Whole-house humidifiers can cause mold in some situations. The most common is if the unit isn’t maintained properly. In these cases, the mold develops on the filter or water pan and blows throughout the house. Humidifiers can also leak into the furnace or other areas where mold can develop. The other possibility is if a model with an automatic humidistat should break, it’s possible for the unit to put too much humidity into the air. High humidity levels can encourage mold or mildew growth throughout the house. Regular maintenance and cleaning are vital to keeping the humidifier clean and effective.5