An in-mouth barrier against entry of insects, pollen and particulates in to the open mouth of a user, particularly in strenuous outdoor exercises, needs to be comfortable and fully effective for various extents of an agape mouth, and ideally should require no conscious effort to re-position in even long durations. A filter frame surrounding a screen is formed to the dental arch so as to envelope both rows of teeth except possibly the molars. A flange on the frame holds the frame on the mandibular teeth and prevents the frame from moving down. The end of the flange lies under the tip of the tongue and prevents the frame from moving up. The frame thus remains comfortably inside the vestibule and covers the mouth for the various extents it may be opened to.