Ventilation rates are also inferred from carbon dioxide measurements.Occupants generatecarbon dioxide, causing indoor carbon dioxide concentrations to exceed outdoor concentrations. According to Equation 1 the ventilation rate can be estimated if the carbon dioxide source strength and the concentrations of supply air and room air are known (ventilation is the only significant process for carbon dioxide removal). Indoor and outdoor CO2 concentrations are measured and the indoor CO2 source strength is based on the number of occupants in a building and an estimate of their CO2 production. However, this method is subject to several sources of error which are described in detail elsewhere (Persily, 1997; Mudarri, 1997) (ASTM D 6245-98) and summarized below: