Q. My company conducts Indoor Air Quality
assessments primarily in residential buildings. Is there
a direct correlation between total airborne mold spore
counts conducted on microscope slide samples and counts
based on collection and growth on agar plates? There
are some industry- accepted standards for culture plate
sampling (CFUs/m3) but none for spore counts. With regards
to assessment and clearance, what constitutes a problem?
What criteria should be used to accurately state whether
a mold problem does or does not exist? How do I interpret
high spore counts with low counts using culture plate
techniques?
- Ron, Florida
|
INFO LINKS |
|
Dehumidifiers Overview - view all Dehumidifiers
sold by AllergyBuyersClub.com
Dehumidifiers Comparison Chart - a comparison with pluses and
minuses of all the best dehumidifiers
Mold Solution Guide - a quick buyers guide of the best recommended
products to solve mold allergy problems
Mold Learning Center - FAQs and Mold Prevention and Cleaning articles |
|
A. Statistically,
total spore counts are always significantly correlated
with counts conducted on agar plate samples. Though
correlation coefficients are significant they are relatively
modest indicating that only about 30% of the variability
associated with the data can be ascribed to the direct
relationship between total mold and culturable-viable
sampling results. The variability that cannot be accounted
for has to be due to other factors.
One of the most important of these is the variation
in apparent viability from one set of results to another.
On average total mold spore to culturable mold ratios
are in the range of 10:1 that is that there are 10X
more dead mold spores airborne than there are live ones.
Sometimes the ratio is as low as 2-3:1. I have seen
ratios as high as 2000:1.
Mold spore viability depends on a variety of environmental
factors. These include (1) how active mold infestation
is and (2) mold spore history relative to the time it
was released, exposure to ultraviolet light and drying
conditions. Most mold spores that are airborne are no
longer alive. As such they cannot grow on culture media.
Spore viability appears to vary from genus to genus
with highest apparent viabilities reported for Cladosporium
and Penicillin. Mold types with low apparent viabilities
appear less frequently on culture plates than they do
on sample slides. This is the case for Epicoccum and
Alternaria.
It is good practice to conduct both culturable-viable
and total mold spore counts in house mold assessments.
The ratio can be used to ascertain how active the infestation
is. A low ratio (on the order of two to five) indicates
a relatively active infestation and a need to re-mediate
both the infestation and its cause. High ratios (typically
greater than 10) indicate that the infestation is less
active with remediation focused on infestation sites
and less on the cause of the initial infestation. A
high spore count and low culture count indicates a relatively
inactive infestation. Since allergenicity is independent
of viability, the high spore count is significant. It
indicates that a significant exposure risk exists.
I use 10,000 spores per cubic meter as my guideline
value in residences. This is based on some research
work in Australia in which we were able to show a correlation
between symptoms and total airborne mold levels. Levels
above 15,000 s/m3 are of particular concern. These are
results that are based on Burkard samplers or their
equivalent (laboratory calibrated Allergenco samplers)
and counting at 1000X. Culturable-viable results should
never be higher than total mold spores sampling results.
What is a "safe level"? That is a difficult
question. In residences concentrations based on the
Burkard total mold spore method (or equivalent) should
not exceed 10,000 S/m3 with 5,000 S/m3 as a target level.
Concentrations dominated by one genus such as Penicillium
or Aspergillus even at 10,000 S/m3 are unacceptable.
In such cases levels should not exceed 5,000 S/m3. Repeated
testing is recommended when as few as one Stachybotrys
spore is found on a slide or culture plate.
One cannot state in regards to a mold problem whether
a house is safe or not. More realistically a house can
be described as relatively safe when total airborne
mold concentrations are less than 5,000 S/m3 and no
evidence of mold infestation is present.
First Published: early November 2002
Updated: September 2003
Send this article to a friend
.................................................. .................................................. .................................................. ..................................................
