The five water-damage indicator species
The five water damage mold species on the HERTSMI-2 panel are Aspergillus penicillioides, Aspergillus versicolor, Chaetomium globosum, Stachybotrys chartarum and Wallemia sebi. They span the moisture range rather than sharing one.
The five species, ordered by the water they need
| Species | Minimum water activity | Where it grows indoors | What finding it indicates |
|---|---|---|---|
| Aspergillus penicillioides | 0.585–0.632 | Settled dust, bedding, mattresses | Dust reservoir and chronic mild damp — not a leak |
| Wallemia sebi | 0.69–0.75 | House dust, dry and salted foods | Dust reservoir; rarely an active wet colony |
| Aspergillus versicolor | 0.78–0.80 | Damp gypsum board, carpet and furniture dust | Sustained mild dampness or condensation |
| Chaetomium globosum | Above ~0.90 | Drywall paper, wet framing, cellulose | Wetting that lasted rather than dried |
| Stachybotrys chartarum | ~0.89–0.94, optimum above 0.98 | Chronically saturated cellulose | Prolonged free water at that location |
What the panel can and cannot establish
| Question | Can a dust panel answer it? |
|---|---|
| Which of these species are present in the dust | Yes — this is what it measures |
| Whether there is active growth somewhere in the building | Partly — composition is suggestive, not conclusive |
| Where the growth is | No |
| Where the water came from | No |
| Whether the moisture problem is still active | No |
| Whether anyone has been made ill by it | No |
Where the list comes from
These five are drawn from the 36 species measured by the Environmental Relative Moldiness Index, a research index developed by EPA scientists using DNA counts in settled dust. ERMI splits its 36 into 26 Group 1 species associated with water damage and 10 Group 2 species found in most homes regardless.
HERTSMI-2 is a five-species subset of that panel, created outside EPA by Ritchie Shoemaker and Mycometrics, scored on a point system rather than as a relative index. It is the source of the grouping that circulates as the definitive water-damage list.
It is worth being exact about status. EPA has stated that ERMI was peer reviewed for research purposes, has not been validated for non-research use, and that the agency does not recommend its routine public use in homes or schools. That does not make the species meaningless — the underlying mycology is sound and useful. It means the score built on top of them is being read as more diagnostic than its authors' own evidence supports.
The mistake almost every version of this list makes
Consumer material routinely describes Aspergillus penicillioides as a high-humidity organism. It is close to the opposite. It is one of the most xerophilic organisms ever characterised, with cell division demonstrated at 0.585 water activity — conditions in which essentially nothing else on the list can begin.
Wallemia sebi is the same kind of case: growth reported down to roughly 0.69 aw, with a halotolerance that lets it colonise salted and sugared foods. Neither species is telling you about a leak. They are telling you about dust, and about a building that has been mildly damp for a long time.
Read correctly, the panel is a gradient. Two dry-end organisms, one in the middle, two that require sustained wetting. Which of them appear, and in what proportion, is a statement about the kind of moisture problem a building has had — and that is considerably more useful than a single number.
How to read a result
Dry-end species alone, with nothing above 0.85, is the signature of dust accumulation and ambient damp. It points at ventilation, at humidity control and at cleaning rather than at demolition.
Mid-range species with the dry-end ones suggests a persistent condensation or humidity problem — cold surfaces, poor air movement, a bathroom or laundry that does not clear.
Wet-end species change the question. Chaetomium globosum and Stachybotrys chartarum do not establish on a surface that dries. Finding either means something stayed wet at that location, and the productive next step is finding the water rather than counting spores.
None of these readings is a health finding. A species list describes a building. What a given exposure means for a given person is a clinical question, and the evidence reviews associate outcomes with dampness itself more consistently than with any named organism.
Questions people ask
- What are the 5 mold species tested for water damage?
- Aspergillus penicillioides, Aspergillus versicolor, Chaetomium globosum, Stachybotrys chartarum and Wallemia sebi. They make up the HERTSMI-2 panel, a five-species subset of EPA's 36-species ERMI index.
- Is Aspergillus penicillioides a high-humidity mold?
- No, and this is the most common error in descriptions of this panel. It is an extreme xerophile, demonstrated dividing at 0.585 water activity. It is associated with settled dust rather than with wet building materials.
- Which of the five indicates the worst water damage?
- Stachybotrys chartarum requires the most water and the longest duration, so it is the strongest indicator of a sustained moisture failure. Chaetomium globosum is close behind. Neither can establish on a surface that wets and then dries.
- Does finding these species mean the house is unsafe?
- It means the building has had moisture conditions permitting growth. It is not a health finding, and species identification does not establish that anyone was exposed to anything or harmed by it. Health questions belong with a clinician.
- Is a HERTSMI-2 score an EPA-approved test?
- No. ERMI, which HERTSMI-2 derives from, is an EPA research tool. EPA has said it has not been validated for non-research use and does not recommend routine public use of it in homes or schools.