Air Quality Index: What do the numbers mean?

PHILADELPHIA (WPVI) -- 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

AQI BY THE NUMBERS

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:

Green - Good: 0 to 50. Air quality is satisfactory, and air pollution poses little or no risk.

Yellow - Moderate: 51 to 100. Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.

Orange - Sensitive Groups: 101 to 150. Members of sensitive groups may experience health effects. The general public is less likely to be affected.

Red - Unhealthy: 151 to 200. Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.

Purple - Very Unhealthy: 201 to 300. Health alert: The risk of health effects is increased for everyone.

Maroon - Hazardous: 301+. Health warning of emergency conditions: everyone is more likely to be affected.

Learn more about the index from the EPA at AirNow.gov.

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