Measurement methods for carbon dioxide are described elsewhere (e.g., IPMVP IEQ-Committee, 1999). The sampling strategy for CO2 is extremely important. The indoor CO2 concentration will generally be spatially non-uniform and measurement protocols should be designed to determine the average CO2 concentration in the breathing zone or in the exhaust air streams. Precautions are necessary to avoid measurements in air directly exhaled by building occupants. The CO2 concentration is seldom at steady state in real buildings because of variations in occupancy and ventilation rates. If occupancy and ventilation rate are reasonably stable, the time required to reach steady state depends on the ventilation time constant which is the reciprocal of the air exchange rate of the space; e.g., if the air exchange rate is 0.5 hª1 the time constant is 2 h. A period of three time constants with a stable occupancy and ventilation rate is required for CO2 concentrations to reach 95% of their steady state value. Three time constants corresponds to 6 h if the air exchange rate is 0.5 hª1 and to 3 h if the airexchange rate is 1 hª1.