ISO 16000-4:2004 specifies a diffusive sampler/solvent desorption/high performance liquid chromatography (HPLC) method for the determination of formaldehyde in indoor air. The test method is applicable to the measurement of formaldehyde in indoor air over the range from 0,001 mg/m3 to 1,0 mg/m3 for a sampling period of between 24 h and 72 h. For sampling periods of 24 h, the applicable concentration range is 0,003 mg/m3 to 1 mg/m3, and for 72 h it is 0,001 mg/m3 to 0,33 mg/m3. The method is suitable for measurements in atmospheres of up to 80 % relative humidity and for monitoring at air velocities as low as 0,02 m/s. Potential interferences, including those due to the presence of other carbonyl compounds, should be eliminated by the chromatographic step in the method. The sampling method gives a time-weighted average result.