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Fungi require water for
nutrient absorption and normal metabolic activity. Water
requirements for fungi and other microorganisms are usually
expressed as water activities (aw). The water activity is equal to
1/100th the equilibrium relative humidity when expressed as a
percent. Relative humidity is an expression of how much moisture is
in the air as a percentage of the total moisture the air can contain
at the current temperature. For example, if the air has all the
water vapor that it can contain at a given temperature, the relative
humidity is 100%; if the air has only half of the vapor that it can
contain at a given temperature, the relative humidity is 50%.
Equilibrium relative humidity is the humidity of a tested material
while at the same vapor pressure as the ambient environment. In
other words, moisture is not migrating to or away from the tested
material.
Water activity of 0.60
is considered the lower limits for microbial growth. Below 0.55 all
metabolic activity ceases and DNA is denatured.
|
<0.9 |
most bacterial activity inhibited |
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<0.8 |
most yeast
activity inhibited |
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<0.7 |
most fungal
activity inhibited |
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<0.6 |
all microbial
activity inhibited |
Fungi Growth and Wood
Moisture Content
Normal moisture
levels for wood building materials typically range from 8 to 14%.
It is widely recognized that significant decomposition by
wood-rotting fungi will occur above the fiber saturation point at
28-30% moisture content. Depending on the type fungi present,
pre-conditioning and limited decomposition may occur at moisture
levels as low as 20%. Several non-rotting fungi, including certain
species of Aspergillus and Penicillium, grow at water
activities as low as 0.70, which correspond to wood moisture
contents below 15%. Optimal water activities for these
fungi range from 0.80 to 0.90, which correspond to wood moisture
contents of 16-20%.
|
Wood Moisture
Content (for a typical softwood) |
|
7% |
9% |
12% |
14% |
16% |
20% |
24% |
30% |
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Normal |
Borderline |
High |
|
no fungal growth |
fungal growth
possible at 16% |
fungal growth and
potential wood decomposition |
A moisture content
of 16% is well-recognized as the limit for surface mold.
Lstiburek, J. 2002. Moisture control for buildings. ASHRAE
Journal. February 2002.
Andrews, S. 2002. Mold: a growing concern. Professional Builder.
April 2002.
Fungi
differ in their specific requirements for water.
Water activities vary
based on the species and substrate in question. Most molds grow
best at 0.80 to 0.95. It should be noted that water activity and
relative humidity are not the same as "percent moisture", which is
simply the percent of water within a given substrate.
|
Fungus
|
Water Activity
(aw)
|
|
Saccharomyces |
0.61 - 0.95 |
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Aspergillus |
0.65 - 0.98 |
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Wallemia |
0.65 - 0.87 |
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Penicillium |
0.78 - 0.86 |
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Alternaria |
0.84 - 0.88 |
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Cladosporium |
0.82 - 0.88 |
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Fusarium |
0.86 - 0.91 |
|
Mucor |
0.90 - 0.94 |
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Rhizopus |
0.90 - 0.93 |
|
Stachybotrys |
0.91 - 0.94 |
Water requirements may
be met by high relative humidity.
Generally speaking, most
molds will grow when the relative humidity of the air is 70% or
greater. Relative humidity is affected by temperature; so when
outdoor air cools from 70° to 50°F, the relative humidity may
increase from 40% to 70%. Differences in ambient temperature (e.g.
indoor versus outdoor temperatures) can therefore cause condensation
to form on cooler surfaces, especially in areas such as windows,
crawlspaces, and attics where cooler and warmer air meet.
Condensation occurs when the dew point is reached (the temperature
at which water will condense from the air if the air mass were
cooled). When the relative humidity is high, the dew point will be
close to the air temperature. At 100% relative humidity, the dew
point is equal to the air temperature. When the relative humidity is
low, the dew point is much lower than the air temperature.
The shaded ranges of
each moisture spectrum represent suitable conditions for fungal
growth.
|
Water Activity (aw) |
|
0 |
0.1 |
0.2 |
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
0.9 |
1.0 |
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0 |
10% |
20% |
30% |
40% |
50% |
60% |
70% |
80% |
90% |
100% |
|
Equilibrium
Relative Humidity |
|
Wood Moisture Content (for a typical softwood) |
|
0 |
3% |
5% |
7% |
9% |
12% |
14% |
16% |
20% |
24% |
30% |
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0 |
10 |
20% |
30% |
40% |
50% |
60% |
70% |
80% |
90% |
100% |
|
Equilibrium
Relative Humidity |
Fungal spores may remain viable even when dried or chemically
treated.
Fungi will start to make
spores when the nutrients of the given substrate are exploited or
when triggered by certain environmental factors. Usually this
occurs within 72 hours under high moisture conditions. Under
unfavorable conditions, mold spores can remain dormant even in
extreme environments. Spores also do not have the same water
requirements as the vegetative cells and hyphae. If left
undisturbed, spores may survive for months or even decades. As
moisture is once again introduced, the spores germinate to give rise
to renewed vegetative growth and a new progeny of spores.
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