ERMI test: your home's mold history, read from its dust

The ERMI test, explained

Where ERMI came from

ERMI grew out of work by EPA scientists who developed a DNA-based method for identifying and counting specific molds, called mold-specific quantitative PCR, or MSQPCR. Instead of growing mold on a dish or counting spores under a microscope, the lab measures the genetic material of each target species directly. That makes it possible to identify molds that are hard to tell apart by eye, and to measure them in dust where most of the material is no longer intact spores.

To give the numbers meaning, the researchers needed a reference point. They built one from dust collected in homes across the United States, then used it to rank how moldy a home’s dust is relative to that national sample. The result is the Environmental Relative Moldiness Index. The word “relative” matters: the score says how your dust compares with other homes’ dust, not whether the building is safe.

How the score is calculated

The 36 species fall into two groups:

Group	How many	What they represent	Examples
Group 1	26	Molds associated with water-damaged buildings	Stachybotrys chartarum, Chaetomium globosum, Aspergillus versicolor, Aspergillus penicillioides, Wallemia sebi
Group 2	10	Molds common in most homes, much of it from outdoors	Cladosporium herbarum, Alternaria alternata, Epicoccum nigrum

The lab takes the logarithm of each species’ concentration, adds up the Group 1 values, and subtracts the sum of the Group 2 values. The result is a single number that commonly falls somewhere between about -10 and 20. Two things follow from that math. First, because it’s logarithmic, a difference of a few points is a real difference in mold load. Second, because Group 2 is subtracted, a home with very little ordinary outdoor mold in its dust can score higher than you’d expect from its Group 1 counts alone. That’s one reason we read the species list, not only the score. Our answer on what a good ERMI score is covers the quartiles and targets.

What changes a result

• Recent cleaning. A deep clean, new carpet or fresh paint removes the dust history the test depends on, and can make a home read better than it is.
• Where the dust came from. A bedroom closet, a living room floor and the top of a door frame collect dust at different rates and from different air.
• Flooring. Carpet holds far more dust than hard floors, so the same house can give different results depending on which rooms are sampled.
• Recent disturbance. Demolition, remediation or moving furniture can stir up and redistribute old dust.
• Lab and method. Vacuum and cloth methods, and different labs, aren’t always interchangeable. Keep them the same for before and after comparisons.

Using ERMI the right way

The EPA has been clear that ERMI was developed as a research tool and hasn’t been validated for routine use by the public. That doesn’t make the test useless. It means the score shouldn’t be the only thing a decision rests on. In practice, ERMI works best when it’s paired with an inspection that finds the moisture, when the collection method is documented so it can be repeated, and when the target is agreed before the sample is taken.

If your doctor is the one asking for the test, find out whether they want ERMI specifically or would rather see a HERTSMI-2 score, which many labs can calculate from the same dust. Our comparison of ERMI vs. HERTSMI-2 explains the choice, and working with your doctor on a moldy home covers how to agree on a target.

An ERMI score describes the dust in a building. It isn’t a medical test and doesn’t diagnose anything. We don’t diagnose or treat medical conditions. If you have symptoms, please work with a doctor, and with your permission we’ll share results with them.