A unidirectional valve for use with a filtering face mask. The valve can include a flap, and a valve seat. The flap can be cantilevered and can flex away from a seal surface of the valve seat to an open position. The valve can further include a housing that at least partially covers the valve seat and the flap. In some embodiments, the height of the housing is no greater than about 1.25 cm, while the distance between the flap and the seal surface when the flap is in the open position is at least 0.35 cm. A projection can be coupled to an inner surface of the housing, and the height of the projection can generally decrease toward the front of the housing. In some embodiments, the projection can have a profile that substantially follows the profile of the flap when the flap is in the open position.