The optimal relative humidity level for a room ranges between 35 and 45 percent. On cold days, humidity levels above about 40 percent can lead to window condensation, so homeowners may want to maintain a lower humidity level during the winter months.
Humidity levels above 50 percent encourage the growth of mold, mildew, bacteria and dust mites. If humidity levels drop too low, the air in a home may become uncomfortably dry and the dryness could cause wooden furniture, household accessories or musical instruments to crack. Generally, humidity rises in the summer and drops in the winter. Heating systems tend to dry out the air, reducing humidity further during the winter months.
Homeowners can use a device called a hygrometer to monitor relative humidity levels inside a home. Household appliances called humidifiers and dehumidifiers help modulate humidity. Humidifiers work by adding moisture to the air, while dehumidifiers remove moisture from the air. Portable humidifiers and dehumidifiers are designed for use in a single room, and the recommended room size is usually denoted on the packaging or on the unit itself. Whole-house humidifiers and dehumidifiers are installed in the heating system of a building, and work in conjunction with the heater to maintain both temperature and optimal humidity throughout all of the rooms in a home or office.