Why is it
that some buildings (like schools) can be tested for mold and come out
“healthy” when many people in the building have moderate to severe SBS
symptoms?-Susan ,
California
There are several reasons why this
may be the case. These include (1) the nature of mold testing and
interpretation of results and (2) SBS symptoms may be associated with
exposures other than mold (to be discussed in a future posting).
In my experience few school buildings
have system-wide mold infestation/exposure problems. In most
instances infestation/exposure problems in school buildings tend to be
localized. There may be one or more problem rooms where water leakage
through roofing has caused damage to ceiling tile with subsequent
infestation, or water seepage into a below ground level part of the
building may result in mold infestation. Water damaged/mold infested
ceiling tiles are quite common and pose a mold exposure risk to
students and teachers in those rooms that have been affected. They do
not in most cases cause a building-wide exposure problem.
Airborne mold levels in school
buildings are typically much lower than those in residential
buildings. This is likely due to the fact that school buildings are
mechanically-ventilated with conditioned air being constantly
re-circulated through HVAC system filters. Though these filters
initially have low collection efficiencies, they become increasingly
effective as they become soiled.
Mold levels determined by the culture
plate method are rarely above 300 CFU/m3 in school building
spaces. When they are, mold infestation of building materials is
likely to be present nearby. As a consequence, I tend to use this
number as a guideline between acceptable/non-acceptable school
building airborne mold levels. It is not a magic number in
distinguishing between a “healthy” or unhealthy school spaces. It is
a reference point for determining whether a mold infestation is
resulting in elevated airborne mold levels and exposures of concern.
The culture plate method has its
limitations, however, in that it only quantifies mold spores that are
alive and can grow on the culture medium being used. A large
percentage of airborne spores are no longer alive and thus cannot be
quantified by this method. This is significant since their ability to
cause allergy-type symptoms is independent of their viability (whether
they are alive or not).
Because of this fact I also collect
samples with a total mold spore sampling device. Such devices collect
airborne mold spores on a microscope slide or coverslip. Most users
of such devices send their samples to commercial laboratories where
they are counted at 400 or 600 magnifications. My counts are
typically conducted at 1000X. Because of the higher resolution of the
latter, count results are usually several times higher than similar
samples collected in the same buildings as compared to those counted
by commercial laboratories. When one samples is also quite
important. I have tested classrooms with students present and after
classroom hours. Counts are typically several times higher when
samples are collected under occupied classroom conditions. The
primary reason for this difference is likely the fact that when
students are present, their activities re-suspend surface dust which
contains mold spores. A second possible reason is that students may
be bringing mold spores/particles into classroom spaces on their
clothing from home and on their shoes as well. It is not uncommon to
smell mold odors on student coats hunk in lockers/closets.
Low mold values can not be
interpreted that as a confirmation that a building is “Healthy”. I
have seen low mold values in air tests in classrooms spaces that
smelled musty. Obviously an infestation problem existed that was not
being manifested in elevated airborne mold levels in samples that were
collected.
August 12, 2004
Indoor Environmental Quality (2000), Thad Godish Ph.D.,
C.I.H
