. AQI values under 100 are considered harmless to most people. 

Safeopedia Explains Air Quality Index

Air pollution levels are measured on a daily basis and ranked from 0 to 500, with 0 indicating good air quality and 500 indicating an immediate danger to health. 

A visual representation of Air Quality Index values, ranging from 0 to 50 AQI indicating good air quality to 301 to 500 AQI indicating hazardous air quality.

Source: Quartz

The air pollution levels are divided into six color-coded categories, each with a corresponding advisory. These categories are, in order from least to most hazardous:

  1. Green (Good)
    • Index value: 0 to 50
    • Advisory: None
  2. Yellow (Moderate)
    • Index value: 51 to 100
    • Advisory: Acceptable air quality, but a possible risk for people who are unusually sensitive to air pollution.
  3. Orange (Unhealthy for Sensitive Groups)
    • Index value: 101 to 150
    • Advisory: Those with asthma and other respiratory issues should limit time outdoors as they may experience adverse health effects. The majority of people are not likely to be affected.
  4. Red (Unhealthy)
    • Index value: 151 to 200
    • Advisory: Some people may experience adverse health effects and those in sensitive groups may experience serious health effects.
  5. Purple (Very Unhealthy)
    • Index value: 201 to 300
    • Advisory: Heightened health risk for everyone. Limit outdoor activities.
  6. Maroon (Hazardous)
    • Index value: 301 to 500
    • Advisory: Emergency health warning, with everyone likely to be affected. 

Major Air Pollutants

The AQI reports the air pollutants most commonly found, which are regulated under the Clean Air Act (CAA). These pollutants are:

The overall AQI for a particular hour is determined by the pollutant that has the highest AQI level at the time. The forecasts depend on factors such as wind, temperature, precipitation and cloud cover. Pollution caused by ground-level ozone is monitored from April to October in almost all reporting areas, while particle pollution is measured year-round. SO2, NO2 and CO levels do not influence air quality as much and are only measured at a few sites.

The calculation for measuring AQI requires converting the measured concentration of a pollutant to a uniform index. The index is based on health effects linked with a pollutant and is established by the EPA through the National Ambient Air Quality Standards. These standards are reviewed by the EPA every five years as required by the CAA.

Alerts for air quality are issued when the AQI is greater than 101, indicating the air quality may be harmful to sensitive populations.

Limitations of the Air Quality Index

The Air Quality Index provides a rough and ready indication of how air quality risks. However, the AQI number taken on its own can be somewhat misleading. This is because the AQI is not based on a tally of all air pollutants currently in the atmosphere. Rather, it represents only the concentration of the single pollutant with the highest value.

Suppose, for instance, that smog levels result in an AQI of 290. Now, let’s say the wind carries smoke from a wildfire in a neighboring state. Although this smoke means there is an additional airborne hazard present, it will not push the AQI any higher than it already is – not unless the smoke risk surpasses the risks associated with the smog.

Another issue with the AQI’s value is a numerical representation of risk, not a representation of the pollutant. In other words, if the AQI climbs from 200 to 400, it does not necessarily mean that the amount of pollutant in the air has doubled. It simply means that the hazard level has increased. Since this is not clear from a glance at the AQI numbers, it can easily be misinterpreted.


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