A: The default display on the PurpleAir map is the United States Environmental Protection Agency (EPA) Air Quality Index (AQI) scale for PM2.5. The AQI allows comparison for different pollutants with an easy-to-visualize color scheme. Hovering over the color scale bar in the map legend reveals a description of each data range. The graphs give further information on different counts, relative weights and sizes of particles in the air.