Part 3
Mold
remediation clearance. What type of air sampling and analysis
provides acceptable clearance? What is an acceptable ratio
(percentage of indoor versus outdoor spore counts)? Should
indoor counts be “no more than 50% of outdoor counts or
significantly lower indoors as opposed to outdoors”? This
seems to be a grey area everywhere I look.-Matthew,
South Carolina I
Mold
Remediation Clearance, Part 3
As a general rule, because of the
very high variability in test results reported by commercial
laboratories for spore trap sampling, I do not recommend their use
for clearance testing. In our studies, it was, of course, evident
that some laboratories were clearly superior to others and it is
quite possible that one can use such results with more confidence
(because it was a research study being prepared for publication it
is not likely that I will reveal any names of the laboratories
evaluated).
What alternatives are there to
satisfy building owners/occupants that a mold remediation was
conducted well enough that the building is now “safe”? Obviously
one can look to other methods. The volumetric culture plate method
has some potential, as it has a long history of use, counts are easy
to conduct, and many laboratories are EMLAP Environmental
Microbiology Laboratory Accreditation Program) accredited and
periodically challenged on the ability of laboratory analysts to
identify major mold types. Guideline values have been recommended
for acceptable levels by a number of organizations. These have been
reviewed in Bob and Gail Brandy’s book “Worldwide exposure standards
for mold and bacteria – Historical and current perspectives”(www.safety-epa.com.)
Volumetric culture
plate methods have one major limitation in that they can only
quantify those mold spore/particles that can grow on the media
used. In the real world because of environmental factors such as
heat, desiccation, ultraviolet light, etc., most airborne mold
spores are no longer viable and as such cannot grow on culture
media. In some cases mold spores/particles are viable and cannot
grow on the media selected for sampling. In my experience
culturable-viable concentrations tend to average about one tenth of
those on spore trap samples I have counted myself. However, there
can be a broad range of variation. I have observed ratios as low as
1:1 and as high as >100,000:1.
Culture plate methods
do have the advantage of providing good data (this is particularly
the case if samples are collected on DG-18 medium) on both
Penicillium and Aspergillus concentrations.. Because
species of these genera are commonly associated with wet building
materials and generally higher indoors than outdoors when a mold
infestation problem exists, the ability to identify and quantify
Penicillium and Aspergillus both indoors and outdoors has
a lot of merit in conducting clearance testing. As indicated
previously, culture plate methods because of viability issues have
the potential for producing false negative results. Based on my use
of both culture and spore trap methods for airborne mold testing in
the same building, experiences in conducting inter-laboratory
comparison studies, and reviewing spore trap results from commercial
laboratories in which I have them conducted both culture plate and
spore trap testing, false negatives are less likely to occur with
culture plate methods than with spore trap samples sent to the
average commercial laboratory.
If one is to use
culture plate methods for clearance sampling, I recommend that a
minimum of two series of samples be taken indoors and one outdoors
using MEA and DG-18 media. DG-18 is an excellent medium for both
Aspergillus and Penicillium and typically results in
higher Aspergillus and Penicillium counts (than MEA)
collected in the same environment.
(to be continued)
July 29, 2005
