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 isn’t an easy one stop shop for answers to the “Explain my ERMI” question!  


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, that doesn’t mean that you’re completely out of luck, 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 built this webpage as a starting point for what we hope can be a collaborative effort to help all of us in our mold journey better understand and address our indoor environment.  We certainly aren’t mold experts, but maybe you are!  If you find something that’s incorrect, please drop us a note with an explanation and a reference (if you have it) so we too can learn and update this site to help spread the knowledge to others.  If you feel you need to drop us a note and tell us how useless the ERMI is and how it has this issue or that feel free, but know that we’ve probably already read that article or heard that position. We still see a value of the ERMI for those who are trying to understand what hidden mold issues may exist in their dwelling.  Further to that point, we don’t suggest you focus so much on the ERMI number but rather the types and relative abundance of the molds that are found.  We think of the ERMI as one option in your mold evaluation tool box and believe it can be a helpful screening tool that can provide a directional indicator as to the mold burden in a home.

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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.

Percentage of U.S. Homes

ERMI Score U.S Distribution

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 that you have a little background, head over to the ERMI Analysis page and input your ERMI or HERTSMI-2 values for additional details.