Most automotive and heavy-duty air filtration medias are papers wet laid from cellulose fibers that are subsequently saturated with polymers. One critical factor in the performance of these filtration media is the retention of such physical properties as stiffness, tensile strength and Mullen burst strength while being exposed to moisture in the form of 100% relative humidity air. Typically this retention of properties is improved through the use of formaldehyde based cross-linking agents. In this paper we demonstrate that the loss in properties is a result of the absorption of moisture into the sheet, and that cross-linking the sheet inhibits this absorption, and thus improves the retention of physical properties.