The issue of mold as it relates to indoor air quality and health has recently received significant media attention. In an attempt to answer questions related to mold and wood products, we have reviewed governmental and other public domain information and have compiled the following set of "Common Questions and Answers about Mold." We have provided this information for the convenience of the public, as users of wood-based products, in accordance with the mission of the US Forest Service, Forest Products Laboratory. Please consult the references noted at the end of this document for additional information.

Mold and moisture

• What is mold?    Answer

  Hide

  Mold and/or mildew are microscopic fungi that are present virtually everywhere, indoors and outdoors. Under the proper conditions, they can grow on a variety of organic materials, including wood products (1, 11). Mold growth requires ample moisture in the form of liquid or high humidity. While there is not universal agreement on the lower threshold of humidity required for mold growth, most literature indicates that the relative humidity needs to be at least 70% to 90% to support mold growth (15). Once growth occurs, molds can spread by production of spores.

• How do molds get in the indoor environment, and how do they grow?    Answer

  Hide

  Molds naturally grow in the outdoor environment. Mold spores may enter a house through open doorways, windows, heating, ventilation, and air conditioning 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 (13). Mold needs oxygen, water, nutrients, and a temperature between 32 degrees and 100 degrees Fahrenheit to grow (12, 18). When mold spores come in contact with a food source where there is excessive moisture, such as where leakage may have occurred in roofs, pipes, walls, in plant pots, or where there has been flooding or heavy condensation, mold may grow. Additionally, a relative humidity of at least an average of 80% for a month's duration will provide sufficient moisture for substantial mold growth (7). Many organic materials (e.g., wet cellulose materials, including paper and paper products, cardboard, ceiling tiles, wood, and wood products) provide suitable nutrients to support mold growth. Other materials such as dust, paints, wallpaper, insulation materials, drywall, carpet, fabric and upholstery commonly support mold growth (13).

• Isn't mold happening now because buildings are too tight?    Answer

  Hide

  This rationale for moisture/mold problems is often presented by the media as the main driving force for the current rash of moisture/mold claims. However, mold needs moisture to grow and moisture can come from multiple sources in a house, many of which have nothing to do with the amount of fresh air exchange a building experiences. A chronic plumbing leak releasing water onto drywall or a leaky foundation wall are two examples. There can indeed be cases where the humidity level in a house would be lower with more fresh air, potentially alleviating some moisture/mold problems, but to characterize this as the root cause of all mold problems is incorrect (6).

• The homeowners had a water leak. What is the potential for growing mold in this area?    Answer

  Hide

  Materials that are exposed to a constant leak or have been soaked and not dried thoroughly can support mold growth. Some molds can form a new colony in one or two days on damp materials. Additionally, molds do not require light, and growth can continue indefinitely without light (2).

• What about moisture in the crawl space?    Answer

  Hide

  A definitive answer about moisture problems in the crawl space does not exist. A symposium on the subject held by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (14) produced the following recommendations:

  • There should be proper drainage, clearance and access.
  • Crawl spaces should have ground covers for moisture control. These should be installed carefully to limit evaporation from the soil.
  • Heated crawl spaces should not be vented with outdoor air.
  • Unheated crawl spaces may be vented, but there is no overriding need to do so for reasons of moisture control if an effective ground cover is present. (Note: Check with local building codes to verify whether these practices are permitted in your jurisdiction.)

  The Building Science Corporation web site also contains crawl space information and recommendations. This information includes discussion of the differences between crawlspace design recommendations for cold climates versus warm (moist) climates in accordance with the International Energy Conservation Code (20).

• At what moisture content should someone be worried about mold?    Answer

  Hide

  A useful rule of thumb is that wood with a moisture content greater than 19% is sufficient to support mold growth (19). More specifically, it is the moisture content at the wood surface and the relative humidity in the air above the wood surface that is important. This detail sometimes causes confusion, since a 19% moisture content (average for the entire cross-section) for kiln-dried lumber results in about a 16% moisture content at the surface - dry enough to minimize even the slow growth of most molds. Much higher moisture levels are required for substantial mold growth. And very high moisture levels are required to support decay producing organisms (8), which, unlike mold, can cause permanent loss of structural strength (3). Wood can achieve excessive moisture content when exposed to direct wetting, or from extended exposure to high humidity.

• Do structural adhesives cause mold to grow more readily or faster?    Answer

  Hide

  A recent study (9) on fungal susceptibility of pine and aspen oriented strand board (OSB) found that the amount of mold growth on both OSB types was approximately equal to the mold growth on solid aspen. These results indicate that the adhesive has little or no effect on mold growth. Molds grow best on freely available sources of nutrients (4), and because the cured resins used in structural adhesives are a poor source of freely available nutrients (and may even block mold access to the cellulose fibers), these adhesives are generally assumed not to encourage mold growth.

• What is wrong with mold?    Answer

  Hide

  Mold and/or mildew fungi do not cause wood decay. However high moisture environments that foster mold growth also have the potential to support decay-producing organisms (3). Molds produce spores, which often become airborne, and may trigger an allergic reaction for some people (13). In addition some molds produce mycotoxins and microbial VOCs (volatile organic compounds). However these have not been linked conclusively to health problems in buildings.

