This International Standard provides comprehensive guidance and specific statistical procedures for uncertainty estimation in air quality measurements including measurements of ambient air, stationary source emissions, indoor air, workplace atmospheres and meteorology. It applies the general recommendations of the Guide to the Expression of Uncertainty in Measurement (GUM) to boundary conditions met in air quality measurement. The boundary conditions considered include measurands varying rapidly in time, as well as the presence of bias in a series of observations obtained under conditions of intended use of methods of air quality measurement. The methods of measurement considered comprise ? methods corrected for systematic effects by repeated observation of reference materials, ? methods calibrated by paired measurement with a reference method, ? methods not corrected for systematic effects because they are unbiased by design, and ? methods not corrected for systematic effects in intended use deliberately taking into account a bias. Experimental data for uncertainty estimation can be provided either by a single experimental design in a direct approach or by a combination of different experimental designs in an indirect approach.