What is Mold?
Mold are micro-organisms (micro-fungi) in the Kingdom Fungi. The Fungi are classified as one of the five great kingdoms of life (Margulis and Schwartz, 1997). The Fungi include larger (macro) organisms such as mushrooms and lichens (symbiotic with algae) and smaller (micro) organisms including molds and yeasts. Unlike bacteria (prokaryotic – no enclosed nucleus), the Fungi are eukaryotic (enclosed nucleus). The Fungi includes a wide spectrum of many forms of life. In general, they reproduce by spores (often airborne). Because they are eukaryotic, mold spores are much larger than bacteria and, unlike bacteria, major mold types can often be easily identified under a microscope (but are still too small for the unaided human eye to see).
Why is Mold A Concern?
Mold can cause a variety of concerns. The presence of visible mold is an indication of dampness. Many international studies have shown increased health problems occur in damp buildings (IOH, 2004, WHO, 2008). In damp conditions, mold can damage or discolor building materials and documents. The presence of mold can cause musty odors and affect property value and sales. Slight amounts of visible mold are often enough to cause a home sale to be canceled. Especially when disturbed, elevated airborne mold levels can pose serious health concerns.
Thousands of lawsuits related to mold have occurred in the last 20 years with some claims in the millions of dollars. Because of this, many insurance companies are refusing to provide mold coverage. That is, the perception of mold can pose liability concerns. The clean-up of mold can sometimes be very expensive. Simply put, the perception of a mold problem (even if not real) can have detrimental consequences.
The presence of minor apparent visible mold (may not really be mold) is often enough to stop a house sale or bank loan for a building. There is increased public concern about mold. People refuse to accept moldy situations that were ignored in the past.
There have been changes in building practices and materials. Modern buildings are often more tightly sealed and insulated (becoming damper). They often make use of building materials that mold like such as paper backed drywall and particleboard. Basements were not used for living space in the past but are now in common use. Basements are often damp and prone to problems.
What Does Visible Mold Look Like?
Visible mold often causes discoloration on surfaces, especially drywall. This discoloration sometimes forms circular expanding ring patterns. A close-up look at mold often shows a fuzzy pattern (especially when actively growing). Mold growth can occur in a variety of colors but many molds often turn black after several days. For this reason, the use of the term “black mold” is often misleading. On damp drywall, the problem black mold Stachybotrys often forms a sooty-looking black deposit. This is sometimes mistaken for soot or dirt.
What May Look Like Mold But Is Not?
Many spotted surfaces are often mistaken for mold growth. Dark spots can include dirt and soot (often from candles). Black rust stains (such as on attic sheathing) is sometimes mistaken for mold. Light surfaces (especially whitish, crystalline looking deposits) are often mineral deposits (called efflorescence) that form along the grout lines on concrete blocks in basements. A simple tape test (microscope check) can help determine what is mold and what is not.
What Does Mold Need?
All growing organisms (excluding bacteria) need oxygen, organic food and water. The key to mold growth is moisture. They need amazingly little water to survive and grow. Mold cell walls are made of chitin (forms the tough shells of arthropods and is in our fingernails). These chitinous cell walls allow many forms of mold to survive with very little water. While bacteria need liquid water to survive, many molds can grow when the relative humidity on a surface exceeds about 70-90%. That is, mold does not need liquid water to grow. In fact, mold cannot grow under liquid water (lack of oxygen). Mold only needs damp conditions to grow.
Relative humidity is a function of the moisture in the air and the air temperature. Cold air cannot hold as much humidity as warm air. For this reason, the relative humidity is often high enough to support mold where cold surfaces occur (such as on windows or in corners). Damp conditions are often cool.
As for food, mold has an amazing ability to eat almost anything. They secrete enzymes which can digest cellulose as well as paint, etc. Because they must secrete enzymes, they prefer ground up cellulose and organics (more surface area exposed to digest). For this reason, mold can quickly eat paper or drywall (paper backed gypsum boards) but finds it much more difficult to eat solid wood.
There is a misperception that mold does not grow in cold temperatures. This is false. Anyone who has left food in their refrigerator for several days often finds lots of visible mold. It is true mold will grow much faster at warmer temperatures.
Because mold can eat almost anything, this means that controlling moisture is the key to handling mold concerns (although it can help to use mold and moisture resistant materials). From a practical perspective, this means controlling humidity and minimizing cold surfaces.
