Experiments are conducted in a full-scale chamber equipped with whole floor and whole ceiling supply or exhaust to form approximately zero and larger temperature gradients corresponding to unstable and stable air conditions. It can be observed that the air with smoke exhaled from a life-sized thermal manikin is locked and stratified at certain heights at stable condition while it mixes well with the ambient air and is diluted quickly through upper openings when the air is relatively unstable. The concentration of contaminant simulated by tracer gas (N2O) is measured both around and 0.35m from the manikin, indicating that the person who exhales the contaminant may not be polluted by himself as the protective effect of the thermal boundary layer around the body, especially in stable condition with two concentration zones and clean air drawn from the inlets. However, other persons facing the respiration some distance away may suffer higher contaminant exposure if the air in room is quite stable and contaminant from the mouth can penetrate a longer horizontal distance. In addition, the air stability slightly changes the velocity profiles, giving higher velocity decay and more turbulent mixing with surrounding air at unstable air condition.