 when using the mass balance equation to model indoor air quality , the primary assumption is that of uniform mixing . 
 different points in a single compartment are assumed to have the same instantaneous pollutant concentrations as all other points . 
 although such an assumption may be unrealistic , under certain conditions predictions ( or measurements ) of exposures at single points in a room are still within acceptable limits of error ( e.g. , 10% ) . in this article , 
 three studies of the mixing of environmental tobacco smoke ( ets ) pollutants are reviewed , and data from several other ets field studies are presented . under typical conditions for both short sources ( e.g. , 10 min ) and 
 the continuous sources of ets in smoking lounges , i find that average exposure concentrations for a single point in a room represent the average exposure across all points in the room within 10% for averaging times ranging from 12 to 80 min . 
 i present a method for determining theoretical estimates of acceptable averaging times for a continuous point source.imagesfigure 1figure 2figure 3figure 6 
Images