A dehumidifier with a built-in humidistat is generally set at a relative humidity level between 30 and 50 percent. If winters are cold and heating systems are needed, a level between 30 and 40 percent keeps windows from fogging up due to condensation.
At levels greater than 50 percent, mold spores, dust mites, bacteria, cockroaches and other pests flourish. Lower levels sometimes harm wood floors and crack ceilings while contributing to skin and respiratory irritation. If a dehumidifier does not have a humidistat, an individually-purchased hygrometer, hung near the appliance, reports the relative humidity. This allows the user to turn the dehumidifier off and on as needed.