rectangular circular triangular elliptical trapezoidal etc when viewed in cross section the valve seat preferably is rigid in structure so that the seal surface maintains its desired configuration although the valve seat desirously is rigid in structure the cross members onto which the valve seat is joined may be sufficiently flexible to enable the mask body to conform to the wearer 39 s face and to allow it to return to its desired configuration when deformed from for example striking another object during use fig4 shows that a valve cover 42 may be placed over the valve seat 38 the valve cover 42 may be integrally joined to the valve seat along one edge in a hinged manner or may be glued welded mechanically joined or secured thereto by a combination of such means the valve cover and the valve seat therefore may be made as a single part examples of valve covers that may be used are shown in u s pat nos des 347 298 and des 347 299 the valve cover may include one or more surfaces that mechanically secure the flexible flap to the valve seat 38 the valve cover may be made from similar or different materials than the valve seat but typically will be made from the same rigid plastic fig5 shows a cross section of an example of a filtering structure 18 that may be used in connection with the present invention as illustrated the filtering structure 18 may include one or more cover webs 70 a and 70 b and a filtration layer 72 the cover webs 70 a and 70 b may be located on opposing sides of the filtration layer 72 to capture any fibers that could come loose therefrom typically the cover webs 70 a.