William B. Rose, in his book Water In Buildings: An Architect’s
Guide to Moisture and Mold, pp. 236-237
spores are the principal means of mold reproduction, and most mold spores are
transported in the air, in order to find new sites for colonization. Spores
may carry toxins capable of disposing of previous colonizing mold organisms
of other mold species.
Penicillium mold, for example, may poison bacteria in the neighborhood
with penicillin. Stachybotrys spores may…contain toxins such as
tricothecenes to wipe out the indigenous local bacteria or simpler fungi.
All organisms digest food; mold does its digestion outside the organism
itself. It excretes enzymes capable of breaking down long-chain
hydrocarbons. Those enzymes need to travel by diffusion in a thin water film
to the food molecules, which they break down into simpler sugars and
starches. These water-soluble simpler compounds then diffuse outward from
the site where they were created, and some of them migrate back to the
hungry mold organism. Mold relies to a great extent on having a thin film of
water several molecules thick that can transport the enzymes outward, but
not too far, and transport the broken-down compounds back to the organism.
The film, of course, is nothing like a flat membrane because the surface of
materials at the microscopic level is mountainous and cavernous. The mold
organism grows hyphae, which are thin strands the tips of which are capable
of conducting the same reactions where they touch the upholstery (or other
cellulose-based surfaces) on which the organism resides. The strands of
hyphae form a mat called a mycelium. The mycelium creates a buffer
that helps to regulate the wetness of the surface. It retards evaporation of
the surface, helping to guarantee a stable moisture film necessary for
digestion. Mold needs an edible substrate, air, and a multi-molecular film
at the surface.
The conditions for initial mold growth on a surface may be quite different
from the conditions under which growth continues,, or under which growth
recurs on affected but scrubbed surfaces. Generally, initial growth requires
higher levels of moisture than is required for continued growth. Mold
continues to grow on surfaces:
►Where the moisture content of the substrate provides a dependable water
►Where the air humidity is high. This ensures that the water film is thick
enough to permit diffusion of enzymes outward and simple hydrocarbons back
to the organism.
►Where there is no water flow on the surface.
►Where sufficient food can be found.
►Where there are no significant inhibiting chemicals or treatments.
Different (mold) organisms require different thicknesses of films. The
phylloplane (leaf-inhabiting) fungi, such as Penicillium and
Cladosporium, get by with rather thin films. Thehy can thrive where the
relative humidity in the air is 80% or greater. Stachybotrys requires more
water and a thicker film; it requires a surface relative humidity around 95%