The Complete Guide To The Dust Test

What a dust test is — and why dust

Airborne spore counts change minute to minute with activity, ventilation and weather, so a single air sample can miss intermittent problems. Settled dust behaves differently: it accumulates over weeks and months, effectively averaging the home’s mold history into one sample. That makes dust an unusually stable and informative material to analyze.

Modern dust tests use qPCR (quantitative polymerase chain reaction) to detect and count the DNA of specific mold species. This is the same DNA-based approach behind well-known indices like ERMI and its shorter cousin HERTSMI-2. Because it reads DNA, it can identify species precisely and quantify them even when the mold is dead — which matters, because dead mold can still be relevant.

The EPA 36 species panel

Many dust tests report a panel of 36 mold species derived from research funded and collaborated on by the U.S. Environmental Protection Agency and the Department of Housing and Urban Development, based on the American Healthy Homes Survey. Each species is reported in spore-equivalents per milligram of dust (often abbreviated “Spores E/mg dust”), and results are typically grouped into two clusters — Group 1 and Group 2 — with a “Sum of the Logs” value for each group.

You will frequently see “ND” next to a species, meaning “not detected.” High numbers for particular species are the signal that draws attention. In the sample report used for this guide, for example, Aspergillus ochraceus was reported around 14,897 and Aspergillus sydowii around 7,827 — far above the low readings of most other species — which is the kind of pattern that points toward an indoor moisture source rather than ordinary background dust.

Water-activity classes: the clever part

The most practically useful feature of a dust test is that it sorts detected molds by how much moisture they need to grow. This is measured as water activity (written as aᵥᵥ, on a 0.0–1.0 scale) or, equivalently, as equilibrium relative humidity (ERH). Because different molds colonize at different moisture levels, the mix of species hints at how severe and how recent a moisture problem may be.

Three important cautions about water activity

The sample report is refreshingly honest about the limits of interpretation, and those cautions are worth repeating:

• Finding molds from all three classes does not necessarily mean you have three separate moisture problems.
• A single moisture source can create all three water-activity environments in the materials around it.
• You cannot determine the total number of water sources simply by counting species within each class.

The Mold Code: how your home compares

To make results digestible, many reports condense everything into a single “Mold Code” — a 1-to-5 ranking of how your home’s mold burden compares to other homes, expressed as percentiles.

How to collect a good sample

Results are only as good as the sample, so follow the lab’s instructions precisely. In general, dust tests ask you to collect settled dust from surfaces that accumulate it over time rather than freshly cleaned areas.

• Choose undisturbed areas — the tops of door frames, shelves, or a room you rarely dust.
• Avoid recently cleaned surfaces, which lack the time-averaged record you want.
• Use the exact cloth/swab and technique the kit specifies; do not substitute.
• Label the location and collection date — the report ties results to that room.
• Follow shipping instructions promptly so the sample arrives in good condition.

Reading a sample report, step by step

Putting it together, here is a sensible way to read a dust-test report without over-interpreting it:

• Start with the Mold Code for the big-picture comparison against other homes.
• Scan the 36-species panel for any standout high numbers versus mostly low/ND readings.
• Note which water-activity classes the elevated species belong to — that hints at moisture severity.
• Cross-reference with what you know: recent leaks, humidity readings, musty rooms.
• Use it to guide inspection — the goal is to find and fix the moisture source, then re-test to verify.

What a dust test cannot do

It is just as important to know the limits. A dust test is an environmental screening tool, not a diagnosis of your home or your health.

It cannot pinpoint the exact location of hidden mold — that requires inspection. It cannot prove that mold is causing any person’s symptoms; that is a medical question for a licensed clinician who considers your history and environment together. And a single elevated reading is not automatically a crisis, just as a low code does not guarantee there is no hidden problem. Treat the numbers as a starting point for investigation, not a final answer.