AQI

Purple Air now displays the AQI instead of the PM2.5 value on the map and in the site details window. This number is used all over the world to represent air pollution and can represent many different types of pollution.

Purple Air tracks particulate matter at the moment, so this number is based on that.

For more details on the AQI please see the Wikipedia page here:
https://en.wikipedia.org/wiki/Air_quality_index