What ERMI actually measures
ERMI, the Environmental Relative Mouldiness Index, is a DNA-based way to measure how mouldy a building is from a single dust sample. Here is what it actually measures, and what the number means for your home.
What the test does
Performing an ERMI is a way to determine whether water damage in a building has caused mould amplification, which can cause negative health consequences for the occupants.
Samples are analysed using Mould-Specific Quantitative Polymerase Chain Reaction (MSqPCR). Fungal DNA is first extracted and purified from the dust, then analysed with this highly specific DNA-based method to identify and quantify selected mould species.
The 36 species, in two groups
The usual sample is settled dust collected from a home or building. While a wide spectrum of moulds can be analysed this way, ERMI detects 36 species. The research behind it selected 26 (Group 1) associated with water-damaged buildings, and 10 common species (Group 2) that were not.
To calculate the index, the individual log10 values of the Group 1 moulds are totalled, the Group 2 moulds are totalled, and the difference between the two totals is taken.
What the number means
Put simply, a building with a high ERMI value has a greater chance of causing a mould problem for its occupants than one with a lower value.
To interpret a result, the ERMI is compared to a national database. The US EPA determined indices for 1,096 homes across the US as part of the 2006 HUD American Healthy Home Survey, ranked lowest to highest to create a national Relative Mouldiness Index scale. An ERMI graph lets you compare one house against another without being a statistician, and a reference table helps interpret the result.
Where it came from
The US Environmental Protection Agency developed ERMI to provide a straightforward, objective, sensitive and standardised way to assess mould, based on dust samples.
In initial studies, mould concentrations in "mouldy homes" (visible growth or a history of water damage) and "reference homes" (no visible mould) were compared, and species were grouped by which were found at higher concentrations in mouldy homes.