Concept of capture efficiency as a design parameter for Local Exhaust Ventilation systems is presented. A theoretical model, based on computer-generated streamline maps, is developed for predicting the capture efficiency of a flanged circular hood operating with a cross-draught blowing perpendicular to the centreline. The model assumes a low flow, point source of gaseous contaminant located on the hood centreline. Measured values of capture efficiency are presented and compared with the theory. Potential applications of capture efficiency to the design and improvement of Local Exhaust Ventilation systems are discussed.