Interpreting an ERMI
Looking Beyond the Number
The ERMI (Environmental Relative Moldiness Index) test produces a single numerical score, intended to indicate how moldy a home is based on the relative presence of 36 mold species in household dust. At first glance, this score might seem like a simple and straightforward way to understand your mold exposure risk. However, professionals who specialize in indoor environmental health rarely use the ERMI score as the deciding factor when interpreting results—and for good reason.
The truth is, while the science behind ERMI is solid, the way the score is calculated can lead to misleading conclusionsabout a home’s health or risk level.
How the ERMI Score Works
The ERMI score is based on the difference between two groups of molds:
- Group I molds are associated with water damage and are more concerning.
- Group II molds are common in homes, even those without water damage.
The total score is calculated by subtracting the total Group II molds from the total Group I molds. The higher the score, the more “moldy” the home is considered.
This seems logical—but the math can obscure the reality.
Why the Score Can Be Misleading
There are several reasons why experienced Indoor Environmental Professionals (IEPs) do not rely on the ERMI score alone to assess indoor air quality or make recommendations.
1. The score is based on a ratio, not absolute levels.
A home with very high levels of both Group I and Group II molds could produce a low ERMI score, even though the total mold burden is high. Conversely, a home with very little mold but a slightly higher Group I count could produce a high ERMI score, even if conditions are relatively safe.
2. Group II molds can artificially lower the score.
Group II molds are common in every home and often vary based on outdoor ecology, building type, and cleaning habits. A dusty home with lots of Group II molds might end up with a misleadingly low ERMI score, despite also having high levels of problem molds.
3. Not all molds are equally relevant to health.
ERMI weighs all Group I molds equally in its score. But some species—like Stachybotrys chartarum or Chaetomium globosum—are much more significant indicators of water damage or potential health risk than others. Experienced professionals focus on which molds are present, not just how many.
How IEPs Interpret ERMI Results
Rather than looking at the final score, skilled Indoor Environmental Professionals focus on:
- The individual mold species present
- The quantities of each species, particularly those associated with water damage
- The home’s history (leaks, floods, humidity issues)
- Symptoms reported by occupants
- Comparison between rooms, when multiple samples are taken
This species-level review provides far more meaningful information than the score alone.
Bottom Line
If you’ve received an ERMI report, don’t panic if the score seems high—or assume everything is fine if it seems low. The score is only one small part of the picture. Instead, focus on which molds were detected, in what amounts, and how that compares to your home’s history and your health concerns.
When in doubt, consult with a medically sound IEP who understands how to interpret the raw data—not just the number at the bottom of the page. With the right guidance, ERMI can be a helpful tool—but it should never be the only tool.