A method of making a filtering face mask, which method includes the steps of: (a) providing (i) a valve seat that comprises an orifice and a seal surface, wherein the orifice allows air to pass therethrough and is surrounded by the seal surface, and (ii) a single flexible flap; (b) supporting the single flexible flap non-centrally and operatively relative to the orifice of the valve seat to form an exhalation valve, the single flexible flap being supported such that there is an imposed curved profile that extends from a first point where a stationary portion of the flap is supported on the valve seat to a second point where a free portion of the flap contacts a seal surface; and (c) attaching the exhalation valve to a mask body that is adapted to fit over the nose and mouth of a person.