An electrostatic filter having an improved dielectric is described. The dielectric constitutes a dielectric mat formed from high porosity non-woven fibers having pores that have a void fraction of at least 97%. The dielectric mat is sandwiched by a pair of porous electrodes. The fibers that form the mat have a maximum cross sectional thickness that is less than 100 microns and a length to maximum thickness ratio of greater than 10. The pores are preferably statistically substantially equally sized and open in three dimensions. In various preferred embodiments, the void fraction of the dielectric mat is greater than 99 percent. The mat preferably has a tortuosity of at least 2. The maximal cross sectional width of the fibers is preferably in the range 0.1 to 50 microns. In many implementations, the fibers that form the dielectric mat are formed from either polyester or polypropylene. In some implementations, the cross section of the fibers will have at least one (and preferably two or more) distinct regions of small transversal radius of curvature along its perimeter.