Pa. measures air quality with colors. Here’s what they mean

This story originally appeared on StateImpact Pennsylvania.

Pennsylvania is under a cloud of unhealthy air caused by smoke from Canadian wildfires.

The state Department of Environmental Protection on Wednesday declared a Code Red Air Quality Action Day for the entire state.

But in some places, actual air quality reached more dangerous purple and maroon levels.

The federal Environmental Protection Agency measures and rates air pollution on a scale from 0 to 500 called the Air Quality Index. The lower the number, the cleaner the air.

The AQI is also broken into colors.

Green (0-50) is healthy. Yellow (51-100) is seen as acceptable.

Orange (101-150) means the air is likely to affect certain groups, such as the very young, old, and people with asthma.

An AQI in the red zone (151-200) means even people without sensitivities could feel health effects from breathing the dirty air. The EPA recommends people limit outdoor activity.

Purple (201-300) is very unhealthy and maroon (301-500) is hazardous. When the AQI is above 300, it ranks as a health emergency and all people in the area should avoid spending time outside.