Requests for ERMI testing have increased dramatically lately.  Clients often hear about the testing from a medical provider.  Typically these providers, while well-intentioned,  have a very limited understanding of the test they are recommending.  The purpose of this article is to clarify the situations when ERMI testing is useful.

If you’d rather skip the technical explanation, the short answer is easy.   The EPA states that ERMI testing should only be used for research purposes.  It was not designed to evaluate the main point of concern for most homeowners – the current level of mold spores in the air and the potential health exposure.  But let’s examine why ERMI suffers from these limitations.

It’s easiest to understand the limitations of ERMI testing by comparing it to the standard test used for IAQ inspections, the non-viable air sample.  Non-viable samplers pull air for a given period of time (often 3-5 minutes) at a set air flow rate.  This produces a consistent quantity of air across the sample media.  By measuring the exact same volume of air on every sample, we’re able to compare different samples with a high degree of confidence.

ERMI testing is very different.  The sampling quantity is highly variable.  Many labs are now promoting a Swiffer style sampling technique to collect the dust.   As you can imagine, it is extremely difficult to quantify the sampling of a Swiffer.  All samples would have to be taken with the exact same wiping pressure and the exact same square inches of the surface.  Additionally, the smoothness of the surface would yield very different results (i.e. high gloss trim paint vs. matte finish).    These are impossible factors to quantify.  Without a reliable, repeatable testing technique, it’s very difficult to draw any conclusion from the results.

Let’s imagine for a moment that a rigorous, quantifiable technique could be found.  The results still cannot tell us anything about the current conditions.  Unlike air sampling, ERMI testing uses settled dust as the sample material.  Unfortunately, there’s no way to know how much time has transpired since the surface was last cleaned. Was it 6 weeks, 6 months?  The difference in time could be enormous, which would yield wildly different results.

Ah, but what if a relative (percent) approach is used?  One could compare the ratio of spores to dust or other particulates, which would theoretically provide good results, regardless of the time frame.  But there is another problem.  You cannot determine when the elevated mold spores occurred.

Let’s say the home had no mold problems for the last 2 months.  However, there was a 2 day period 3 months ago when a large amount of mold spores were released when the homeowner opened a moldy tent inside the home.  Or perhaps the outside air had a high quantity of mold spores during a one week period and the windows were left open.  The elevated mold spores were clearly not part of an ongoing mold issue.  But they would show up in the ERMI sample.

Or consider a home where the sampling location (door trim) hadn’t been cleaned in 6 years.  During the first 5.5 years, the home had no mold problems.  6 months ago, a much larger family moved into the home.  They kept the heat low and didn’t use the bath fans.  Mold levels spiked quickly and remained high.  Because this only represents 1/12 of the timeframe of the sample, the critical recent spike in mold would be lost.

Yet another challenge related to ERMI is the difficulty in interpreting the results.  One of the biggest problems is the way the formula treats outdoor spores. Without delving too deeply into the technical aspects, the formula groups mold spores into those that typically grow outdoors, and those that are often associated with water damage.  Unfortunately, this is an impossible distinction.  Cladosporium, for example, is one of the most common types of mold growth found with indoor water damage.  Yet the ERMI formula places it as an “outdoor mold spore”.  This is true as well.  Cladosporium is a rather productive mold and it’s found both indoors and outdoors.  Therefore, if a home had a high amount of Cladosporium growing indoors, these would actually be subtracted from the data and the house would receive a clean bill of health.

Michael Pinto has an excellent explanation of the difficulties related to the formula here, on the bottom of page 4.

This doesn’t mean the ERMI testing is worthless.  It can be effective when trying to determine the long term history of a home, which was the original intent.  The test was designed to examine the settled dust beneath carpeting to determine if childhood exposure to mold is correlated to the development of health problems (asthma, allergies).

• EPA Notice on ERMI Testing