the disclosed method provides an electret filter having a permanent electrostatic charge and formed of polymer fibers having an intrinsic viscosity of 1 . 5 or less . fibers of the requisite viscosity are formed into a non - woven fabric and fed , at room temperature , through a gap between a discharge electrode and a ground electrode while being tightly pressed against the ground electrode . the ground electrode preferrably presents a polished metal surface for contact with the web .