This paper discusses the aerodynamic behaviors of a gas mask canister with a complex inner structure and two porous materials in the filter layer and the activated carbon layer. The effects of the distribution and area of holes in the main sieve diaphragm and the thickness of the activated carbon layer on the pressure drop and the flow structure were determined using computational fluid dynamic (CFD) tools. The momentum loss of porous flow calculated by Forchheimer's equation was added to the source term in the momentum equation. Streakline flow visualization was employed to observe gas flow structures within the empty canister and to identify the shortcomings of the prototype canister. Simulation results for the estimated inertial and viscosity parameters in Forchheimer's equation agree closely with experimental values. The porosity of the canister for intake flows of 15-135 L/min causes the flow behavior to transition gradually from linear (viscous effect) to slight non-linear behavior (slight inertia effect). This study uses air age as an index of the time that air resides within the canister to displace the adsorption time of toxic gas. This approach conveniently elucidates overall filter capacity and the positions of dead zones in the activated carbon layer. The simulation results reveal that the channel design of the main sieve diaphragm dominates the aerodynamic behavior of the fluid within the activated carbon layer. Better hole distribution and a larger hole area correspond to a lower pressure drop, a smaller dead zone, and a higher adsorption time. The results in this study provide a valuable reference for designing channels in the main sieve diaphragm, and will be helpful in designing gas mask canisters. (C) 2009 Elsevier Ltd. All rights reserved.