Typically, dehumidifiers should be set to a relative humidity, or RH, level between 30 and 50 percent. This indicates how much water vapor is actually in the air in contrast to the amount the air is able to hold at a specific temperature.
In cold weather when a heating system is in use, the RH level should be kept between 30 and 40 percent. Otherwise, condensation can form on windows. Relative humidity levels above 50 percent are more likely to allow bacteria to grow. Some dehumidifiers do not have components that monitor humidity level. The Energy Star program recommends the purchase of a hygrometer, which independently reads the humidity level of a room and guides the homeowner to turn the dehumidifier to the on or off setting.