A method for measuring leakage of vapors into a chemical-cartridge half-mask facepiece during simulated use is described. The respirator fitted with high-efficiency cartridges, is worn in a dichloro-difluoromethane (ccl2f2)-air test mixture while a series of timed activities (facial movements, varied breathing, exercise, etc.) is performed. Complementing other tightness tests, this method is useful particularly for studying face fit and the effect of facial movements on leakage. The concentration of ccl2f2 within the facepiece is monitored continuously to observe the immediate effect of physical activity on leakage. A composite sample is collected simultaneously in a plastic bag as a measure of average leakage during the 18-minute test period. Leakage, reported as percent, is calculated from the concentrations of ccl2f2 within the facepiece and in the test atmosphere.