McIlvaine et al. (1992) explain
The patent [for the two-stage filtration device] covers the combination of a HEPA filter and an ionizer. The ionizer has alternate wire and plate electrodes connected to a dc high voltage low current source and is located upstream of the HEPA filter remote from the air inlet base.
It consists of two filter stages, an ionizer stage, and a special absolute filter collector stage. This combination enables the following: (1) Increase of the efficiency of absolute filters to ratings which heretofore were unobtainable; and (2) reduction of initial pressure drop by up to 50 percent for efficiency ratings in comparable ranges featured by conventional absolute filters operating on a mechanical basis. A further advantage is considerably slower increase in the pressure drop, as dust collects over the course of operation.
In addition to the separation phenomena featured in a purely mechanical absolute filter, forces of attraction are created in the electrostatic HEPA/ULPA filter which act between the charged or polarized particles, and the polarized fibers of the filter medium. By virtue of these electrostatic forces, a filter medium with a lower intrinsic mechanical efficiency rating—with the important low specific pressure drop associated therewith—can be stimulated to perform with a higher efficiency rating.
In the case of purely mechanically operating absolute filters, the accumulation of particles in and on the filter medium and its microfine fibers results in formation of a filter cake. With the electrostatic HEPA/ULPA filter, however, the separated particles form dendrites on the filter medium in the form of treelike structures. These particular particle structures—as well as the fact that a portion of the particles deposited will remain on the separation plates of the ionizer and the collector—having the significant advantage of a prolonged initial penetration of the filter medium and resulting in a considerable reduction of the pressure drop.