What Types of Mold Are There?
There are many different types of mold and they can be classified in many different ways. One simple classification based on health is whether molds are likely to be problem indoor molds or a non-problem indoor mold. The main concern from mold is airborne exposure. For practical purposes, MTS considers mold to include a few major groupings: (1) common non-problem indoor molds; (2) problem damp indoor molds; (3) problem indoor wet molds; (4) common outdoor molds (often not found in large numbers indoors) and; (5) wood rot and macrofungi (large mushroom-like structures).
While there are tens of thousands of mold species, most indoor mold problems are found to be caused by a fairly small number of types of molds. Here is a partial listing of the most common problem and non-problem molds detected in indoor airborne sampling. There are many sources of information on types of mold on the internet. Please refer to other sources for more information.
It should be emphasized small amounts (visible or airborne) of ANY type of indoor mold may not be a concern while LARGE amounts (visible of airborne) of indoor mold ARE often a concern (regardless of what type).
Alternaria (problem damp mold). Often present as a non-problem mold at low levels. However, where elevated airborne levels are present, it may pose a concern. It is considered an important allergen.
Ascospores (common outside mold). This is a broad grouping of molds that may occur at very high levels in damp weather.
Aspergillus (problem indoor damp mold). This is one of the most important indoor problem molds. High levels may cause serious health effects. It can be a serious concern in hospitals at low levels. It has small spores that are easily airborne. It is often grouped together with Penicillium in spore trap tests as Aspergillus-Penicillium. Buildings with mold problems typically have elevated levels of Aspergillis-Penicillium (often >5,000-10,000 spores/m3).
Aureobasidium (problem wet mold). This is a problem mold often found in wet conditions.
Basidiospores (common outside mold). This is a broad grouping of molds that often shows very high outside levels in damp weather. High indoor levels may indicate open windows. Wood rot may sometimes cause elevated levels.
Chaetomium (problem wet mold). This is a common problem mold requiring wet conditions. It has large and sticky spores. These spores tend to settle out of the air quickly and are not usually found at high levels indoors. High levels of this mold suggest wet conditions and recent disturbance.
Cladosporium (common, non-problem mold). This is one of the most common molds in the world. It is widely found in the air both inside and outside of buildings. It’s levels are often highest outside in warm, drier weather. On wood surfaces, it may sometime contribute to minor staining (called blue staining – Schmidt, 2006,p.125). In non-problem buildings, Cladosporium is usually the dominant mold present (may not be true in winter. While usually not a concern, high levels of Cladosporium inside a building may indicate dampness and a possible health concern (allergen). High levels may also indicate open windows.
Penicillium (problem damp mold). This is one of the most important indoor problem molds. High levels may cause serious health effects. It has small spores that are easily airborne. It is often grouped together with Aspergillus in spore trap tests as Aspergillus-Penicillium. Buildings with mold problems typically have elevated levels of Aspergillis-Penicillium (often >5,000-10,000 spores/m3).
Stachybotrys (problem wet mold). This is an important problem mold that requires very damp to wet conditions to grow. It is often the “black mold” found on drywall. It has large spores. These spores tend to settle out of the air quickly and are not usually found at high levels indoors. High levels of this mold suggest wet conditions and recent disturbance.
Ulocladium (problem wet mold). This is a problem mold requiring wet conditions for growth.
References
Flannigan, Brian et.al., Editors, 2001, Micro-organisms in Home and Indoor Work Environments, Taylor and Francis, New York, New York, 490p. Useful guide to mold. (p.39 –water activity required for different types of molds).
IOH, 2004, Damp Indoor Spaces and Health, Institute of Medicine, National Academy of Sciences Press, Washington,DC
Margulis, Lynn and Schwartz, Marlene, 1997, Five Kingdoms – An Illustrated Guide to the Phyla of Life on Earth, 3rd Ed., Freeman & Co., NY
Schmidt, Olaf, 2006, Wood and Tree Fungi – Biology, Damage, Protection and Use, Springer, NY (wood rot and wood stain molds)
Yang,Chin, 2008, Fungi in the air: What do results of fungal air samples mean? – handout, HYPERLINK "http://www.emlabpk.com" www.emlabpk.com (see also Fungal Library in this web site).
WHO, 2009, Dampness and Mould – WHO Guidelines for Indoor Air Quality, World Health Organization, Copenhagen, Denmark (www.euro.who.int/document/e92645.pdf)
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