where, D is the age–specific dose rate (μg/kg); BR is age–specific breathing rate (L/min); BW is age–specific body weight (kg); C(t) is the concentration of each pollutant (μg/m3); OF(t) is occupancy factor (percentage of population likely to be in the building at a given interval of time). For each pollutant, we assume that the indoor and outdoor concentrations to be equal. Thus, OF(t) equals to 1. The uncertainties due to this assumption will be discussed hereinafter. Since there are no data in China on age–specific breathing rates, age–specific body weights, the values reported by Cerna et al. (1998) are used as shown in Table 1.