BACK PRESSURE MODELING OF INDOOR AIR CONCENTRATIONS FROM VOLATILIZINGSOURCES

Indoor air pollution models estimate the concentration and potential e xposure (concentration . time) to persons from sources of airborne con tamination. The typically describe the independent variables of contam inant generation and control that determine and predict exposure. An i mportant source of airborne contamination is volatilization. Volatiliz ation is driven by the difference between the equilibrium or saturatio n partial pressure of a compound and the partial pressure present in t he receiving air volume. Given a relatively small volatilizing source the resulting concentration in room air will most likely be a small fr action of its equilibrium or saturation partial pressure. Under these conditions a relatively large gradient is maintained and the volatiliz ation rate is maximized. The model is simplified since any decrease in the gradient can be considered insignificant and thus ignored in the estimation. For large volatilizing sources (e.g., rugs, painted walls, spills, etc.), however, the concentration can become a significant po rtion of the equilibrium partial pressure, thus reducing the gradient and the net volatilization rate. This significant retarding pressure i s termed ''back pressure.'' A back pressure model is presented for ind oor volatilizing sources of greater than a few hundred square meters i n large rooms (i.e. about one-tenth of the floor surface area of small rooms (i.e., rooms smaller than about 30 m3). The model predicts that dilution ventilation will be relatively ineffective in controlling ex posure from sources with large vaporizing surface area-to-room volume ratios.