Abstract: A porous bacteria filter medium of reduced binder content is provided in flexible fibrous sheet form comprised of micron-diameter glass fibers and less than about 30 percent by weight of a polyolefin heat sealable binder fiber randomly dispersed throughout the sheet and fused therein. This filter medium has a bacteria filtration efficiency of at least 96 percent and a porosity in excess of 25 cfm at only two thirds the weight resulting in breathability within comfortable limits over prolonged periods of time. The polyolefin fibers used in the sheet exhibit a surface area greater than 25 square meters per gram and are formed of polymeric material having a molecular weight of 500,000 and a melt index less than 0.5.