Some parts of New York City and Philadelphia reported Code Purple air qualities early Wednesday. The Wilmington area in Delaware and Bridgeport, Conn., were reporting Code Red air qualities. What does that mean?

The categories are part of the U.S. Air Quality Index, which is the Environmental Protection Agency’s index for measuring and reporting air quality. The index measures five major pollutants regulated by the Clean Air Act: ground-level ozone, particulate matter including PM2.5 and PM10, carbon monoxide, sulfur dioxide and nitrogen dioxide.

The EPA likens its AQI forecasts to a weather forecast, saying it “lets you know when you may want to change your outdoor activities to reduce the amount of air pollution you breathe in.” The higher the U.S. Air Quality Index, which ranges from 0 to 500, the worse the air quality.

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An Air Quality Index value of 50 or below represents good air quality — Code Green. Code Yellow signifies moderate concern, while Code Orange means that the AQI is between 101 and 150, and while members of sensitive groups can experience health effects, the general public is less likely to be affected.

Code Red refers to the tier between 151 and 200, and it means the air is unhealthy for some members of the public. But everyone in these areas should reduce exposure to pollution, especially those at increased health risk, such as people with asthma.

“Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects,” the Environmental Protection Agency says.

The tier above Red, Code Purple, refers to the range from 201 to 300, and the risk of health effects is increased for everyone. Code Maroon, 301 and up, means the air is hazardous and serves as a “health warning of emergency conditions.”

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