Predicting room vapor concentrations due to spills of organic solvents

Relatively small spills of volatile liquids can result in short-term, hi g h-concentration exposures. Because of the transient nature of these exposures, air sampling may be precluded. As an alternative, exposure assessment can be done by mathematical modeling. The vapor emission rate from small spills is highest immediately following the spill and decreases as the surface area available for mass transfer decreases and evaporation cools the liquid. This decreasing emission rate is not described by any of the existing evaporation rate models. The authors present an evaporation rate model that describes the changing emissions as exponentially decreasing. The rate of decrease is governed by an evaporation rate parameter alpha, which has the unit of min(-1) and can be estimated based on experimental measurements. The authors measured alpha for a suite of compounds and different sizes of spill. They found that alpha can be estimated for hydrocarbons containing only C, H, and O with the equation: alpha=0.000524 VP + 0.0108 SA/Vol, where VP is the vapor pressure of the liquid and SA/Vol is the surface area to volume ratio. Next, the authors integrated the exponentially decreasing emission rate into a well-mixed room versus a near field/far field dispersion construct to predict vapor concentrations. A preliminary experiment was conducted in a test room to compare measured concentrations with the concentrations predicted by the models. The well-mixed room model performed well based on ANSI indoor air model evaluation criteria. The predicted near field concentrations showed a poor fit to the measured values based on the ANSI criteria, although overall they did capture the observed time profile.