A fan pulls in air, which passes over cooling coils, causing water vapor to condense. The condensed water drips down into a pan, which must be emptied regularly. Some units contain a humidistat that turns off the compressor when a desired level of humidity is reached. Dehumidifiers are meant to operate during warm weather, when humidity is high. Most of the less expensive cleaners on the market are of the fan-filter type.
Air is drawn into the appliance by a fan, and the impurities are trapped in a filter.
Electrostatic precipitators draw in air with a fan. The air passes an electrode that gives particles in the air an electrical charge. Then the air passes a collector plate with an opposite charge, and the particles are drawn off and stick to the plate. This kind of air purifier can be installed as part of a forced-air heating system in a home. Negative-ion generators have proved to be the best room air purifiers.