This International Standard provides a method for the quantification of the uncertainty of a time average of a set of air quality data obtained at a specified location over a defined averaging time period. The air quality data may have been obtained by continuous or intermittent monitoring. The uncertainty of the time average depends on both the uncertainty of the measurement results and the uncertainty due to incomplete time coverage of the data set. The set of air quality data shall be representative of the temporal structure of the measurand. Information on the uncertainty of the measurement results shall be available. The influence of limited spatial coverage of data set on the uncertainty of the time average is not considered.