Environmental Engineering Reference
In-Depth Information
Table 6.1
Settling Velocities for Particles of
Different Aerodynamic Diameters
Particle diameter (
µ
m)
Settling rate (cm/s)
1
0.004
5
0.08
10
0.30
20
1.20
viability. Loss of viability during dispersion explains, in part, the often large
differences in mold concentrations observed in concurrent airborne mold
sampling using culturable/viable and total mold spore sampling methods.
In the latter case, concentrations of total airborne mold vary from several
times to one or more orders of magnitude higher than culturable/viable
mold. Spore viability ratios (concentration of live spores divided by total
number of spores) for species such as
Aspergillus, Penicillium,
and
Cladospo-
rium
are in the range of about 0.10 to 0.15 (10 to 15%) and are much lower
in species such as
.
Spores must fall on suitable substrates for germination and subsequent
infestation to take place. If environmental conditions are not suitable, spores
will, in time, lose their viability. Spores can become resuspended so that they
may have more than one chance to land on a substrate with suitable envi-
ronmental conditions for growth. Indoors, resuspension occurs as a conse-
quence of occupant activities, e.g., moving or dusting objects, children and
pets playing, and air currents generated by forced air mechanical systems
such as fans and heating/cooling systems, etc.
Different species respond to a variety of environmental conditions that
affect their dispersal. As a consequence, the composition of airborne mold
samples reflects not only the presence of individual species but also dispersal
mechanisms and periodicity factors. The proximity of sources significantly
affects sample concentrations. Samples collected near wood lots or in houses
with significant structural deterioration tend to have high basidiospore (pro-
duced by members of the
Epicoccum
) concentrations. Samples collected
near crop harvesting operations may have enormously high concentrations
of
Basidiomycetes
Alternaria, Epicoccum,
and
Cladosporium
. In houses, samples collected near
sources of
will have high concentrations of these
two fungal types. In other cases, airborne mold spore concentrations may
be low despite the fact that significant infestation is present. This is the case
with
Penicillium
or
Aspergillus
a toxigenic species with large, initially sticky
spores, which may cling together and settle out rapidly (
Figure 6.6
)
.
Stachybotrys chartarum,
C. Nutrition
Fungi obtain food by parasitizing other organisms (mainly plants), or from
decomposing organic matter, or, in specialized cases (lichens), mutualistic
symbiosis with algae. Most fungi are saprobes, obtaining their nutrient
