The fitness of the half facepiece of a gas mask to the face of a worker handling organic solvents was evaluated through the quantitative determination of the concentrations of both organic solvent vapors and airborne dust inside and outside the facepiece. Leakage rates for dust were measured using a mask-fitting tester on a facepiece with a dust filter-covered respirator cartridge of organic solvents. The same facepiece was evaluated for the leakage rate for organic solvent vapors through the determination of the concentrations of the air inside and outside the facepiece. Highly significant correlations were found between the leakage rates obtained for dust and those for organic solvent vapors. The leakage rates for organic solvents were found to be slightly higher than those for dust. The present finding suggested that the leakage of organic solvent vapors from the interstice between the facepiece of gas masks and workers' faces can be evaluated using the leakage rate for dust for the same facepiece of the gas mask attached with the cartridge covered with a high efficiency particulate air (HEPA) filter.