Pleated nanoweb structures

1. A filter comprising a filter medium with a pleated structure comprising a nanoweb layer and a scrim, wherein the nanoweb layer comprises fibers with a number average diameter less than 1 micron and layer thickness less than 50 microns, wherein the ratio of total medium thickness-to-pleat-spacing is less than 0.15, wherein the nanoweb layer has a basis weight of more than about 0.6 gsm and wherein the scrim is a nonwoven layer made of fibers selected from the group consisting of spunbond fibers, dry-laid fibers, wet-laid fibers, cellulose fibers, melt blown fibers, glass fibers, and blends thereof.

2. The filter of claim 1 in which the ratio of total medium thickness-to-pleat-spacing is less than 0.12.

3. The filter of claim 1 in which the ratio of total medium thickness-to-pleat-spacing is less than 0.08.

4. The filter of claim 1 in which the scrim is bonded to the nanoweb layer.

5. The filter of claim 1 in which the ratio of filter resistance to the flat sheet resistance of the medium is less than about 35 Pa/mm where flat sheet resistance is measured at an air flow rate of 5.3 cm/s and is reported in units of mm water gauge and filter resistance is measured at an air flow rate of 3344 m 3 /h and is reported in units of inch water gauge and converted to units of Pascal.

6. The filter media of claim 1 in which the filter efficiency does not decrease over a 20 hour treatment after application of a loading of 0.02 micron potassium chloride aerosol at a density of 3.6×10 5 to 3.8×10 5 particles per cm 3 at an air flow rate of 3344 m 3 /h.