A dehumidifier is a household appliance that reduces the level of humidity in air, usually for health reasons, as humid air can cause mold and mildew to grow inside homes, which has various health risks. Relative humidity is preferably 30 to 50%.1 (From the U.S. EnergyStar website. Retrieved 2008-09-29.) Very high humidity levels are also unpleasant for human beings, can cause condensation and can make it hard to dry laundry or sleep.

Mechanical/refrigerative

Mechanical/refrigerative dehumidifiers, the most common type, usually work by drawing moist air over a refrigerated coil with a small fan. Since the saturation vapor pressure of water decreases with decreasing temperature, the water in the air condenses, and drips into a collecting bucket. The air is then reheated by the warmer side of the refrigeration coil. This process works most effectively with higher ambient temperatures with a high dew point temperature. In cold climates, the process is less effective.Frequently Asked Questions (from a reseller website. Retrieved 2007-10-22.) They are most effective at over 45% relative humidity, higher if the air is cold. Desiccant Dehumidification vs. Mechanical Refrigeration (from a manufacturers website. Retrieved 2007-10-22.)

Desiccative

A desiccant dehumidifier is a device that employs a desiccant material to produce a dehumidification effect. As they are more effective for low-temperature and low (relative) humidity levels, they are generally used for these conditions instead of mechanical/refrigerative dehumidifiers - or are used in tandem with them.

Desiccant materials have a high affinity for water vapor. Typically their moisture content is a function of the relative humidity of the surrounding air. Exposed to low relative humidities desiccant materials come to equilibrium at low moisture contents and exposure to high relative humidities results in equilibrium at high moisture contents. The process involves exposing the desiccant material to a high relative humidity air stream, allowing it to attract and retain some of the water vapor and then exposing the same desiccants to a lower relative humidity air stream which has the effect of drawing the retained moisture from the desiccant. The first air stream is the air that is being dehumidified while the second air stream is used only to regenerate the desiccant material so that it is ready to begin another cycle. Note that the first air stream's water vapor content is reduced while the second air stream's water vapor content is increased. Typically the low relative humidity air stream is air taken from any available source and heated to reduce its relative humidity. Hence desiccant dehumidifiers consume heat energy to produce a dehumidifying effect.