What is Wallemia sebi?

Wallemia sebi is a xerophilic mold, meaning it grows at low water activity. It does not need a leak, standing water or high humidity in the way most indoor molds do, which is why conventional moisture-driven inspection often looks in the wrong places for it. It is one of the five species scored by HERTSMI-2 and one of the 36 species quantified by ERMI, so it is measured routinely on dust reports. Its spores are small and are not reliably identified by direct microscopy, so a spore trap air sample will not tell you whether it is present.

Why does a high Wallemia result barely change my HERTSMI-2 score?

Because the scoring thresholds are not the same across the five species. Chaetomium globosum and Stachybotrys chartarum reach the maximum 10 points above roughly 125 spore equivalents per milligram. Wallemia sebi does not reach 10 points until it exceeds 2,500. A building can carry roughly twenty times the Wallemia and contribute the same number of points to the total. That is deliberate, since the species are not equally potent, but it means a total score that reads comfortably under the safe threshold can still sit on a substantial Wallemia load.

My HERTSMI-2 score is under 11 but I still feel sick at home. What now?

Look at the individual species values rather than the total. A Wallemia sebi result in the hundreds or low thousands contributes only a few points to the total while representing a real presence in the dust. The next step is not another dust test but a building investigation aimed at low water activity conditions, since a xerophile lives where the moisture record does not point. Actinobacteria, endotoxins and persistent mycotoxins are also worth ruling in or out, because none of them appear on a spore trap either.

Will a mycotoxin panel detect Wallemia?

No. Standard environmental mycotoxin panels target compounds such as aflatoxins, ochratoxins, trichothecenes and gliotoxin. Wallemia's metabolites are not on those panels, so a mycotoxin report can come back unremarkable while Wallemia is present in the dust. Mold testing and mycotoxin testing answer different questions, and neither substitutes for the other.

Reference · Xerophilic Mold

Wallemia sebi: the HERTSMI-2 number almost nobody reads

Q: Is Wallemia included in HERTSMI-2 and ERMI?

Yes. Wallemia sebi is one of the five HERTSMI-2 species, alongside Aspergillus penicillioides, Aspergillus versicolor, Chaetomium globosum and Stachybotrys chartarum. It is also one of the 36 species quantified in ERMI. Both are qPCR analyses of settled dust. So Wallemia is not an exotic add-on test. It is already on reports that thousands of people receive, which is precisely why the value gets passed over.

If your HERTSMI-2 came back under 11 and you still can't live in your house, this page is for you. Wallemia sebi is already on your report. The reason it gets passed over has nothing to do with whether it was measured, and everything to do with how it is scored.

Written & reviewed by W. David Winstead, NORMI Certified Microbial Inspector · HBIA Certified Microbial Biotoxin Assessor · 22+ years in indoor environmental quality · Last updated August 2026

What Wallemia is

Wallemia sebi is a xerophilic mold. Xerophilic means it grows at low water activity - it does not need a leak, standing water, or sustained high humidity the way most indoor molds do. That single property is responsible for nearly everything that goes wrong in finding it.

Conventional mold inspection is built around water. Find the intrusion, find the wet assembly, find the growth. That logic works well for Stachybotrys and Chaetomium, which need materials that are genuinely wet. It works poorly for an organism that is content in conditions a moisture meter reads as normal.

It is already on your report

This is the part worth being precise about, because a lot of writing on the subject gets it wrong.

Wallemia sebi is one of the five HERTSMI-2 species, alongside Aspergillus penicillioides, Aspergillus versicolor, Chaetomium globosum and Stachybotrys chartarum. It is also one of the 36 species quantified by ERMI. Both are qPCR analyses of settled dust.

So Wallemia is not an exotic add-on that only a specialist can order. It is measured routinely, on reports thousands of people receive every year. Which raises the obvious question.

Then why does nobody talk about it?

Because of the scoring. The five HERTSMI-2 species do not share a threshold - each is scored against its own scale, reflecting differences in potency. Here is what that looks like in practice, in spore equivalents per milligram of dust.

Species	4 points	6 points	10 points (maximum)
Chaetomium globosum	5 - 24	25 - 124	above 125
Stachybotrys chartarum	5 - 24	25 - 124	above 125
Aspergillus penicillioides	10 - 99	100 - 499	above 500
Aspergillus versicolor	10 - 99	100 - 499	above 500
Wallemia sebi	100 - 499	500 - 2,499	above 2,500

Thresholds as published in the HERTSMI-2 scorecard. Individual species scores are summed to produce the total. Interpretations in common use place a total under 11 in the generally acceptable range, 11 to 15 as borderline, and 16 or above as high; published wordings vary slightly, and your practitioner's reading of your own result governs.

Read the bottom row against the top two. Chaetomium and Stachybotrys max out above roughly 125. Wallemia does not max out until it passes 2,500 - about twenty times higher.

