But an area exposed to a small leak, a surface often wet from condensation, or the water pan of an appliance may support mold growth. Condensation on wood surfaces cooled by heat loss at night is an important moisture source for mold. Convective transfer of indoor moisture into the wall cavities due to holes in the vapor retarder also can deposit large quantities of moisture in the wall cavity, creating mold growth in the spring and summer months. Other factors that contribute to indoor mold levels include weather conditions, the amount of shade near a home, organic debris levels outside a home, and the presence of landscaping. Human activities such as walking and cleaning can increase airborne concentration of mold and other microorganisms 20 times or more.
Many appliances produce airborne mold spores while they operate.
Unless humidifiers are properly maintained, mold can grow in the units and its spores can become airborne. Refrigerators and clothes dryers also can create problems. If the refrigerator is not operating properly, however, significant quantities of stagnant water can accumulate. The exhaust pipes of clothes dryers often contain a variety of microorganisms, including mold. If the dryer is vented to the inside of the home, the spores, along with the excess moisture, are brought into the home.