Back Pressure Modeling of Indoor Air Concentrations From Volatilizing Sources

Abstract

Indoor air pollution models estimate the concentration and potential exposure (concentration · time) to persons from sources of airborne contamination. They typically describe the independent variables of contaminant generation and control that determine and predict exposure. An important source of airborne contamination is volatilization. Volatilization is driven by the difference between the equilibrium or saturation partial pressure of a compound and the partial pressure present in the receiving air volume. Given a relatively small volatilizing source the resulting concentration in room air will most likely be a small fraction of its equilibrium or saturation partial pressure. Under these conditions a relatively large gradient is maintained and the volatilization 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 portion of the equilibrium partial pressure, thus reducing the gradient and the net volatilization rate. This significant retarding pressure is termed “back pressure.” A back pressure model is presented for indoor volatilizing sources of greater than a few hundred square meters in large rooms or 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 exposure from sources with large vaporizing surface area-to-room volume ratios.