The phenomenon of boundary layer separation is an important factor in determining a worker's breathing zone concentration. This article presents the results of flow visualization and tracer gas studies, conducted in a wind tunnel with a mannequin, designed to examine this phenomenon. A simple conceptual model, based on mass transport by vortex shedding, provides a reasonable estimate of the mannequin's breathing zone concentration. An empirical model is developed which relates the measured concentration to the distance from the source to the breathing zone for the situation when the contaminant is released downstream in a uniform flow. Applications of the results are discussed.