Potential of Ventilation System Components to Affect Indoor VOC Concentrations
Air provided to mechanically ventilated buildings passes through heating, ventilation and air conditioning (HVAC) systems that contain many different materials including particle filters, sheet metal ducts, fiberglass batt duct liners, and gaskets. The quality of air passing through HVAC systems can be altered by: (1) primary emissions of VOCs from materials; (2) sorption and desorption processes between contaminants and surfaces; (3) contaminant removal by deposition and chemical reaction at surfaces; and (4) production and release of chemically transformed compounds (i.e., secondary sources). Of particular interest with respect to (3) and (4) are ozone-surface interactions, which may tend to reduce ozone concentrations in the supply air, but may generate aldehydes or organic acids that can be released into the air. Such compounds can be odorous at low concentrations and often are relatively potent chemical irritants.

Study of Contaminant Production in Ventilation Ducts With and Without Ozone
A study was conducted to measure the primary emissions of VOCs from materials typically found in HVAC duct systems, such as duct liners, sealing caulks and neoprene gaskets, and to measure the secondary VOC emissions from these materials when exposed to 100 ppb ozone (Morrison et al., 1998). In the absence of ozone exposure, many of the materials exhibited low VOC emission rates. A duct system for a 1.4 x 104 m³ (e.g., 4,700-m² floor area with a 3-m ceiling height) building operating at 0.7 air changes per hour was modeled. The duct system contained 34 m² of duct liner and 0.7 m² of exposed duct sealant. For this system, the contribution to VOC concentrations in the building was predicted to be only a few percent of typical indoor concentrations. Exposure of the duct liner, neoprene gasket and duct sealant to 100 ppb ozone primarily increased the production of C5 to C10 aldehydes from these materials. In the modeled duct system, the production of these aldehydes would be expected to contribute about 1.4 µg/m³ to their combined concentration in the building. This represents a small, but nonnegligible fraction of the odor thresholds for some of these compounds such as nonanal and decanal.