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The present invention is directed to an air filter medium comprising nanofibers, the filter medium having a thickness of less than 0.25 millimeters, a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in

The present invention is directed to an air filter medium comprising nanofibers, the filter medium having a thickness of less than 0.25 millimeters, a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size. A coating of the nanofibers may also be used to enhance performance of existing filter media by increasing the FOM of the existing filter media. Preferably, the nanofibers are fibrillated.

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1. A filter medium comprising:fibrillated fibers having a Canadian Standard Freeness of less than about 45, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater t

1. A filter medium comprising:fibrillated fibers having a Canadian Standard Freeness of less than about 45, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to provide a filter medium that meets or exceeds the performance criteria.2. A filter medium of claim 1 wherein said fibrillated fibers have a Canadian Standard Freeness of less than about 10.3. A filter medium of claim 1 further including a support layer comprising a material selected from the group consisting of wovens, nonwovens, spunbonds, meltblowns, cellulose, and other fibrous materials.4. A filter medium of claim 1 wherein said fibrillated fibers are wet laid to form said filter medium.5. An air filter comprising the filter medium of claim 1.6. A filter medium comprising:fibrillated fibers having a Canadian Standard Freeness of less than about 45 and microglass fibers, said filter medium having: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size, and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample size 100 cm2; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, meets or exceeds the efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size, and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample size 100 cm2.7. A filter medium comprising:fibrillated fibers having a Canadian Standard Freeness of less than about 45 wherein said fibrillated fibers are electrostatically charged or produce a triboelectric charge, said filter medium having: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size, and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample size 100 cm2; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to meet or exceeds the efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size, and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample size 100 cm2.8. A filter medium comprising:fibrillated fibers having a Canadian Standard Freeness of less than about 45 and an additive selected from the group consisting of activated carbon, activated alumina zeolites, diatomaceous earth, silicates, aluminosilicates, titanates, bone char, calcium hydroxyapatite, manganese oxides, iron oxides, magnesia, perlite, talc, polymeric particulates, clay, iodated resins, ion exchange resins, ceramics, or combinations thereof, said filter medium having: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size, and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample size 100 cm2; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to meet or exceeds the efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size, and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample size 100 cm2.9. A filter medium of claim 8 wherein said additive is chemically treated to impart microbiological resistance or biocidal capabilities to said filter medium.10. A filter medium comprising:fibrillated fibers comprising lyocell, acrylic, nylon or combinations thereof, having a Canadian Standard Freeness of less than about 45 admixed with microglass fibers, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to provide a filter medium that meets or exceeds the performance criteria.11. A filter medium comprisingfibrillated fibers comprising lyocell, acrylic, nylon or combinations thereof, having a Canadian Standard Freeness of less than about 45 admixed with microglass fibers, wherein the microglass fibers have a fiber diameter of less than about 1 micron, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of: about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, can provide a filter medium that meets or exceeds the performance criteria.12. A filter medium comprising:wherein the fibrillated fibers comprising lyocell, acrylic, nylon or combinations thereof have a fiber diameter of about 250 nanometers, a Canadian Standard Freeness of less than about 45, and a chopped fiber length of about 1 to about 8 millimeters, admixed with microglass fibers, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, can provide a filter medium that meets or exceeds the performance criteria.13. A filter medium comprising:fibrillated fibers comprising lyocell, acrylic, nylon or combinations thereof, having a Canadian Standard Freeness of less than about 45 admixed with microglass fibers, wherein the fibrillated fibers comprising lyocell, acrylic, nylon or combinations thereof are present in an amount of about 5% to about 40% by weight of said filter medium, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in sire and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to provide a filter medium that meets or exceeds the performance criteria.14. An air filter comprising a filter medium comprising:fibrillated fibers comprising lyocell, acrylic, nylon or combinations thereof, having a Canadian Standard Freeness of less than about 45 admixed with microglass fibers, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to provide a filter medium that meets or exceeds the performance criteria.15. A filter medium comprising:fibrillated fibers having a Canadian Standard Freeness of less than about 10, said filter medium having performance criteria of: a thickness of less than or equal to about 0.25 millimeters and a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to provide a filter medium that meets or exceeds the performance criteria.16. A filter medium according to claim 15 wherein said fibrillated fibers comprise lyocell, acrylic, nylon or combinations thereof.17. A process of making an air filter medium comprising the steps of:dispersing fibrillated fibers; and forming a layer of the fibrillated fibers having a thickness of less than about 0.25 millimeters such that said filter medium has performance criteria of: a Figure of Merit of greater than about 0.075, and an efficiency of greater than about 99.9% when capturing aerosol particles of about 0.18 microns in size and a pressure drop of less than about 40 millimeters water column at a flow rate of about 32 liters/minute through a sample 100 cm2 in size; and wherein if said filter medium is thinner than 0.25 millimeters, then the efficiency and pressure drop of the thinner filter medium is adjusted to account for a difference in thickness up to 0.25 millimeters thick, to provide a filter medium that meets or exceeds the performance criteria. 18. A method of enhancing performance of an existing filter medium comprising the steps of:providing a plurality of fibrillated nanofibers; wet laying or coating from a fluid suspension the fibrillated nanofibers onto the existing filter medium wherein the fibrillated nanofibers are loaded onto the existing filter medium at a rate of about 0.5 g/m2 to about 11.0 g/m2; and increasing a Figure of Merit of the existing filter medium by at least about 100%. 19. A method of claim 18 wherein in the step of providing a plurality of fibrillated nanofibers, the nanofibers comprise lyocell nanofibers, acrylic nanofibers, nylon nanofibers or combinations thereof, and further including microglass fibers.20. A method of claim 18 further including the steps of providing a binder wherein the fibrillated nanofibers and binder are wet laid or coated from a fluid suspension onto the existing filter medium.