Written by: Dr. Charles Hodges
A September 10, 2011 headline in the Raleigh News and Observer read “Irene leaves Aurora sodden in mold, misery”. Similar headlines were probably common in other newspapers and on radio and television as Hurricane Irene took her heavy rains northward, resulting in flooded homes and businesses - and heavy mold development - in her wake.
Mold development is a common occurrence whenever flooding occurs. However, it does not take hurricanes and flooding to cause mold development in homes and other buildings. Mold can develop anywhere there is adequate moisture and a suitable substrate on which to grow.
Water from leaks in the roof and from broken pipes is the most common and serious cause of mold in buildings. However, there are several other sources of moisture in homes that can result in the occurrence of mold. Some of these include crawl spaces where the soil has become wet, poorly ventilated bathrooms, kitchens where steam from cooking condenses on the cabinets or walls, small closets in which are placed wet clothes or shoes, window sills wet by water that condensed on the inside of window panes, and condensation of moisture within HVAC ducts and on metal vents during the summer when the air conditioning is on.
Two mold samples received in the Plant Disease and Insect Clinic during the winter came from old houses that did not have wall insulation. The houses were occupied by elderly residents who kept the temperature very high, causing water to condense on the inside walls and leading to very heavy mold development.
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| Mold inside wall (Photo: PDIC Database) |
Molds belong to a group of organisms called fungi. There are many thousands of different kinds of fungi. Some of these cause diseases of plants and animals, some are responsible for the decomposition of dead organic matter, and others are important in fermentation processes or in the production of antibiotics. The basic vegetative phase of molds (threads called hyphae), and the reproductive phase (spores), are microscopic. When large colonies of mold develop and produce spores, however, they are often visible to the naked eye. The colonies can be of many different colors – white, black, blue, green, pink, or brown - depending on the individual mold species that produce them.
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| Variety of household molds (Photo: PDIC Database) |
Most molds are spread primarily as spores that move through the air on even the most gentle of air currents. Upon landing on a suitable substrate, and with enough moisture, spores germinate and form a colony. The rate of development of the colony depends on the individual mold species, the substrate, and temperature and moisture conditions. Many of the most common household molds prefer substrates containing cellulose; e.g., the cardboard covering of sheetrock, ceiling tiles, wood, paper, and fabrics. Dead leaves of ornamental plants are a common source of mold spores in buildings. The leaves fall from the plant into the container where they are wet when the plant is watered, often resulting in heavy spore production. In the outdoors, mold spores are formed in large numbers on all types of dead organic material, are spread through the air, and often enter homes through open doors or windows. These are often the same species of molds that are found indoors.
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| Mold on wood wall (Photo: PDIC Database) |
Molds can and do cause various kinds of damage in buildings, but for the most part the damage is cosmetic, such as stains on walls, floors, ceilings and furnishings.
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| Heavy mold on wall (Photo: PDIC Database) |
Molds also can produce unpleasant odors. Most mold fungi are unable to decompose wood, thus do not cause structural damage. Any structural damage in buildings where mold is present usually is caused by water per se rather than molds. At times, the presence of surface molds can indicate that wood decay fungi are also present. These fungi are capable of decomposing wood and, like mold fungi, are active at high levels of moisture. Often there is no visual evidence of wood decay fungi on the surface of the wood, but the presence of surface molds indicates conditions that could lead to damage by wood decay fungi.
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| White mold on window sill (Photo: PDIC Database) |
Most people are concerned about the health risks associated with household molds, and in some cases these concerns are valid. Many people have allergenic reactions such as headache, sneezing, runny nose, skin rash, etc. to high concentrations of certain mold spores. Other individuals can be in the same room at the same time and have no reaction. Some mold fungi produce toxic compounds and exposure to these can pose more serious health hazards, especially in individuals that have a compromised immune system such as from radiation treatment or chemotherapy. Unfortunately there have been numerous reports in the media about the danger to human health from the so-called black toxic mold Stachybotrys chartarum, especially in buildings that have been flooded. More recent research has shown that this fungus is not nearly as dangerous as once reported.
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| Mold on windowsill (Photo: PDIC Database) |
Most mold problems in homes can be detected visually when the colonies develop on walls, ceilings and floors. However, molds can develop in other areas where they may not be easily seen, such within walls, attics, crawl spaces, within HVAC ducts, or inside cabinets, especially cabinets under sinks where water leaks often occur. The pile of carpets made with artificial fibers is usually resistant to mold growth; however, the backing of most of these carpets support mold growth if sufficient moisture is present. One should automatically assume that when water leaks occur, there is an excellent chance of mold development until the water problem is corrected and the wetted material has dried out.
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| Mold on wooden chair. This chair was packed for shipping before it dried completely, resulting in mold development in transport. (Photo: PDIC Database) |
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The identification of household mold fungi, especially to the level of species, is often difficult and time consuming. One method of sampling mold spores in buildings uses an air sampler that draws air over a microscope slide coated with a sticky substance onto which the spores adhere. The slide is then examined under the microscope and the spores identified, usually to the level of genus or groups of genera. It is not possible to identify the spores to the level of species using this method. This is understandable when one considers that the genera Aspergillus and Penicillium, some species of which are common household molds, each contain more that 200 known species. This method does, however, give a relative measure of the number of spores in the air at the time of sampling.
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| Colony of Penicillium (Photo: PDIC Database) |
Another method uses culture plates containing a general purpose growth medium that can be purchased at stores such as Lowes or Home Depot or your local hardware store. The lid from the plate is removed and the bottom containing the medium is placed on the floor. Plates are usually placed in different rooms in the house, and in the attic and crawl space. Since the spores fall onto the plates by gravity, it is helpful to do the sampling when very little activity is going on in the house and the ventilation fan is off. After a prescribed period of time, perhaps one hour, the lids are replaced and the plates are left at room temperature for several days.
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| Culture plate submitted by homeowner (Photo: PDIC Database) |
At this time the mold colonies are beginning to develop, usually one colony develops from one spore. The molds on the plate can be identified by sending the plates to private laboratories as indicated by the instructions in the package.
The Plant Disease and Insect Clinic also provides mold identification services. To submit a sample, collect mold by swiping a clean cotton swab through the colony, and immediately placing the swab in a clean plastic bag and sealing it. A separate sample should be taken for each type of mold colony seen in each room or from different rooms. Submit the samples to the Plant Disease and Insect Clinic, where they will be cultured on growth media. Identification will be made at least to the level of genus and to the level of species for the more common molds. This method gives only a relative indication on how much mold is present. We also accept culture plates collected as described above.
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| Several mold/ yeast varieties identified by the PDIC. Culture plates were streaked with a cotton swab and sealed. After several days, the diagnostician examines the colony types for identification. (Photo: PDIC Database) |
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Most mold can be removed by wiping the moldy area with a cloth wet in a ten percent solution of common household bleach. Finished surfaces or fabrics should be tested to see if the bleach will harm the surface. If the mold growth is heavy, a face mask should be worn while cleaning. Household bleaches have little residual activity, and it may be necessary to clean again until the substrate is too dry to support mold growth. If the mold is covering a large area which requires removal of water-damaged floor, walls or ceiling, the room should be sealed by closing the door or sealing the opening with plastic while the renovation is being done so that the spores do not spread to the rest of the house. There are a large number of private mold remediation companies, which can safely remove mold from buildings.
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| Mold growing on a fabric cap (Photo: PDIC Database) |
The most important thing that the public can do about molds is to prevent water problems or quickly correct them when they are found.
