INTRODUCTION Particulate matter (PM) diameter spans many orders of magnitude@ from a few nanometers to tens of micrometers@ and exposure to particles has been associated with harmful effects on human health. Fine particles (lt;2.5 ?? in diameter) can penetrate deep into the alveolar regions of the lung (Hinds 1999). In a review of studies in the last 20 years on associations between exposure to particles and increased human mortality@ Pope and Dockery (2006) conclude that fine particles have adverse effects on cardiopulmonary health. Also@ exposure to ultrafine particles (lt;0.1 ?? in diameter) have an adverse effect on cardiopulmonary function that is independent of fine particle exposure (Pekkanen et al. 2002). Most epidemiological research on the influence of particle exposure on human health has focused on the outdoor environment. However@ the average American spends 18 hours indoors for every hour outdoors (Klepeis et al. 2001)@ and particle concentrations indoors are often much higher than outdoors@ due to the strong influence of indoor sources (Wallace 2006). Common indoor sources of fine particles include cooking (Wallace et al. 2004)@ cigarettes (e.g. Waring and Siegel 2007)@ and vented clothes dryers (Wallace 2005). Another source of indoor fine particles are chemical reactions. Weschler and Shields (1999) and other researchers (e.g.@ Wainman et al. 2000; Long et al. 2000; Rohr et al. 2002; Sarwar et al. 2003; Sarwar and Corsi 2007) showed that gasphase reactions between ozone (O3) and various terpenoids yield particles in the form of secondary organic aerosol (SOA). The reactions proceed at fast enough reaction rates to compete with loss due to air exchange (Weschler and Shields@ 1996)@ and the reactants both commonly occur indoors. Ozone infiltrates the indoors from the outdoors (Weschler 2000)@ and is directly emitted indoors from office electronics (Lee et al. 2001)@ portable ionizers (e.g. Waring et al. 2008) or in-duct electrostatic precipitators (ESPs) (Viner et al. 1992). Also@ terpenoids are commonly emitted indoors from consumer products@ cleaners@ and fragrances (Wallace et al. 1991; Singer et al. 2006b; Corsi et al. 2007). The SOA is composed of condensed products that either nucleated or partitioned onto preexisting seed particles@ and SOA is in the ultrafine and lower portion of the fine particle size ranges. Further@ the health effects due to SOA may be different and more deleterious than those from primary aerosols (Rohr et al. 2002 and 2003). The heating@ ventilation@ and air-conditioning (HVAC) system of a building can affect the concentration of SOA formed by influencing the reactant and seed particle concentrations@ as well as the indoor air temperature and relative humidity (RH). For example@ ventilation rates influence indoor ozone (Weschler 2000) and seed particle concentrations (Riley et al. 2002) by affecting the rate of transport between the indoors and outdoors. Recirculation rates affect ozone@ seed particle@ and SOA concentrations by altering both the removal to filters in the recirculation air stream and the deposition to indoor surfaces (Sabersky et al. 1973; Lai and Nazaroff@ 2000; Zuraimi et al. 2007). Seed particle and SOA concentrations are reduced by HVAC filters (Hanley et al. 1994) and in-duct ESPs (Wallace et al. 2004). Ozone is either removed by particle-laden filters (Zhao et al. 2007) or can be generated by in-duct ESPs (Viner et al. 1992). The HVAC heat exchanger (or coil) alters both the temperature and RH in the space@ and the temperature affects the chemical reaction rates and resulting product vapor pressures and the RH the mass of SOA yielded (Leungsakul et al. 2005). Since the HVAC system design and operation can affect the mass of SOA formed@ we explore its effects by developing a simulation that predicts the size-resolved mass of SOA formed in typical residential and commercial spaces with HVAC systems. In both the residential and commercial models@ we vary the (i) ventilation and recirculation rates@ (ii) the HVAC filter efficiency@ (iii) the ozone removal on HVAC filters due to particle loading@ and the indoor (iv) temperature and (v) relative humidity. A parametric analysis is used to explore each of these factors on SOA formation.