Engineers are expected to design ventilating systems that satisfy governmental and industrial health standards, but they do not have methods that predict whether their designs reduce contaminant concentrations to satisfactory levels. This paper describes a computational procedure to predict contaminant concentrations at arbitrary points in the vicinity of open vessels containing volatile liquids. Push-pull and suction lateral exhaust systems for freestanding open vessels located in a workplace in which there are transverse air currents (drafts) are analyzed. The velocity and concentration fields are predicted for a variety of exhaust and blowing flow rates, room air currents, and liquid temperatures.