What Is an ERMI Mold Test and What Does Your Score Mean?
What Is the ERMI and Where Does It Come From?
ERMI stands for Environmental Relative Moldiness Index. The EPA developed it because existing mold tests were inconsistent and hard to compare between homes. By standardizing the sample method and testing against a fixed database, the ERMI gives you a number you can actually place in context.
The test extracts DNA from household dust and identifies which mold species are present and at what concentration. Since dust builds up over time, the score reflects months of mold history rather than a single moment.
That long memory is also the ERMI's biggest limitation. DNA lingers whether the mold is alive, dormant, or long dead, so an old roof leak or a previous owner's water damage can push your number up even when nothing is growing today. The test shows what has accumulated, not what is happening now.
This is why Claro never reads an ERMI score on its own. We pair it with a full building walkthrough, moisture readings, and real-time air sampling where it helps, so we can tell a live problem from an old ghost in the dust.
It’s worth noting that EPA itself describes ERMI as a research tool and does not formally recommend it for assessing individual homes. That said, it is widely used by environmental physicians, industrial hygienists, and mold inspectors as one part of a broader assessment.
How Samples Are Collected
How to Interpret Your Score
Low mold burden, consistent with a clean home.
No action needed.
Borderline, worth monitoring.
Investigate moisture sources.
Elevated, suggests past or current water damage.
Professional inspection recommended
Significant contamination.
Remediation likely needed.
Serious problem.
Immediate action recommended.
SPECIES MATTER MORE THAN THE NUMBER
The Score Is Only Half the Picture
The number matters, but the species breakdown matters more. A score of +4 with common outdoor molds driving it is a different situation from a score of +4 with Stachybotrys chartarum present. Stachybotrys is heavy, sticky, and hard to aerosolize. Catching even one spore in a dust sample is a serious finding, regardless of what the overall score says.
The five species below are also the basis of the HERTSMI-2, a shorter test some doctors use when evaluating patients for mold-related illness. If you have an ERMI report, you already have the data to calculate a HERTSMI-2 score from it.
Any species marked in the highest concentration quartile (Q4) in your report should be investigated, regardless of your overall ERMI number.
KEY GROUP 1 SPECIES TO WATCH
Stachybotrys chartarum
Toxic at any level. One spore is a red flag regardless of overall score.
Chaetomium globosum
Associated with water damage and mycotoxin production. Should not be present in a healthy home.
Aspergillus versicolor
Produces sterigmatocystin, a mycotoxin linked to chronic illness. Common in water-damaged materials.
Aspergillus penicillioides
Thrives in sustained high-humidity environments. Elevated counts suggest ongoing moisture problems.
Wallemia sebi
Found in very dry, salty conditions. Its presence is unusual and warrants further investigation.
What the ERMI Does and Does Not Tell You
What it tells you
The historical mold burden in your home, identified down to species level.
How your home compares to a national database of over 1,000 U.S. households.
Useful objective data for doctors investigating chronic health symptoms.
A baseline record for homebuyers who want to know a property's mold history.
What it does not tell you
Where the mold is. A high score tells you something is wrong, not which wall or room.
Current airborne spore levels. Dust-based testing reflects history, not what is in the air right now.
Whether mold is currently active. Past damage that has been fixed can still show a high score.
Whether remediation worked or if any of the spores are viable or culturable mold.