The practical consequence. A dust sample can return Wallemia sebi in the several hundreds, contribute 4 points to the total, and leave a HERTSMI-2 score sitting comfortably in the acceptable range - while several hundred spore equivalents per milligram of a mold you react to are in the dust of your home.

The score is not wrong. It is doing what it was designed to do, which is weight species by potency. But a score is a summary, and summaries lose information. If you are the person still getting sick in a building that scored fine, the individual line is the one that matters.

Three other ways it slips past

A spore trap cannot identify it. Wallemia spores are small and are not reliably distinguished by direct microscopy. On an air sample they land in the small-spore or Aspergillus/Penicillium-like category. An air test that comes back normal has told you nothing about Wallemia either way.
Mycotoxin panels do not target it. Standard environmental mycotoxin analysis looks for aflatoxins, ochratoxins, trichothecenes and gliotoxin. Wallemia's metabolites are not on those panels. An unremarkable mycotoxin report is not evidence of absence.
The moisture survey points the wrong way. Because it tolerates low water activity, Wallemia is not reliably found where the water was. Shoemaker's own HERTSMI-2 work makes this point directly - the five species stratify by water activity, and where elevated Wallemia appears without an obvious matching condition, the search for that condition has to continue rather than stop.

What we actually do with it

Nothing exotic, and nothing that costs you extra. We read the individual species values, not just the total. When the Wallemia sebi line is elevated relative to the rest of the profile, that changes where we look and what we look for: dust reservoirs, textiles and stored goods, intermittently conditioned spaces, and materials sitting at a water activity that a standard moisture survey would call dry.

It also changes what we recommend next. Repeating a dust test rarely helps. A building investigation aimed at low water activity conditions does. And because a client reacting in a building that scored well is usually reacting to more than one thing, we look at Actinobacteria, endotoxins and persistent mycotoxins in the same pass - none of which appear on a spore trap either.

We perform no remediation and accept no referral fees, so there is nothing behind any of this except the finding.

Common questions

Is Wallemia included in HERTSMI-2 and ERMI?

Yes. Wallemia sebi is one of the five HERTSMI-2 species and one of the 36 quantified by ERMI. Both are qPCR analyses of settled dust. It is not an add-on test - it is already being measured, which is exactly why the number gets passed over rather than acted on.

Why does a high Wallemia result barely move my score?

Because the five species are scored against different scales. Chaetomium and Stachybotrys reach the maximum 10 points above roughly 125 spore equivalents per milligram; Wallemia sebi does not reach 10 points until it exceeds 2,500. The weighting is intentional, since the species are not equally potent. But it means a total that reads comfortably safe can sit on a real Wallemia presence.

My score is under 11 and I still feel sick at home. What now?

Read the individual species values rather than the total, and look specifically at the Wallemia sebi line. Then stop testing dust and investigate the building - a xerophile lives where the moisture record does not point, so the survey has to be aimed at low water activity conditions rather than at the leak history. Actinobacteria, endotoxins and persistent mycotoxins are worth ruling in or out at the same time. If you already have reports from another inspector or laboratory, we will read them and tell you what was never looked for.

Where does Wallemia grow in a house?

In conditions dry enough that a standard moisture survey does not flag them. Because it tolerates low water activity, its habitat tends to be settled dust reservoirs, textiles and stored goods, and materials in intermittently conditioned spaces, rather than an obviously wet building assembly. That is why a clean moisture report and an elevated Wallemia value are not a contradiction.

Will an air test or a mycotoxin panel find it?

Neither will answer the question. Wallemia spores are not reliably identified by direct microscopy, so a spore trap cannot confirm or exclude it. And standard mycotoxin panels target aflatoxins, ochratoxins, trichothecenes and gliotoxin - Wallemia's metabolites are not among them. Dust DNA is the measurement that actually addresses it.

Does a high Wallemia number mean my house is unsafe?

No, and we would not tell you that. A dust value is one input, not a verdict, and people differ enormously in how they respond to the same environment. What an elevated Wallemia line does mean is that the building has conditions supporting xerophilic growth that have not yet been located, and that the total score alone is not a sufficient summary of your situation. Whether it explains your symptoms is a question for you and your physician; whether the building supports it is a question for us.

Scored safe and still reacting?

Send us the report. Reading someone else's data and identifying what was never looked for is a service we provide on its own - and the conversation that starts it is free.

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Educational & informational use. AMOLDBUSTER is an indoor environmental health and assessment service. HERTSMI-2 and ERMI thresholds are described as published by their sources; scoring guidance and published interpretations change, and your own result should be read with your treating practitioner. Environmental testing identifies what is present in a sample. It does not diagnose illness or establish medical causation. Always consult your physician or qualified healthcare provider regarding any medical condition.