Air filter assembly having an electrostatically charged filter material with varying porosity

An air filter assembly includes a filter media that is electrostatically charged. The filter media has a first porosity at the inlet side and a second porosity at the outlet side. An electrostatic charge preferably is maintained across the entire filter media to enhance the particle gathering qualities of the media. In one example, multiple layers of filter materials having different porosities are used. In another example, a single filter material having a progressively decreasing porosity is used. One example includes a conductive filter layer that serves as one of the electrodes for the electrostatic field generating portion of the assembly.