They work by passing an air stream through a wet medium, such as a sponge or dampened grill. Some have fiber honeycomb-type panels that wick water upward from a reservoir others employ a foam or cloth drum that spins through a water-filled trough as air passes by.

Still others blow air through a woven aluminum pad that is saturated by a constant stream of water. In some cases, filters are treated with a special compound that retards bacterial growth.