ERMI stands for Environmental Relative Moldiness Index. It is a dust-based DNA test that measures 36 mold species and produces a single score. It was developed by the US Environmental Protection Agency as a research tool for comparing housing stock, and the EPA has been explicit that it was not developed or validated for diagnosing individual homes — a caveat routinely dropped when the test is sold to consumers.
How it works
You collect settled dust, usually from carpet or with a swiffer-style cloth, and send it to a laboratory. The lab uses quantitative PCR to identify and count DNA from 36 specific species, split into two groups: 26 associated with water damage, and 10 common in most buildings regardless.
The score is derived from the difference between the two groups, then placed on a scale built from a national housing survey. A higher number means the dust profile looks more like a water-damaged home than a typical one.
What it is good at
- Detecting history. Dust accumulates over time, so ERMI can surface evidence of past water damage that air sampling on a single day would miss.
- Species detail. You get a species-level breakdown rather than broad spore categories.
- Comparison over time. Testing the same house the same way before and after remediation is more informative than a single reading.
What it is not good at
- Diagnosing a specific home against a threshold. This is the EPA's own caveat and the most misused aspect of the test.
- Locating the problem. A score tells you something is off, not where.
- Distinguishing active growth from history. DNA persists after the organism dies, so a high score can reflect a problem that was resolved.
- Consistency. Results vary meaningfully with where and how the dust was collected. Two samples from one house can differ.
- Mycotoxins. ERMI counts mold DNA. It says nothing about mycotoxin residue, which is a separate test entirely.
Reading a score sensibly
Treat it as one input, not a verdict. A high score alongside a known water event, visible growth and a musty smell is corroboration. A high score with no moisture history and nothing visible is a reason to look for a source, not a reason to gut the house.
If someone quotes you a number and immediately quotes you a price, that pairing is worth being sceptical about.
Where it fits
ERMI is most useful as a before-and-after comparison, sampled the same way both times, alongside a visual inspection and moisture readings. For confirming that remediation worked, standard post-remediation verification is the more established instrument.
And if your concern is what remains on your belongings rather than what is in the structure, neither test covers it — that requires dust sampling for mycotoxins specifically.
Common questions
What is a good ERMI score?
The EPA did not validate the index for individual-home diagnosis, so there is no authoritative pass mark. Comparisons over time in the same house are more meaningful than a single number.
Is ERMI testing accurate?
The laboratory method is sound. The contested part is interpreting a single score for one home, which is not what it was built for.
Does ERMI detect mycotoxins?
No. It counts mold DNA. Mycotoxin detection is a separate dust analysis.
Should I get an ERMI test?
It is most useful as a before-and-after comparison, or where you suspect past water damage you cannot see. It is not a substitute for finding the moisture source.