• If homeowners think there may be mold in their houses, should they test for it?    Answer

  Hide

  According to the U.S. Environmental Protection Agency (EPA), in most cases, if visible mold growth is present, sampling is unnecessary. Because no EPA or other federal limits have been set for mold or mold spores, sampling cannot be used to check a building's compliance with federal mold standards (1). Additionally, testing for mold is difficult because mold is everywhere; therefore, testing will not prove that a house is free of mold (6). The U.S. Centers for Disease Control and Prevention (CDC) advises that it is not practical to test for mold growth in a house, because large mold infestations can usually be seen or smelled (13). When testing is done, it is usually to compare levels of mold spores inside the house with levels outside the house (2). However, results from air sampling are prone to large errors, and different sampling methods produce different results. A thorough inspection of a house, looking for signs of moisture problems or active mold growth, is likely to be more effective than testing as a way to size up potential problems (6). According to the Centers for Disease Control, it is not necessary to identify the species of mold growing in a residence, and CDC does not recommend routine sampling for molds (13). Current evidence indicates that allergies are the type of disease most often associated with molds (13). Since the susceptibility of individuals can vary greatly either because of the amount or type of mold, sampling and culturing would not be reliable in determining the health risk. CDC recommends that if the occupants are susceptible to mold allergy and mold is seen or smelled indoors, there is a potential health risk, and the homeowner should arrange for its removal (13).

• A qualified environmental lab took samples of the mold inside a home and Returned the results. Can CDC or anyone else interpret the results?    Answer

  Hide

  Standards for judging what is an acceptable, tolerable or normal quantity of mold have not been established. If the homeowner decides to pay for environmental sampling for molds, several questions must be addressed before the work starts. For example, decisions must be made regarding how sampling will be accomplished (i.e., surface wiping by tape or swab, air sampling for spores or bioaerosols, etc). Additionally, the CDC recommends addressing several other issues before the work starts (13):

  • Who will establish the criteria for interpreting the test results?
  • What are their qualifications?
  • What will be done, or what recommendations will be made based on the sampling results?

  It is important to keep in mind that the results of samples taken in a unique situation cannot be interpreted without physical inspection of the contaminated area, or without considering the building's characteristics and the factors that led to the present condition (13). Returned the results. Can CDC or anyone else interpret the results?

• How do you determine if a mold is toxic?    Answer

  Hide

  Unfortunately, it is impossible for homeowners to distinguish between so-called "toxic" and "non-toxic" molds because they all look like black or gray sooty patches (16). Press coverage about lawsuits and health studies involving mold has focused on one type of mold called Stachybotrys chartarum, which is often referred to as "toxic mold." There is no particular reason why this mold should be singled out. All molds and their remediation should be treated with the same precautions (12, 13). (Note: Stachybotrys chartarum is also known by the name Stachybotrys atra. Throughout this document, this mold will be called Stachybotrys for convenience.)

• What is Stachybotrys, and what should people do if they determine they have Stachybotrys in their building or home?    Answer

  Hide

  Stachybotrys is a greenish-black mold. It can grow on material with a high cellulose and low nitrogen content, such as fiberboard, gypsum board, paper, dust, and lint. It prefers very wet conditions and its spores do not readily become airborne. It is not necessary, however, to determine what type of mold may be present. Any mold growing in homes and buildings, whether it is Stachybotrys or other molds, indicates that there is excessive water or moisture present. This is the first problem that needs to be addressed. CDC does not believe that one needs to take any different precautions with Stachybotrys than with other molds. Any extensive mold growth should be treated the same with respect to potential health effects and removal (13).

• How common is mold, including Stachybotrys, in buildings?    Answer

  Hide

  Molds are very common in buildings and homes and will grow anywhere indoors where there is sufficient moisture. The most common indoor molds are Cladosporium, Penicillium, Aspergillus and Alternaria, but these are the most common outdoor molds as well. The scientific community does not have accurate information about how often Stachybotrys is found in buildings and homes. While it is less common than other mold species, it is not rare (13).

Structural Integrity and mold remediation

• Will mold affect the structural integrity of the wood in my home?    Answer
  Hide

  Surface mold on wood from short-term wetting does not structurally damage the wood (3, 6). Mold and mildew are of a different type of fungi than those that cause wood to rot. Unlike wood decay fungi, mold and mildew do not cause significant loss in the strength of wood products. Nonetheless, mold and mildew on wood indicate high moisture, and prolonged periods of high moisture may also support the growth of wood decay fungi (10).

