A gas mask having a filter canister mount including an inhalation valve and a self-sealing mechanism that prevents air inflow when a filter canister is removed from the canister mount. In one embodiment, the self-sealing valve includes an elastomeric diaphragm mounted in the inlet port and includes a hinge and a skirt, the skirt having a sealing surface that is biased into contact with a valve seat. The filter canister includes a projection adapted to act on the hinge to pivot the skirt away from the sealing surface as the canister is fitted to the canister mount. In one embodiment, the canister comprises a stacked-radial-flow arrangement of particulate and carbon filtration media. In one embodiment, the gas mask includes a visor having a pair of spaced optical panels with a hinge mounted therebetween for relative rotational movement.