I am the
Director of engineering for a manufactured home builder. I sometimes
receive mold test results from our customers. Is there any
training/seminars/reading that you could recommend to help me better
interpret and understand the numbers? IAQ “experts” interpreting the
results seem to vary in their opinions as to what is significant. I
have a fair background in statistics and building science, and these
sampling methods by themselves (1 per room compared to the outside) do
not seem to be very rigorous, nor does there seem to be a common
interpretation of the data.Any suggestions on how I could get a little
smarter on this subject would be greatly appreciated?-Tom
Unfortunately, most mold test
results, particularly if they use total mold spore sampling procedures
are often uninterpretable by lay individuals for whom the testing is
done as well as the professionals who collect the samples.
Many samples are collected by professionals who
as a practical matter do it in an off-the-shelf time of manner. They
buy a sampling pump, collect samples on cassettes using the factory
recommended volumes and then send the samples to a laboratory. Many
of these individuals may have never seen a mold spore under a
microscope. Some have read a little of the mold literature, but most
have not.
Analytical laboratories
use a variety of counting protocols that may give different results
even on the same sample. One of the most notable is the magnification
used in counting. Most laboratories use 600 or 400X. Our studies show
that the lower magnifications result in undercounts by an order of 2
or 3.
There are other issues
as well including the portion of the sampling slide counted and
problems, such as particle bounce and movement of mold spores away
from the particle trace as a result of the application of a
staining/mounting media. These factors also contribute to
undercounts.
I generally recommend
that a minimum of two to three total airborne mold samples be
collected indoors with a fourth outdoors. Mold types in indoor
samples should then be compared to outdoor samples to determine
whether mold types present are from the same or different
populations. Typically if there is an indoor infestation, mold
samples will be high in terms of Penicillium and Aspergillus
with outdoor samples dominated by such species as Cladosporium,
Epicoccum, Alternaria, Fusarium, etc.
I also recommend the use of culturable-viable
sampling on plates in the sampled environment since it allows
identification of mold types present and provides an indication how
active an infestation may be. Unlike total mold spore methods,
concentrations on sample plates are more interpretable. Levels above
1000 CFU/m3 are high if predominant mold types represent a
distinctively different mold population than that outdoors.
A single sample is virtually useless in
determining whether a building has a mold problem. Unfortunately many
homeowners believe that sample results that show any mold present
indicates that they “have mold”. That is not necessarily true as mold
spores can be found almost anywhere. They are organisms of decay and
in doing the job nature intended for them, you can expect to find mold
spores airborne almost anywhere.
The issue is not whether
mold spores are present. The issue is the kind of mold, the presence
of an active infestation, and elevated exposure levels.
March 5, 2004
Indoor Environmental Quality (2000), Thad Godish Ph.D.,
C.I.H
Direct E-mail
00tjgodish@bsu.edu
