The data obtained from EPA's monitoring stations are averaged over various averaging times depending on pollutant, as listed in the 'Pollutants, standard levels and calculation averaging times' table. These averages are calculated every hour by EPA.
The calculation is done separately for each station. There are two stages in the calculation. First, each pollutant measurement is converted into a pollutant index, and second, all the pollutant index values are combined together to form a station index.
In order to get an overall appreciation of air quality at a particular EPA monitoring station, the index for each pollutant is calculated. The maximum of these figures is taken to be the index for that monitoring station.
See also 'Determination of an overall station summary'.
Assume the values listed in the table below have been measured at an EPA monitoring station and reported on the hourly air quality update web pages (table and map) at 9.30am.
||Maximum measured level
|Overall station index (the maximum of the figures is taken to be the index)