INTRODUCTION Due to deficiencies in natural ventilation or air exchange in the indoor environment of buildings@ mechanical ventilation equipment may be installed to maintain quality standards@ and health and comfort levels of the indoor air. However@ when excessive indoor related materials or chemical-based products are present in the indoor environment@ harmful chemical substances may be produced (Chiang@ 2006)@ including volatile organic compounds (VOCs) from solvent-based paint and formaldehyde from wooden or other construction materials. These substances are just two indoor factors that may affect human health. A common pollutant of the indoor environment is Volatile organic compounds (VOCs) and formaldehyde (Zhang@ 2007)@ which may originate from smoking by building occupants and/or from construction or related materials. In studies of the relationships between temperature@ relative humidity (RH)@ and formaldehyde concentration indoors@ it has been reported that formaldehyde can be removed by photocatalysts (Yang@ 2007). However@ Matthews et al. (Matthews@ 1986) showed a seasonal pattern@ with formaldehyde concentrations in summer 6?C9 times higher than in winter. Yu et al. (2006) studied air change rates (ACR)@ RH@ and photocatalytic filters in a simplified heating ventilation air-conditioning (HVAC) system. Their results showed that first-order decay of toluene and formaldehyde ranged from 0.381 to 1.01 h-1 under different total ACR@ from 0.34 to 0.433 h-1 under different RH@ and from 0.381 to 0.433 h-1 for different photocatalytic filters. However@ increasing outdoor airflow rate increased the cooling load of the HVAC system (Amal@ 2001). In order to effectively solve the above problem@ it is possible to increase purification equipments of indoor is needed@ as to reduce the volume of external air induction@ and then to save energy and enhance indoor air quality.