top of page

Thanks for stopping by!  Like many of you, after getting our ERMI results back we were left wondering what all the numbers really mean and had to search out the advice of others to help us put the numbers in context.  Let’s face it, there hasn't been an easy one stop shop for answers to the “Explain my ERMI” question, until now!  


After a lot of research trying to answer that seemingly basic question, we understand why that is - it’s not easy to generate a straightforward answer that can take into account all the variables in your individual environment.  However, what we did learn is that there are some guiding attributes of the 36 mold types within the ERMI that can help you better understand the types of water events your home or business may have experienced. 

We created the Explain my ERMI analysis tool to compile a wealth of information on each of the molds in the ERMI test into a simple to use tool that will help each of us in our mold journey better understand and address our indoor environment.  ERMI, like any sampling tool has its limitations, but there is value in the ERMI for those who are trying to understand what hidden mold issues may exist in their dwelling.  The Explain my ERMI analysis tool doesn't focus on the bottom line ERMI score but rather the types and relative abundance of the molds that are found.  With your Explain my ERMI analysis report you'll turn your ERMI results into a helpful report that you can use to aid you in determining what your next steps should be.  Now, before we tell you more about the tool, we thought you might like a little background on the ERMI.

So what is an ERMI Anyway?

a little background

ERMI stands for

Environmental Relative Moldiness Index

The ERMI process was developed by researchers at the United States EPA to “objectively describe(s) the home’s mold burden” by sampling settled dust and evaluating it for mold DNA.

Since it is impractical to measure all the molds in a building, the researchers began the analysis with a total of 82 species of molds found in water-damaged and control homes.  From the 82 species only 36 were found to be widely distributed.  These 36 species were divided into 26 Group 1 species associated with water damage and 10 Group 2 species that are not associated with water damage.  By subtracting the sum of the log-transformed concentrations of Group 2 species from the sum of the log-transformed concentrations of Group 1 species the ERMI score is obtained.

"It is also important to note that the ERMI is a mold index not a health index.  Each individual's genetic make-up and health status makes their particular response to mold exposures unique."  Development of an Environmental Relative Moldiness Index for US Homes, Vesper et al.

The comparison then assesses a number ranging from -10 to 20 or higher divided into four quartiles. The lower the score for each quartile the better because it means the mold burden is low. Ideally, the score should be zero or less.

ERMI Score U.S Distribution


Percentage of U.S. Homes

Relative Moldiness Index Values

let's nerd out a bit more

I promise this is helpful!

Okay so we learned some ERMI basics, but how do all those number give us some idea as to the source of our mold problem?  Part of that answer lies in an understanding of the water requirements of different mold species.  So let's take a look at the three different groupings of molds based on their water requirements.

Molds can be classified into three groups based on the amount of moisture they need to colonize.  Knowing which colonizer group an elevated mold belong to can help you determine where your moisture issue may be located.  The three colonizer groups are:

1.  Primary Colonizers [Xerophilic/Xerotolerant]

Sometimes referred to as Opportunistic Molds, these molds have the lowest moisture requirements to grow and can survive on normal humidity. These molds only need a water activity (Aw) of around 0.75 before fungal growth starts. These molds can be found growing on dusty furnishings and textiles in poorly ventilated areas or areas with elevated relative humidity >70%.  If you find elevated levels of Xerophilic molds in your ERMI it may be an indication of a humidity issue that needs to be addressed. Some of these molds may also grow well under Mesophilic conditions too.

2.  Secondary Colonizers [Mesophilic]

Mesophilic molds are classified as those needing a water activity (Aw) of between 0.8-0.9 to support fungal growth.  These molds can often be found growing in areas with excessive condensation.  Elevated levels of Mesophilic molds in an ERMI sample could point to issues of condensation around windows, air conditioning vents, humidifiers, inside the HVAC plenum adjacent and downstream of the evaporator coil, and in bathrooms where condensation may form from water vapor and surface temperature differentials.

3.  Tertiary Colonizers [Hydrophilic] 

Sometimes referred to as Water Indicator Molds, these molds require an active water source such as a leaking pipe. Hydrophilic molds are classified as those needing a water activity (Aw) greater than 0.9 to support fungal growth. Elevated levels of Hydrophilic molds in an ERMI sample point to a current or previous water leak.

Water activity (Aw) is also referred to as “equilibrated relative humidity” and it is the single, most important factor in determining whether mold growth can be initiated on building materials.  Water activity (Aw) is defined as the partial pressure of water relative to the vapor pressure of pure water at the same temperature or a measurement of the water that is available for biological and chemical reactions. The water activity (Aw) scale starts from 0 (dry) and goes to 1.0 (pure water).

Now for what you came for

So all of that background is really helpful, but I'm sure you're ready to learn what your ERMI results mean, right?!  That's why we created the "Explain my ERMI" analysis tool, so you can have a detailed explanation of your ERMI results including answers to the following questions:

  • How bad is my mold problem?

  • Which molds in the ERMI results are elevated and by how much?

  • What Colonizer Group do the elevated molds belong to, and what do the characteristics of the molds found in my ERMI report tell me about where potential areas of concern might be?

  • Do any of the molds in my ERMI report have the potential to create Mycotoxins?

What's really cool about the Explain my ERMI Analysis tool is that you can enter multiple ERMI's and get a report on each, all for the same low cost!  Let's face it, one ERMI test is expensive enough, but if you've run multiple ERMI's in your home, the last thing you want to do is pay for multiple interpretations.  We know some online consultants that will charge a few hundred dollars just to tell you "yep, your ERMI is bad."  That's why we give you unlimited access to the Explain my ERMI Analysis Tool for a full week for only $23!  You can analyze as many ERMI's as you want during the week long period!  Ready to get started? Just click below for instant access.


  • One week access!

  • Analyze multiple ERMI tests!

  • One low price!


bottom of page