ERMI and HERTSMI-2: What Dust Testing Can and Cannot Tell You

What these tests actually measure

ERMI (Environmental Relative Moldiness Index) uses qPCR DNA analysis to quantify 36 mold species in settled dust — 26 associated with water damage and 10 common outdoor species — producing a single comparative index. HERTSMI-2 is a simplified scorecard built from five of the highest-signal water-damage species.

Both are dust tests. They characterize the reservoir of what has settled in the living space over time, which is a fundamentally different question than what is airborne at one moment — and often a more useful one for chronic exposure.

Why dust beats a single air sample

Airborne spore counts swing enormously hour to hour with activity, HVAC cycles, and weather. A single air cassette is a snapshot that can easily miss a real problem or flag a harmless spike. Settled dust integrates weeks to months of what the building sheds, which better matches how occupants are actually exposed.

Air sampling still has a role — it is useful for clearance testing under controlled conditions and for isolating specific rooms — but it should not be the primary screening tool for a chronic-exposure question.

Collection determines whether the result means anything

Sample from areas that collect dust over time and are not cleaned frequently: top of door frames and trim, top of the refrigerator, ceiling fan blades, book spines. Avoid floors, recent renovation dust, and freshly cleaned surfaces.

Composite thoughtfully. A whole-house composite answers 'is this building a problem,' while room-specific samples answer 'where.' Decide which question you are asking before you swab.

Reading the numbers without panic

HERTSMI-2 scoring commonly used by practitioners treats under 11 as acceptable for sensitized occupants, 11 to 15 as borderline, and over 15 as unsuitable pending remediation. ERMI values are comparative rather than absolute, and single species results matter more than the composite when one water-damage marker dominates.

One elevated species with an identifiable source is a solvable problem. A broad elevation across multiple water-damage species suggests a building-wide reservoir or HVAC distribution and warrants a full envelope assessment.

The honest limitations

ERMI was developed as a research and screening tool, and no dust index is a pass-fail certificate for health. Results vary with collection technique, lab, and location sampled. Scores cannot tell you where the source is, whether growth is active, or what remediation will cost.

Use dust testing the way an engineer uses instrumentation: as one input alongside moisture readings, visual inspection of concealed spaces, and building history. The test is a flashlight, not a verdict.

When to retest

Retest after remediation and small-particle cleaning are complete, with the same collection method and locations as your baseline so the comparison is valid. Testing mid-project measures construction dust, not outcomes.