In order for mold to grow it needs a food source with a high organic content. This includes wood, paper, cotton, wicker, and other plant-derived materials. Non-organic material, like porcelain and tile are not food sources for mold. When mold is found on these surfaces, as in bathrooms or sinks, it is usually feeding on other organic material that has accumulated on the surface. Thus mold growth in toilets, sinks, or showers can generally be prevented through regular cleaning or maintenance.
Mold also needs moisture to grow. This moisture can come from leaks, floods, condensation, steam, or from high humidity. If mold is present on a surface, then it either currently or in the past had access to moisture. Sometimes the source of the moisture may not be obvious, but it has to be there.
Microbiological contamination is common in high humidity climates. High humidity climates are climates with a relative humidity above 40%. However, microbiological contamination can be present within buildings in any climate and is often a function of moisture incursion from sources such as rainwater, stagnant water in HVAC air distribution systems (e.g. condensation drip pans), and cooling towers. If mold growth is found on a wall it may be the result of a roof or pipe leak. If it is on an exterior wall then there may be water intrusion from outside or there may not be enough insulation so that condensation is occurring on the wall surface inside. The moisture source may not always be from a structural or design problem with the building. It may be due to human activity inside, like steam from showers or cooking, a spilled fish tank, or a leaking waterbed.
The key to preventing mold grow indoors is to limit the opportunities for moisture to come into contact with potential food sources for mold.
In addition to these conditions for growth, it should be noted that a typical indoor air sample should have approximately one-third of the spore count found in an outdoor air sample. This indicates that the filtration component of the HVAC system is performing properly.