The modeling of the indoor concentration distribution produced by sources and sinks of pollutants is complicated by nonuniform mixing in a building or large room. Two common approaches to predicting the concentration distribution are to treat either the indoor volume as containing multiple microenvironments with uniform mixing within each, or to treat the entire volume as a single uniformly mixed compartment with an empirical mixing factor m that is introduced to correct for nonuniform mixing. We review the literature on the use of m and show that this empirical approach violates a basic principle of mass conservation.
We propose a new conceptual model for the case of an episodic source of pollution in a building or room by defining the source operating conditions within three periods, tα, tβ, and tγ, where tα is the time while the source is emitting; tβ is the time after the source stops emitting, but while the concentration distribution is nonuniform in the building; and tγ is the time from the point where the indoor concentration becomes uniform until it becomes nondetectable above the background value. We define the state of uniform concentration as when the coefficient of variation of concentration (standard deviation/mean) throughout the volume becomes less than 0.1. We show that, with this definition, the assumption of uniform mixing for the entire volume may not lead to serious errors in predictions of exposures if tγ > > (tα + tβ).