between one quarter inch and one inch in length and have a diameter which is determined primarily by the convenience of producing these fibers in the continuous filament manner typically these fibers are between eight 8 microns and twenty five 25 microns in diameter the lower diameter limit is set by process restraints the upper limit is determined by material usage considerations as well as the hand or feel of the final mat material the coarser fibers result in an abrasive and irritating feel which would make such a mat less desirable the other basic type of fiber in the mat is microfibers microfiber is a term of art referring to fibrous materials having a mean diameter in the neighborhood of one 1 micron these fibers are preferably flame attenuated glass fibers although other compositions could be used depending on the relative costs of such microfibers their ability to disperse evenly in the water slurry of the wet process apparatus and their ability to modify the permeability of the resulitng fibrous mat to the extent in the manner as will be set forth in detail more particularly the microfibers are of the type and are formed in the manner as set forth in u s pat no 4 167 404 which patent is hereby incorporated by reference for the purposes of this invention however microfibers are defined as fibers having mean diameters ranging from 0 05 3 50 microns more typically 0 1 to 0 7 microns these fibers are formed in a flame attenuating process as set forth in the above referenced and incorporated patent after being collected the microfibers as a mat.