Comment; Scott gives a very clear, coherent, easily understood review of the limitations of the ERMI/HERTSMI2 testing. Despite the limitations, personally I DO believe it provides some useful information.
Keywords; Concentration, Loading, ERMI, HERTSMI2, Shoemaker, Loading Factor, ERMI variability, Cost, Home Mold Assessment
Concentration is spores per gram of dust (or some other weight).
Let’s assume you have a situation where you have an ERMI =10 for two identical homes – same
size, same layout, same contents. BUT, one house is very dirty and the other is very clean.
Now let’s assume you collected 1 gr. of dust from each home. BUT, remember, one house is very
dirty so you only need to wipe 1 square foot to get 1 gram of dust – about the size of a typical
floor tile (12in x 12in). And the other is very clean, so to collect 1 gram of dust you will need to
wipe 100 square feet – an entire small bedroom floor!
We send these for ERMI lab analysis. The lab finds you have 1000 spores per gram of dust, and
report to you the ERMI = 10.
This score is not based on what is the risk, nor what is the concentration. It is based on a
comparison of OTHER houses in a very limited specific study of dust in homes. It compares one
house to the houses in that study and only that study (See my other discussion on ERMI and the
source for the score.)
So to interpret ERMI from the lab: we now know that the dirty house has 1000 spores on one
We can not say anything else about the house. But remember, it’s dirty, a dirty house, there is
dust everywhere, so we must assume there are also 1000 spores on every other single square
foot of floor everywhere. If the house is 1000 sq ft (a small house), you probably have a total of
1,000,000 spores on all the floors of this dirty house. (I will save the discussion for later on
whether this million spores is a lot or a little. But consider this: mold spores are so small that you
can actually fit 1 million of them in a single grain of table salt! Just sayin’.)
For our discussion, we also tested a clean house; and remember, we got the same ERMI = 100.
But, in the case of the very clean house, it was so clean we had to collect dust from 100 square
feet of floor, not just one like in the dirty house. Since the concentration of spores in both houses
was the same, the lab analysis of 1000 spores per gram is actually telling us there are 1000 spores
on 100 square feet. The dirty house had 1000 spores on only 1 square foot!
The clean house has a total of 10,000 spores on the floors; the dirty house has a total of 1 million
spores on the floors – 100 times more spores! That is a big difference.
WHICH is worse? WHICH house has more exposure risk per foot of floor (like when you walk
around and kick up the dust, or run a vacuum, or broom?
Clue: BOTH had the same ERMI of 10. Sounds like, according to Shoemaker recommendations,
that they are equally risky to the occupant. Maybe not, you say? Probably not? Definitely NOT.
Question: Do the dirty house and clean house with the same ERMI Score have the same risk of
Answer: No. The dirty house has 1 MILLION spores – which is 100 times MORE than the clean
A huge problem with using ERMI or HERTSMI is that it does not report mold in the correct units. It
reports spore equivalents per weight, or “concentration:” In order to properly assess exposure,
hazard risk, or potential impact, you need to know the loading factor.
Loading factor would be reported as spores per weight per square area of surface from where the
dust is. Such as a spores per gram of dust per square foot of surface (such as a table, floor, or
shelf). Neither ERMI nor HERTSMI do this. So the scores can NOT be used for exposure
assessment. But we have no other scores reported. Hence, useless waste of money and effort.
And remember, the loading sampling and analysis only work properly if the surface dust is
1. getting re-introduced into the breathing air (unlikely), or
2. is somehow a valid representation of the air you were breathing in the past.
One good way of understanding the misinterpretation is to consider what happens before and
During the “Before” situation, there is a large amount of dust which contains a certain
measurable concentration of mold DNA, aka ERMI score, and which probably has been there a
long time as it settled out of the air coming from many locations, not just the mold damage
growing in the home.
Then, it is cleaned – but, and this is important, the concentration of DNA in the dust residue that
did not get removed has not changed. So any dust left over is exactly the same as before
cleaning. When you sample it, it’ going to be the same as the before sample – it’s the same dust –
only now it is a much thinner layer. So, ERMI before = ERMI after!
Thus, there is a high likelihood that the before and after samples are analyzed by the ERMI lab as
being the SAME. But does Before actually equal After?
The result changes only if some of the sample locations have a new type of dust, one with
different concentrations of mold. Perhaps, this occurs because the growth is still growing – or the
door and window were left open, or a dirty dog or child ran through the house! There is no way
of telling what explains the difference if only samples are being interpreted. And unfortunately,
this is usually what gets done when a doctor, or website, or blog, tells the patient to “take an
ERMI sample”. Hopefully, it is obvious that without a full physical assessment of the actual
conditions, functions, and operations of the building, the ERMI means little except to confuse and
frighten the occupant.
Here’s the rub, you are not ever going to expect to see mold distributed evenly.
ERMI variability from surface to surface is likely to be high….the second problem – COST. It is
impractical to collect ERMI/HERTSMI in a way that is scientifically reliable and valid. (Note: if you
don’t do science for research or sampling methodology, I recommend you look those terms
up….they are very important in sampling and analysis; sorry, not time or space here.)
HOME MOLD ASSESSMENT
The most important thing is the assessment:
1. physical inspection of every nook and cranny of the home,
2. building history of damage, water, repairs,
3. building function/operations assessment, and
4. occupant health.
Putting these 4 things in place before sampling will get you more and better info. There are so
many reasons not to do a home test of any kind. There are equally so many not to pay an
inspector to do tests of any kind. Once you do the other four steps, then you can figure out a
“hypothesis”, which is a question, for the sampling: if you can’t predict the sample will provide a
useful answer to your question, you should not take it.
All sampling must increase the power of your decision-making.
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