| abstract |
A high efficiency filtration medium comprised of a nonwoven filter web of electret charged fibers of a nonconductive thermoplastic resin having a resistivity greater than 1014 ohm-cm, preferably polypropylene. The nonwoven filter web has a basis weight (BW) of less than 60 grams/m2, an effective fiber diameter (EFD) of less than 5 microns and a penetration (PEN) of less than 0.03%, wherein the ratio); BW/(EFD.PEN) is greater than 200. The invention filter medium can be easily used in applications requiring HEPA performance at relatively low pressure drops. |