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In 1993 Brian
Flannigan 23 , a "Reader" in Microbiology at the Department
of Biological Sciences in Heriot-Watt University (Gothenburg, Sweden)
published, “Approaches to Assessment of Microbial Flora of
Buildings”. He cites the Block’s experiment on the role
of the substrate:
“Block
24 (1953) proposed that fungi were incapable of obtaining the
moisture needed for their development directly from the atmosphere.
He considered that they obtained it from the substrate, which
had absorbed it from the atmosphere. That is, that the RH of
the atmosphere had an indirect effect on fungal growth, and the
more hygroscopic a material was, the more susceptible it was to
mold growth. The minimum moisture content at which mold growth
occurred depended on the material and ranged from 10% to 14%.”
Flannigan points
out that buildings contain a mixed community of yeast, each of which
may experience optimal conditions [temperature and relative humidity
(Table #1) and for nutrients which extend the viable range. Even
strains among species have different requirements.
R. Rylander
25 , Professor, Department of Environmental Medicine, University
of Gothenburg, Sweden confirms Flannigan’s observation and
cites “cold walls on which humidity in indoor air condenses,
in humid carpets, and between inner and outer walls” as problem
areas. Flannigan goes through the mathematics for condensation and
identifies concrete, brick, and gypsum as containing interstitial
water:
“They
[people] have the impression that if RH of the atmosphere is less
than 70% then little or no mold growth will occur. However, the
temperature is not constant in buildings. There are generally
surfaces that are at a lower temperature than the bulk
of a room. Consequently, although the moisture content will be
the same as in air in the center of the room, the RH of the air
adjacent to a cooler wall will be higher. In a poorly insulated
[or leaky ] building, the temperature differential between the
ambient air and an outer wall may be 5°C (9°F); for an
ambient air temperature of 20°C (68°F) and a wall temperature
of 15°C (59°F) this could mean a difference between 60%
and 80%, a difference between a RH that reportedly would not support
mold growth and one that would. Where the temperature of a surface
is at or below the dewpoint, water condensing on that surface
will allow germination and mold growth on it, irrespective of
the ambient humidity. Where moist air permeates a porous
material like concrete, brick or gypsum, condensation may be interstitial.
This can then act as a reservoir of water which will permit mold
growth to continue under ambient conditions that would have dried
the surface and prevented growth.”23
The widespread
practice of shutting down ventilation systems during unoccupied
hours should be stopped. Instead buildings should be continually
pressurized to prevent infiltration (Figure #2).
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