Molds naturally grow in the environment. Their spores are aerodynamic and carried by wind currents to a suitable place to grow. They enter your house through open doorways, windows, and heating, ventilation, and air conditioning (HVAC) systems. Spores in the air outside also attach themselves to people and animals, making clothing, shoes, bags, and pets convenient vehicles for carrying mold indoors.
(Mold growing on the surface of a unit ventilator)
A source of constant moisture is required for mold growth. Controlling moisture stops mold propagation.
(Photo: Mold growing on the backside of wallpaper) When mold spores drop on places where there is excessive moisture, such as where leakage may have occurred in roofs, pipes, walls, plant pots, or where there has been flooding, they will grow. Many building materials provide suitable nutrients that encourage mold to grow. Wet cellulose materials, including paper and paper products, cardboard, ceiling tiles, wood, and wood products, are particularly conducive for the growth of some molds. Other materials such as dust, paints, wallpaper, insulation materials, drywall, carpet, fabric, and upholstery, commonly support mold growth.
Mold exposure does not always present a health problem indoors, although some people are sensitive to molds. The common health concerns from molds include hay fever-like allergic symptoms. Certain individuals with chronic respiratory disease (chronic obstructive pulmonary disorder, asthma) may experience difficulty breathing. Individuals with immune suppression may be at increased risk for infection from molds.
(Photo: mold growth on fiberboard) Some people may have more severe reactions to molds. Severe reactions may occur among workers exposed to large amounts of molds in occupational settings, such as farmers working around moldy hay. There are very few case reports of toxic molds inside homes causing unique or rare health conditions such as pulmonary hemorrhage or memory loss. A causal link between the presence of toxic mold and these conditions has not been proven.
Centers for Disease Control and Prevention State of the Science on Molds and Human Health. 10/07/03
US Environmental Protection Agency Indoor Air Quality Molds (link no longer live). 10/07/03
Minnesota Department of Health Mold in Homes. 10/07/03
New York City Department of Health Facts about Molds. 10/07/03
See also http://www.onlinetips.org/instances-mold