Air Quality Index: What do the numbers mean?

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Wednesday, June 28, 2023
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PHILADELPHIA (WPVI) -- With smoke blowing into the Philadelphia region from the wildfires in Canada, you may have heard a lot about the Air Quality Index or AQI.

But what it is, exactly?

The Air Quality Index is a scale used for daily reporting of how clean or polluted the air is, and what health effects might be of concern.

The AQI focuses on what you might experience if exposed within a few hours or days after breathing polluted air.

The Environmental Protection Agency calculates the AQI using 5 major air pollutants regulated by the Clean Air Act:

  • Ground-level ozone
  • Particle pollution (also known as particle matter)
  • Carbon monoxide
  • Sulfur Dioxide
  • Nitrogen Dioxide


The AQI runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern.

The index is divided into six categories, each corresponding to a different level of health concern.

An AQI of over 100 triggers an alert for vulnerable groups.

Levels over 300 signal a health emergency and everyone is more likely to be affected.

Full breakdown:

Good: 0 to 50

Moderate: 51 to 100

Sensitive Groups: 101 to 150

Unhealthy: 151 to 200

Very Unhealthy: 201 to 300

Hazardous: 301+

Learn more about the index from the EPA at

6abc Air Quality Tracker