The PurpleAir Map displays the United States Environmental Protection Agency (US EPA) Air Quality Index (AQI) by default, however there are other options you can choose to see also. An AQI is a color-coded scale intended to convey the severity of unhealthy air. The US EPA AQI ranges from 0 to 500, and is partitioned into six categories to report levels of five major pollutants; including particulate matter (PM) such as \text{PM}_{2.5} and \text{PM}_{10}. (The subscript refers to the diameter of the particle in microns.)
(Source: AQI Basics | AirNow.gov)
The US EPA AQI for PM is calculated from a 24-hour average: this is based on hourly averages of PM concentrations and is updated daily. The EPA also reports a real-time AQI called the NowCast. The NowCast scale for PM is calculated using a weighted 12-hour average. This is based on hourly averages and rate of change of PM concentrations, and is updated hourly depending on data availability.
Visitors to the PurpleAir Map have the option to view different averaging time periods in terms of the visitor-chosen AQI on the PurpleAir map legend. This allows visitors to see data for current conditions in the familiar terms of the AQI, yet at a finer resolution of changes in air quality throughout the day. This finer resolution is not a replacement for the correlations to public health concerns described by an AQI’s default time interval.