Reference

What ERMI doesn't measure.

EPA developed ERMI as a research tool for comparing groups of homes. EPA says it has not been validated for use outside research. Reading one result as a verdict on one home goes beyond what the method was designed for. This page lays out the limits as the EPA and the indoor-mycology literature describe them.

The EPA's own framing

ERMI was developed by EPA researchers in the mid-2000s as a population-level research tool for comparing the mold burden of indoor environments against a national reference distribution (the American Healthy Homes Survey, AHHS). It was not designed as an individual-home diagnostic certainty, and the EPA itself has consistently maintained this framing.

Individual-home interpretation (what most homeowners want from a test result) is a downstream application. It can be useful, but it sits on probabilistic ground that the population-level method does not directly address.

What ERMI does measure

  • DNA presence in settled household dust of 36 specific mold species via qPCR (quantitative polymerase chain reaction).
  • Concentration in spore equivalents per milligram of dust (SE/mg) for each species, calibrated against a known-quantity DNA standard.
  • An index score derived from the sum of the log concentrations of the Group 1 (water-damage indicator) species minus the sum of the log concentrations of the Group 2 (environmental reference) species. The score is compared against the AHHS national distribution to place the home in a quartile or percentile band.
  • A HERTSMI-2 derivative score using a subset of five Group 1 species weighted by their building-mycology relevance (developed by Dr. Ritchie C. Shoemaker for re-entry decisions in sensitized individuals).

What ERMI does not measure

Mycotoxin presence or concentration

ERMI detects organism DNA, not the secondary metabolites (mycotoxins) those organisms can produce. A species capable of producing aflatoxin can be present without active aflatoxin biosynthesis at the time of sampling. Whether an organism was actively producing mycotoxin in a sampled location requires a different methodology (substrate sampling, direct mycotoxin measurement) that is outside ERMI's scope.

Active growth versus historical residue

Settled household dust integrates spore material from weeks to months of indoor mycoflora. A positive ERMI detection means species DNA is present in the home; it does not mean the organism is currently growing somewhere on the structure. Conversely, an active growth site can produce relatively low dust counts if the spores are not aerosolizing (sealed wall cavities, behind vapor barriers, under flooring).

Airborne exposure

Settled dust is a different sample medium than air. Dust-DNA concentration does not directly translate to airborne spore exposure. Air sampling (Burkard, cassette, or sticky-trap methods) measures a different and more transient signal.

Species not on the 36-panel

The standard ERMI does not test for Fusarium, Memnoniella echinata, Ulocladium species, Aspergillus parasiticus, Aspergillus terreus, or most extended-panel species. Most of these (Memnoniella, A. parasiticus, A. terreus, several Penicillium species) were in the EPA's original 82-species MSQPCR panel but were not selected for the 36-species ERMI index. Fusarium is a notable exception. It was not on the 82-species panel at all, which is why extended labs that target the genus are filling a gap that predates the ERMI selection. See the 82-species story for the methodology, and Beyond the 36 for the individual extended-species pages.

Allergens, yeasts, bacteria

ERMI is mold-DNA-specific. It does not measure dust mite allergens, animal dander, pollen, bacterial markers (endotoxin, β-glucans at the bacterial level), or yeast concentrations. Indoor air quality is a multi-factor problem; ERMI addresses one of the factors.

Health outcomes

ERMI is an environmental measurement. It does not predict, diagnose, or rule out any specific health outcome. Whether mold exposure documented by an ERMI is contributing to a specific person's health concern is a clinical question for a qualified physician, and the science of mold-related illness in sensitized individuals is itself an evolving area.

Sources of variability

  • Sampling location within the home (whole-house vacuum vs. specific room vs. HVAC return)
  • Sampling technique (Swiffer cloth vs. cassette, filter media, dust quantity collected)
  • Lab processing: qPCR standards differ slightly between labs; species-level qPCR also has known cross-reactivity constraints
  • Time of year and weather: outdoor mold load varies seasonally, and an ERMI sampled in autumn near a wet outdoor environment will read differently than the same home sampled in winter
  • Recent disturbance: renovation, HVAC maintenance, or thorough cleaning can shift dust composition for weeks to months

How to use ERMI well

  • Read it as a pattern, not as a single number. Which species are elevated matters more than the score itself.
  • Pair it with building inspection. The dust signal tells you what species are present; the building inspection tells you where moisture is allowing it.
  • Consider climate context: outdoor dew point exposure, building age, HVAC configuration. A high Group 2 (reference species) load typical of a wet climate is not the same finding as a high Group 1 load in the same numerical range.
  • Don't expect ERMI to locate the moisture source. That's what a building professional does.
  • Don't over-interpret single-sample changes. Repeat sampling after remediation is more informative than a single before-or-after.

Further reading