If you've spent any time in mold-illness communities, you've run into ERMI and HERTSMI-2. They're cited constantly — and misunderstood almost as constantly.
Both are DNA-based tests run on a single dust sample from your home. Both can be genuinely useful. Neither is a verdict on whether your home is "safe," and neither should be read in isolation.
What these tests actually measure
ERMI (Environmental Relative Moldiness Index) was developed by the EPA. It quantifies 36 mold species in a dust sample and compares them to a national index. HERTSMI-2 is a slimmer five-species scorecard drawn from the same underlying data, favored by clinicians treating mold-illness patients.
What you get back is a number — and a list of species. What you don't get back is context.
The dust problem
Dust is a history book. It tells you what has settled over weeks or months, not what's in the air you're breathing right now. A high ERMI can reflect a resolved problem that was never cleaned up. A normal ERMI can miss an active source in a room you didn't sample.
This is why a dust test alone is not an assessment. It's one data point.
When the numbers are genuinely useful
For a clinician working with a sensitized patient, HERTSMI-2 can help track whether an environment is tolerable. For a prospective buyer, ERMI can flag a home that deserves a closer look. For an assessor, either test becomes far more meaningful when paired with:
- Live air sampling
- Moisture mapping and thermal imaging
- A physical inspection of the building envelope and HVAC
How to read your results
Don't panic over a single elevated species. Don't dismiss a borderline score, either. Bring the report to someone who reads them regularly and who has no remediation contract to sell you. The right interpretation is almost always more boring than the internet suggests — and far more useful.