When the exhaust air from a building ventilation system contains an undesirable contaminant such as toxic or corrosive gases, vapors, or odors, the system designer must consider the possibility that the wind around the building will recirculate this contaminated exhaust gas to an air intake, or to ground level where it may become a hazard. For this reason a designer should be able to estimate the amount of dilution that the wind will provide for a given exhaust-intake confi gura ti on.

With this problem in mind ASHRAE has supported two research projects, the first completed in 1962 by Halitsky (1) and the second in 1975 by Wilson (2), to determine a rational procedure for estimating the dilution of exhaust gases as they are carried around a building by the wind. Both of these investigations showed, as predicted by theory, that the concentration of a contaminant at a given distance from a vent was inversely proportional to the wind speed U_H, and directly proportional to the volume flow rate Q of the contaminant from the vent.