Environmental Engineering Reference
In-Depth Information
are consistent with human glucan challenge studies which have shown an
increase in nose and throat irritation, nasal congestion, and increased airway
responsiveness.
V. Mold infestation — risk factors
Infestation of building materials by a variety of mold species depends on
conditions that favor the germination and growth of viable mold spores and
hyphae. These conditions include nutrient sources, viable mold
spores/hyphae, temperature conditions in the range of 40 to 130°F (5.5 to
55°C), and moisture (R.H.
70% or the presence of liquid water on surfaces)
for 24 hours or more.
Sources that provide the minimum nutritional needs of mold spores
abound in building spaces and structures. These include wooden structural
materials and furnishings and a variety of paper products, textiles, etc. Mold
growth will even occur on the surface of materials that provide no nutrients,
such as steel, plastic, synthetic fibers, etc. In such cases, mold utilizes depos-
ited organic dusts or other natural organic substances as nutrients. These
dusts allow mold to grow on fiberglass filters and ductwork and synthetic
organic textiles (such as those used in carpeting). Mold also grows on metal
and plastic furniture that has been subject to human handling (hand
oils/skin scales apparently serve as the nutrient source).
A variety of studies indicate that in the absence of liquid water, high
relative humidity (
70%) is needed for germination of mold spores and
subsequent optimal infestation conditions on substrate surfaces. Once infes-
tation has taken place, mold growth can continue at lower relative humidity,
particularly for xerophilic species (e.g., Aspergillus spp.). High relative
humidity (if not wetted materials) is required for both the germination and
optimal growth of hydrophilic species (e.g., Stachybotrys ).
Initial mold infestation and subsequent mold growth is favored in build-
ing environments with moisture problems. These include persistent high
relative humidity, episodic condensation on or in building surfaces, water
intrusion through the building envelope, and flooding.
A. High relative humidity
High relative humidity (
70%) is not uncommon in building environments,
particularly residences. It occurs episodically during rainy or foggy weather.
Such conditions in many cases are transient and not sufficiently persistent
to cause mold infestation. However, buildings, most notably residences,
constructed on sites that have been historically wet and have not been
provided with adequate drainage, experience chronic elevated humidity.
Other sites are subject to seasonal high water tables, which cause wetness
in crawlspace, basement, and slab substructures. This wetness is associated
with vaporization and subsequent convective movement of water vapor into
building interiors, causing high relative humidity in building spaces. In some
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