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 method is applicable to air quality data obtained by continuous or intermittent monitoring by means of a specified measuring system. 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. This International Standard is only applicable if: a) the set of air quality data used to calculate the time average is representative of the temporal structure of the measurand over the defined time period, b) appropriate information on the uncertainty of the measurement results is available, and c) the measurement results have all been obtained at the same location. This International Standard implements recommendations of the Guide to the expression of uncertainty in measurement (GUM).