EPA says ERMI is not validated for judging your house. Almost nobody selling it says so.

EPA researchers developed the index as a way to compare the mold burden of groups of homes in research studies. A dust sample is analysed by mold-specific quantitative PCR, which counts DNA from named species rather than growing anything on a plate. Thirty-six species are measured: twenty-six associated with water damage, ten common background organisms.

The score is the difference between those two groups on a logarithmic basis, normed against a national sample of roughly 1,096 homes. That last detail is the one that gets lost. An ERMI value is a position relative to a housing stock, not a quantity of mold — a percentile wearing a number's clothes.

HERTSMI-2 is a five-species subset developed outside EPA, scored on a simple points system, and calculable from an existing ERMI report without new sampling. That convenience is much of why the two travel together.

EPA's position is public and has been consistent. ERMI has been peer reviewed for research purposes but has not been validated for non-research purposes, and for that reason the agency does not recommend routine public use of it in homes, schools or other buildings. EPA also does not recommend that homes be routinely tested or sampled for mold at all.

The agency's Office of Inspector General went further, raising the concern directly: that members of the public may make inappropriate decisions or take unnecessary actions about indoor mold in the belief that the tests they bought rest on tools fully validated and endorsed by EPA.

It is worth being precise about what this is and is not. EPA is not saying the mycology is wrong, that quantitative PCR does not work, or that the thirty-six species are badly chosen. It is saying that an instrument built to compare populations has been repurposed to judge individuals, and that the evidence supporting the first use does not transfer to the second.

The easy version of this story is that mold testing is a racket. It is not the version we are telling, because it is not true and because the opposite error does real harm too.

Over-reading a score frightens people out of sound homes, funds demolition that fixes nothing, and occasionally ends a house sale that should have proceeded. Under-reading it — concluding that because a score is not EPA-validated, sampling is worthless — leaves people with a real moisture problem and no information at all.

The defensible position is narrower than either. A dust panel tells you which of the named species are in the dust you sampled, and the composition of that result genuinely is informative about what kind of moisture history a building has had. It cannot tell you where the growth is, whether it is still active, where the water came from, or whether anyone has been affected. Those are the questions that actually decide what to do.

Start with the questions that cost nothing and cover the whole property: what the building is, when it was built, what its climate and drainage load look like, and what water events it has been through. That is an unselected view, and it is available without anyone entering the house.

Then, if something in that picture warrants it, sample with a hypothesis — a specific location, a specific question. Sampling without one is how a homeowner ends up holding a number and no decision.

And when a laboratory result arrives, ask what the denominator was, whether an outdoor control was taken, and what threshold defined elevated. For most of these methods there is no health-based standard in the United States to compare against, so the comparison being made is usually against a laboratory's own historical sample book.