What is a good ERMI score?

Why there’s no official passing number

ERMI stands for Environmental Relative Moldiness Index, and “relative” is the key word. The score tells you how your home’s dust compares with dust from a national sample of homes. It doesn’t tell you whether a building is safe, because no federal agency has set a safe level for mold indoors.

The EPA researchers who developed the method built it for studies, and the agency has cautioned that ERMI was designed as a research tool and hasn’t been validated for routine public use. That doesn’t make the score useless. It means a single number shouldn’t be treated as a verdict. The test itself is explained on our ERMI testing page.

How scores are commonly read

Most lab reports place a result against the national comparison, usually in quartiles: four equal groups from the lowest-scoring homes to the highest. Exact cutoffs vary between labs and report versions, so read the ranges printed on your own report. In general terms:

Where the score falls	How it’s commonly read	What people often do next
Lowest quarter of the comparison homes	Less water-damage mold in the dust than most homes	Check the species list anyway, then keep the report as a baseline
Middle two quarters	Similar to a typical home	Look at which molds drive the score and whether there is a known water history
Highest quarter	More water-damage mold in the dust than most homes	Inspect for the moisture source before deciding on remediation

Scores can be negative, and because the math uses logarithms, a change of a few points reflects a real change in the amount of mold in the dust.

Why the species list matters as much as the number

The score adds up 26 molds associated with water damage and subtracts 10 common household molds. That means a home with little ordinary outdoor mold in its dust can score higher than its water-damage molds alone would suggest, and a high reading of one important mold, such as Stachybotrys or Chaetomium, can be hidden inside an unremarkable total. We always read the individual results, not just the headline score. Many labs can also calculate a HERTSMI-2 score from the same sample, and the ERMI vs. HERTSMI-2 comparison explains why a doctor might prefer one.

Targets for mold-sensitive households

If a physician asked for the test, the physician decides what number is acceptable for their patient. Some doctors who work with mold-related illness set targets well below what would be typical for the general population. Get the target in writing before the sample is collected, so everyone is measuring against the same line. Working with your doctor covers how to have that conversation.

If your score came back high

A high ERMI shows that water-damage molds have built up in the dust. It doesn’t show where they’re growing. The next step is an inspection that finds the moisture. After any remediation, retest with the same lab, the same collection method and the same rooms, so the before and after numbers can be compared fairly. Hold off on deep cleaning, new carpet or fresh paint in the weeks before a retest unless it’s part of the planned work, because each one changes the dust the test depends on.

We test and remediate buildings. We don’t diagnose or treat medical conditions, and a home test result isn’t a medical diagnosis. If you have symptoms, please work with a doctor. With your permission, we’re glad to share test results with them.