Abstract: An air ionizer comprises an ion emission wire and a metallic reflecting shield partially surrounding it, the wire and the shield being connected to a high voltage source. The shield is insulated from ground and its two edges define therebetween a discharge region for the emitted ions. The reflecting polarity of the wire and reflecting shield potentials are the same. The high voltage potential at the shield is at least 3000 V, preferably 5000-10,000 V and does not exceed that at the wire. The potential of the high voltage source to which the shield is connected is at least equal to the potential prevailing at the shield edges due to the electrical field generated by the electrical charge at the wire in the absence of a reflecting shield.