• How is mold growth prevented?    Answer
  Hide

  Controlling moisture is the most important factor. Mold will not grow if moisture is not present. A properly constructed building is designed to keep the inside of a home dry, which will not allow mold growth (1, 2, 3). As part of routine building maintenance, buildings should be inspected for evidence of water damage and visible mold. The conditions causing mold (such as water leaks, condensation, air infiltration or flooding) should be corrected to prevent mold from growing (13). Specific Recommendations adapted from CDC recommendations (13)

  • Keep the overall humidity level in house low enough to minimize the occurrence of localized relative humidity greater than the 80% threshold that will support active mold growth. The appropriate indoor humidity will depend on the climate and, to some extent, on the level of insulation in the building.
  • Use an air conditioner or a dehumidifier during humid months. Some air conditioners may not remove sufficient moisture from the air and an additional dehumidifier may be required.
  • Be sure the home has adequate ventilation, including exhaust fans in kitchen and bathrooms.
  • Use paints that have mold inhibitors in them.
  • Clean bathrooms with mold killing products.
  • Do not carpet bathrooms.
  • Remove and replace flooded carpets.

• How do you get rid of mold?    Answer
  Hide

  It is impossible to get rid of all mold and mold spores indoors; some mold spores will be found floating through the air and in house dust. The mold spores will not grow if moisture is not present. Indoor mold growth can and should be prevented or controlled by controlling moisture indoors. If there is mold growth in a home, it must be cleaned up and the moisture problem must be fixed. If the mold is cleaned up, but the moisture problem is not fixed, then, most likely, the mold problem will reoccur (1).

• How do I determine whether a wood-based product has suffered permanent damage due to high moisture exposure?    Answer
  Hide

  In an initial assessment, the key question that must be answered is: Does this appear to be a "mold only" condition, or does it appear to have become a potential decay situation? Mold and mildew cause no structural damage to wood other than unsightly discoloration (3). Mold growing on solid lumber or other structural wood products is most likely a surface contamination issue, not a structural issue, so the product can be cleaned, dried, and used. However, prolonged exposure to high moisture conditions will also support the growth of decay fungi. If there is any evidence of decay, the product should be replaced (17). In all cases, moldy, porous materials such as celing tiles, gypsum wallboard and carpeting should be discarded (10).

  If the wood is badly decayed, this will generally be quite visible. Two common visual results of decay are a bleached and stringy appearance to the wood, or a darkened surface with cubical cracking. If decay fungi are visible on the surface, the wood has quite likely already suffered strength loss. However, do not rely on visual clues alone. Wood can appear stained and be sound, and can appear normal yet have already suffered significant strength loss due to decay. Use the pick test to determine if the wood is, in fact, sound. Insert the point of a knife at a shallow angle to the surface and attempt to lever up a thin splinter. If the wood splinters, it is sound. If instead it breaks just above the blade like a carrot snapping in half, it is decayed (5).

• Does cleaning have any negative effects on the structural performance of engineered wood products?    Answer
  Hide

  The cleaning procedures recommended by the EPA (i.e. scrubbing with a detergent and water solution) or by others (i.e. scrubbing with a dilute bleach solution) will not degrade the structural characteristics of structural wood products. However, if a remediator wishes to use another type of cleaning solution, the remediator should be asked whether the treatment exhibits any detrimental effects to wood-based products.

• The homeowners had mold growth and have fixed the moisture problem. How do they clean up the mold?    Answer
  Hide

  A detergent and water solution (1) or a 10% bleach to water solution (1-1/2 cups bleach per 1 gallon of water) (13) has been suggested to clean mold and kill fungi. Note that the EPA recommends the detergent and water solution, probably because it has less potential than the bleach/water solution to be used in an unsafe manner. Also note that this cleaning process will not prevent future growth--only environmental change (i.e. eliminating the moisture) can prevent future growth. The CDC recommends that large mold infestations be addressed by a professional who has experience in cleaning mold in buildings and homes (13). As noted by the EPA and others, those who choose to use bleach solutions are cautioned never to mix bleach with ammonia (or detergents that may contain ammonia), to protect eyes and skin from contact with bleach solutions and to provide ventilation if using bleach indoors. Finally, High Efficiency Particulate Air (HEPA) vacuuming after cleaning will remove remaining mold spores (even dead spores may be allergenic).

Health Concerns

• Toxic mold is receiving a lot of media attention. Should homeowners be concered about a possible health risk?    Answer
  Hide

  The CDC provides the following answer to this question:
  
  "The hazards presented by molds that may contain mycotoxins should be considered the same as other common molds which can grow in [a] house. There is always a little mold everywhere - in the air and on many surfaces. There are very few case reports indicating that toxic molds (those containing certain mycotoxins) inside homes can cause unique or rare, health conditions such as pulmonary hemorrhage or memory loss. These case reports are rare, and a causal link between the presence of the toxic mold and these conditions has not been proven. A common-sense approach should be used for any mold contamination existing inside buildings and homes. The common health concerns from molds include hayfever-like allergic symptoms. Certain individuals with chronic respiratory disease (chronic obstructive pulmonary disorder, asthma) may experience difficulty breathing when exposed to mold. Individuals with immune suppression may be at increased risk for infection from molds. If [the homeowner or any family members] have these conditions, a qualified medical clinician should be consulted for diagnosis and treatment. For the most part, one should take routine measures to prevent mold growth in the home." (13)