A build-up of moisture in the joists, the sill, the walls or the floor of a basement or crawl space for more than 24 hours causes mould. On damp, unpainted concrete walls, a white powdery substance, which is not mould, may appear. The mould on painted concrete walls may appear as green or red, and other moulds may piggyback them.
Although mould can grow in the lowest moisture environments, it must also have food (organic materials) and oxygen. Allergens, irritants and toxic substances can be produced by moulds. Although many moulds are harmless to someone who is not allergic, a few can be fatal. Mold on unpainted concrete basement walls and floors will only grow if there is organic material, such as damp boxes, wooden forms or framing, in touch with it, and once started, may attract other moulds.
Black mould may be either toxic or cosmetic. Toxic black mould may cause breathing or health-related problems, and may cover large seen and unseen areas. These species include Stachybotrys chartarum and Memnoniella echinata. Non-toxic black mould is more localised, and creates no health issue. These include Sapstain Wood Fungi, Dematiaceous moulds, Black yeasts, and Ophistomoid fungi. Some of these arrive in the lumber used to build the concrete forms and framing.
Mineral efflorescence is not mould, but is sometimes mistaken for mould. It is a white or brownish-white, powdery substance on cement, plaster or breeze block walls or chimneys. Moisture flowing through the walls leaches out salts to form the white powder on the wall’s surface. It indicates that there is, or has been, moisture present, and may indicate the presence of mould elsewhere. Other than a musty smell, it is harmless.
Green mould usually appears on wallboard, boxes and furniture stored in a damp basement. It may also appear on painted concrete with other colours of mould. These include Aspergillus sp., Penicillium sp., and Trichoderma sp. mould.
Red moulds are yeasts, such as Aspergillus versicolor, and appear on surfaces subjected to high humidity. Because they are associated with other moulds, they are sometimes hard to spot.
The medical symptoms caused by damp buildings are many and varied. Molds can cause allergic reactions that include hay-fever like symptoms, and may also irritate the eyes, skin, nose, throat and lungs, whether or not a person is allergic. Existing asthma symptoms may be triggered by moulds. Damp buildings also attract insects and rodents. Mold does not have to be alive to cause health problems. That is why mould identification and removal should be performed by qualified contractors and not by inexperienced homeowners.