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Molds have been part of the environment
since humans have walked the planet. References to mold can be found
in the Bible and scientists estimate molds evolved more than 400
million years ago.
Molds are part of the diverse group of
organisms called fungi, which include a wide range of species from
tiny molds to large mushrooms. Fungi are part of nature's recycling
system and play a key role in breaking down organic materials such
as plants, leaves, wood fiber and other natural materials. Fungi
extract food from organic material, and they grow and reproduce
by way of spores.
Conservatively, more than 100,000 species
of mold exist in the world and at least 1,000 species are common
in the U.S. Molds and other fungi are estimated to comprise a quarter
of the entire biomass on earth.
As such, molds and mold spores are everywhere.
The air we breathe is a virtual jungle of fungal spores and we regularly
encounter mold spores as part of everyday life.
There are a host of materials in and
around a house that, under proper conditions, can become a breeding
ground for mold. Molds require four things for growth: food, suitable
temperature, oxygen and moisture. In certain situations, wood can
provide the necessary elements to allow mold to grow.
Wood is organic, consisting of biological
materials such as cellulose and lignin. Since wood fiber comes from
a living tree, it also contains sugars, starches and proteins that
can serve as a food source for mold.
Molds have broad temperature requirements,
but grow best between 70 and 85 degrees Fahrenheit. Oxygen is readily
available in most areas where wood is used, as are favorable temperatures.
The key element for mold growth on wood
is moisture. All wood contains moisture. But once the moisture content
of wood falls below 20 percent, mold growth cannot be supported.
Depending on the climate, framing lumber will dry to below 20 percent
moisture content during construction and before the building is
enclosed.
Under proper conditions, a variety of
molds can form on wood. Research by Oregon State University revealed
that Douglas Fir sapwood was colonized by more than 45 species of
fungi within six weeks of sawing. Of the mold species identified,
none were Stachybotrys, which is commonly referred to as
a "toxic mold."
Molds that grow on wood typically discolor
the wood through production of pigmented spores that can be yellow,
green, orange, black and an array of other colors. The discoloration
is usually confined to the surface of the wood.
Stain fungi are different than mold and
penetrate more deeply into the wood surface. These fungi darken
as they age, creating a "blue stain" in the wood.
Decay fungi may also grow when wood products
are exposed to chronic moisture. Decay fungi, unlike molds, attack
beyond the surface of the wood into the structural polymers of the
fiber, reducing its strength. Generally, decay fungi invade wood
only after prolonged exposure to moisture, such as what occurs with
plumbing leaks or seeping from outside water sources.
Humans have coexisted with molds for
thousands of years. So why are molds a concern today? Recent court
cases involving mold, sensationalistic media coverage and publication
of questionable scientific research have increased public awareness
of the issue.
To date in the U.S., there are no regulations
or exposure limits for molds or mold spores, and the mycotoxins
that may be present. While there are many claims that so-called
"toxic molds" like Stachybotrys cause severe health effects,
the U.S. Centers for Disease Control (CDC) states: "At present,
there is no test that proves an association between Stachybotrys
chartum (Stachybotrys atra) and particular health symptoms."
You can find additional information about
mold online at these sites:
Mold, Housing and Wood
Western Wood Products Association
www.wwpa.org/lumberandmold.htm
Mold and Mildew
Restoration Consultants
www.restcon.com/links/articles/mold_and_mildew.html
Facts About Mold
American Industrial Hygiene Association
www.aiha.org/content/accessinfo/consumer/factsaboutmold.htm
Mold: Causes, Health Effects and Clean-Up
Building Science Corporation
www.buildingscience.com/resources/mold/mold_causes.pdf
Also, see the other Fast Facts on mold and
wood products in this series:
No. 1 - What is Mold?
No. 2 - Preventing and Controlling
Mold
No. 3 - Cleaning Mold on
Wood
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