The AQIs reported by purple air are US EPA PM NowCast[0] AQIs for PM 2.5. NowCast, uses weighted averages over the past 12 hours of observations, rather than a

The AQIs reported by purple air are US EPA PM NowCast[0] AQIs for PM 2.5. NowCast, uses weighted averages over the past 12 hours of observations, rather than a simple arithmetic mean over the past 24 hour like the regular EPA AQI standard. NowCast was developed to be more sensitive to current conditions in order to help people make better immediate descisions about their health. The normal AQI standard was developed for long term air quality monitoring. The other thing that can effect the numbers is that AQIs are defined across a range of pollutants (usually PM 2.5, PM 10, CO, NO2, O3, and SO2) PurpleAir sensors only measure PM 2.5 and PM 10, but I think the numbers they show are only PM 2.5. [0] https://en.wikipedia.org/wiki/NowCast_(air_quality_index) https://en.wikipedia.org/wiki/NowCast_(air_quality_index)