HEALTHYBUILDINGSCIENCE is committed to facilitating the accessibility and usability of its website,, for everyone. HEALTHYBUILDINGSCIENCE aims to comply with all applicable standards, including the World Wide Web Consortium's Web Content Accessibility Guidelines 2.0 up to Level AA (WCAG 2.0 AA). HEALTHYBUILDINGSCIENCE is proud of the efforts that we have completed and that are in-progress to ensure that our website is accessible to everyone.

If you experience any difficulty in accessing any part of this website, please feel free to call us at (415) 785-7986 or email us at [email protected] and we will work with you to provide the information or service you seek through an alternate communication method that is accessible for you consistent with applicable law (for example, through telephone support).

Request an Inspection

Call us at (415) 785-7986 or click the button below to schedule your building health assessment.

Request a Site Visit

Sign up for our Quarterly newsletter

Subscribe to our e-mail newsletter to receive helpful updates and articles from Healthy Building Science.

Sign Up For Newsletter

We value your privacy.
Your email is never shared or sold.

What is ERMI Testing?

by / Thursday, 22 April 2021 / Published in News

Scientists always needed a reliable process to understand the amount of mold burden in a specific area. The conventional way of analyzing it today is by measuring mold concentrations in an airborne mold spore trap sample. For practical purposes, air sample readings have been limited to very brief periods, typically 5 minutes, with some limitations in the understanding of long-term mold exposures.

While air testing is limited to collecting spores from the air, another sampling tool, the ERMI (Environmental Relative Moldiness Index), which we will discuss here, collects spores and their DNA from dust. Adding ERMI testing to indoor air quality assessments provides additional data useful in determining the moldiness of a home.

What is ERMI testing?

The ERMI was developed by the U.S. Environmental Protection Agency to help analyze settled dust in carpets to assess if childhood mold exposure may be associated with the development of health problems (asthma, allergies). It provides another clear, objective, and standardized way to perform mold testing. It developed as a ranking system based on dust samples collected from homes and can help to effectively predict the moldiness level of homes.

The primary goal of measuring moldiness is to help prevent mold associated adverse health effects that may be affecting occupants. The ERMI test uses state-of-the-art DNA testing, mold-specific quantitative polymerase chain reaction (MSQPCR), to pinpoint those molds associated with water intrusion and numerous respiratory disorders such as Chronic Sinusitis, Asthma, and Infant Wheezing.

Measurements acquired from ERMI testing are compared to a national database in order to more effectively interpret collected information. As part of the 2006 HUD American Healthy Home Study, indices were calculated using this approach for 1,096 homes throughout the U.S. Individual indices ranked from the lowest to the highest have been used to develop a National Relative Moldiness Index (RMI) scale.

The results of the test assign an ERMI score to each home. Homes and businesses with a high ERMI score are more prone to have unwanted indoor mold growth than those with a low ERMI score.

The ERMI panel includes 26 species of mold known to grow in water-damaged homes, and 10 species which are typically found in all homes, whether water damage exists or not. Each species is identified and quantified from DNA extracted from dust samples taken from the living and sleeping areas of the home.

Concentrations of each of the 36 molds are used to obtain an “ERMI score” that assesses the moldiness of each sample against those tested by the US-EPA. These values vary from approximately minus 10 (low moldiness) to 20 (high moldiness).

How are samples collected for ERMI testing? 

ERMI testing depends on settled dust, which behaves as a mold breeding ground. The ERMI sample is a combined sample of dust, typically taken from the living room and master bedroom. The required dust samples can be obtained by vacuuming about 2 square meters each of the bedroom and living room carpet for five minutes each. The outdoor control is integrated into the ERMI sample.

How can we best utilize ERMI Scores?

During a mold and moisture inspection an ERMI analysis offers data useful to assist in the assessment of the home by a qualified professional.

If the ERMI score indicates that there may be water damage in the house, further detailed inspection, including additional testing such as bulk or mold swab testing may reveal a concealed source of water damage and/or mold.

Does living in an environment with a high ERMI score cause health issues?

There are no explicit guidelines for human health with an ERMI score. The symptoms of mold exposure vary from person to person, depending on the sensitivity of each individual and their degree of mold exposure.

The ERMI score should be used in combination with individual mold species quantifications and symptoms of home occupants in order to make a decision. The ERMI score is simply a guideline for the determination of mold exposure levels for home occupants.

Drawbacks of using only ERMI testing alone

Lacks quantifiable sampling methods

It is easy to understand the limitations of the ERMI test by comparing it to the non-viable air sample test used for Indoor Air Quality inspections. Non-viable samplers (airborne spore traps) draw air for a specified period (typically 5 minutes) at a set air flow rate.

This generates a consistent volume of air across the sample media. By measuring exactly the same volume of air in each sample, we are able to compare different samples with a high level of success.

The ERMI test is quite different. The amount of sampling may be highly variable. Many laboratories are now promoting Swiffer style sampling techniques to collect dust. As might be expected, it’s extremely difficult to quantify Swiffer sampling.

All samples should be taken with the same wiping pressure and same total sampling surface area on all the surfaces tested. In addition, the texture of the surface can produce very different results (i.e. high gloss trim paint vs. matte finish). These factors are almost impossible to accurately measure. Without a reliable, repeatable test technique, it is very hard to draw useful conclusions from the results.

Inability to determine current conditions

Unlike air sampling, the ERMI test uses settled dust as the sample material which doesn’t necessarily tell us about the current conditions.

It may be hard  to know how much time has passed since the last time the sampled surface was cleaned. Has it been 3 weeks, or maybe 3 months? The difference in time could be significant, which would naturally produce different results.

Let’s say a home hasn’t had any mold problem for the last 2 months. However, there was a 3-day period 3 months ago when many mold spores were released while the property owner opened a moldy tent within the home to dry it out.

Perhaps the outside air had a high amount of mold spores for a week and the windows were left open. Clearly, the elevated mold spores were not part of the ongoing mold issue. But they’re still going to show up in the ERMI sample.

Challenges with interpreting test results

Another challenge related to ERMIs is difficulty in interpreting results. One of the main issues is how the formula treats outdoor spores. The formula separates the mold spores that usually grow outdoors from those that are associated with water damage.

Unfortunately, this is a difficult distinction to make. Cladosporium, for instance, is one of the most common forms of mold growth found in places with existing indoor water damage. Yet the ERMI formula positions it as an “outdoor mold spore.”

Cladosporium is a ubiquitous mold found both indoors and outdoors. Thus, if a home had significant levels of Cladosporium growing indoors, it would simply be subtracted from the data and the house would achieve a clean bill of health.


ERMI testing definitely has its place if used strategically. ERMI testing can be useful in trying to ascertain the long-term history of a house and an overall sense of the quantity of settled mold spores in the home. ERMI testing is a viable option for defining mold built up by offering comparable information to a baseline from which to start or support a more thorough investigation.

Many times, a single ERMI sample is not going to be enough. Usually, a home should be dissected in a few areas, and airborne spore trap samples including outdoor control samples should be collected to get a better idea of what the real impact of the outdoors are.

Industrial hygienists work to improve indoor environmental quality by focusing on multiple areas like excess moisture, volatile organic compounds, allergens, and mold.

Having an experienced, professional environmental inspector interpret ERMI results along with performing a more complete mold, moisture and indoor air quality inspection will provide you with a more accurate and complete picture of your home’s mold situation and overall health.

Leave a Reply



Subscribe to our e-mail newsletter to receive helpful updates and articles from Healthy Building Science.

You have Successfully Subscribed!