The heat load, imposed by air-permeable NBC (nuclear, biological, and chemical)-protective suits, can be reduced by improving the air permeability of the suit. However, increased air permeability will reduce the chemical protective performance, since increasing the air permeability of the NBC-protective material will result in higher air velocities through the material. In this study the relation between the chemical protective performance and air velocity through NBC-clothing is evaluated. A theoretical model was developed that describes the chemical protection of air permeable NBC-protective clothing material under various conditions. The initial breakthrough concentration and the 50% breakthrough time are modeled as function of parameters like the air velocity and the challenge concentration. Using this model, the effect of airflow through the material on the breakthrough concentration of mustard vapor was calculated and compared with results of breakthrough experiments. The predictions of the model are in good agreement with the experimental results. The air velocity through the material and thus the air permeability of the material appear to be parameters of critical importance. High air velocity through the material results in high breakthrough concentrations, and therefore poor protective performance of the material. To describe the total breakthrough curve, a semiempirical model of experimental breakthrough results was made. This model describes the total breakthrough concentration of vapor through NBC-protective material as a function of parameters like the air velocity and the challenge concentration. This model can be used as a tool to optimise the protective performance of NBC-protective clothing material.