Air conditioning for comfort means the provision of warm, filtered air. High moisture content or humidity is oppressive, but a low humidity may cause irritation by excessive loss of moisture from the skin. In some climates steps may be taken to add or remove water vapor from the air. The air is cleaned, usually by passage through a fabric filter which may be dry or moistened with a viscous liquid, and heated electrically or by banks of finned tubes supplied with steam or hot water over which the air is blown. Electrostatic precipitation provides an alternative method of air cleaning. The fine particles entrained in the air are charged by the absorption of electrons as they pass between two electrodes. The charged particle then migrates in the electrical field and is finally arrested on one electrode.