Abstract: A portable electrostatic air cleaning device having a minimal depthwise air-flow dimension, and particularly adapted to be substituted in an air conduit for a typical furnace or air conditioner filter. Ionizer wires and narrow collector plates are alternately spaced widthwise of the air cleaner, preferably between a pre-filter screen and an after-filter screen. The narrow collector plates are mounted within the device at an angle to the air flow direction to effectively lengthen the air flow through the air cleaning device.The pre-filter screen and the after-filter screen preferably contain electrically conductive materials which are charged to the same potential as the collector plates in order to maximize the electrostatic field strengths.