The Environmental Relative Moldiness Index

What Is ERMI?

The Environmental Relative Moldiness Index (ERMI) is an indoor air quality testing method developed by the U.S. Environmental Protection Agency (EPA) together with the U.S. Department of Housing and Urban Development (HUD). In essence, ERMI was designed to be a sensitive and standardized method to assess air quality and mold contamination. ERMI uses DNA-based technology to differentiate and quantify certain indoor mold species.

Furthermore, the test also statistically compares the results from your household to results from one thousand U.S. households, gathered from a national home survey. This then provides you with the ERMI rating of your home. Since it had been implemented in 2007, ERMI metrics have been used in several studies as an indicator and a predictor of mold contamination, moisture damages, and potential respiratory conditions.

ERMI vs. Visual inspections

Screening for molds and mold contamination in households can be done in many ways. When done by experts, visual observations can be a reliable method, especially when visual assessment is supported by microbial verification and performed by an experienced building inspector or engineer. Unfortunately, not all visual inspections can be performed by appropriately qualified personnel. Furthermore, mold contaminations can often be hidden inside structures.

ERMI vs. Air sampling

Another method for assessing possible indoor mold damage is air sampling. This is probably is the most commonly used method for quantifying indoor microbial species. The test is done by exposing special vacuum-pumped cartridges to air in several rooms in your home, usually 5 minutes or less. The trapped mold particles are then prepared and analyzed. The drawback of this system is the accuracy because short-term exposure to the indoor air might not be enough to access the air quality fully. You might say sort of like comparing a snapshot to a video. The limited reproducibility, particularly considering short-term air samples, might also be a downside.


The HERTSMI-2 test (an acronym for Health Effects Roster of Type-Specific Formers of Mycotoxins and Inflammagens – 2nd Version) is a smaller, more targeted indoor mold test that only searches for five particular mold species. These species can be connected with more serious respiratory conditions and are most commonly involved in Chronic Inflammatory Response Syndrome (CIRS). HERTSMI-2 is a derivative test of the ERMI, and although it might be cheaper, it is less robust because it searches for only five types of molds. This means that even though HERTSMI-2 uses similar technology, it is not as accurate in assessing indoor air quality as ERMI.

ERMI vs Tape sampling

Tape sampling is among the most commonly used techniques to test surfaces during mold inspections. Sampling is performed using packaged kit specifically designed for tape mold sampling, but it can also be performed using standard, clear and clean cellophane tape. The tape is pressed against a visible and moldy surface and is then sent for laboratory analysis. This method is non-invasive and non-damage to materials or surfaces. The sampling is also not complicated to perform. However, its drawback is that you can only perform this on visible mold colonies, while some light-spored, airborne genera may be easy to miss. Furthermore, not all mold spores settle onto flat surfaces easily. Because of these disadvantages and the fact that the sampling method covers only a small representative sample, the tape sampling method might be better suited only for limited qualitative and not quantitative mold analyses.

The science behind ERMI

The ERMI test uses mold-specific quantitative polymerase chain reaction (MSqPCR) technology to (first) determine which of the 36 mold species may exist in your home and (second) quantify them. The tested samples come from the dust collected from your household. Settled dust represents an excellent repository of the overall microbial condition of a home. Therefore, this method is fairly accurate because certain surfaces, such as rugs, carpeting, or wooden floors, capture mold microflora particles due to the long-term propagule (spore) precipitation.

House dust under high magnification
Figure 1. House dust under high magnification. (Photo Source: NIAID)

The sampled mold particles first need to be “opened” and their DNA extracted in the laboratory. This DNA is then amplified many thousand times (qPCR technology), so there are enough copies of certain genes for experts (or machines) to read and detect exactly which mold species are present. The qPCR technology also allows to approximately quantify (in real-time) the number of certain mold species present in your home. This can provide an insight into possible critical mold damage located somewhere in your home.

ERMI molds

The MSqPCR technology is homed to recognize one of the 36 mold species divided into two categories. The first category (Category 1) contains 26 mold species typically found at higher levels in water-damaged homes. The second category (Category 2) contains 10 common species not associated with water damages (common indoor molds). These 36 species have been chosen among the 82 species analyzed in samples of water-damaged and control homes. The quantified result can then be compared to a national database. This database was composed of results gathered from 1.096 households across the United States during the 2006 Housing and Urban Development (HUD) American Healthy Home Survey. Individual results, ranked from lowest to highest, were used to compile the national Relative Moldiness Index (RMI) Scale (Fig. 2).

ERMI Molds Table – First Category (Common in Water-Damaged Homes)

Name Ecology & habitat Humans and Animals Pathogenicity Plant Pathogenicity Toxins Importance
Aspergillus flavus Soil and decaying wood Invasive and non-invasive Aspergillosis; Impaired food consumption, stunted growth, immune suppression, and possible liver cancer development Opportunistic pathogen of oil-containing crops such as maize, tree nuts, peanut seed and cottonseed Aflatoxin B1 and B2 None
Aspergillus fumigatus Saprophytic (decaying organic and plant materials), opportunistic fungal pathogen Invasive aspergillosis in immunocompromised humans; allergen Opportunistic, non-specific phytopathogenicity Fumitremorgens, verruculogen and gliotoxin None
Aspergillus niger Soil and decaying organic matter Non-pathogenic to humans (unless in cases of severely immunocompromised persons) None None Important in food industry for production of citric acid. Also pectinase, protease and amyloglucosidase
Aspergillus ochraceus Soil fungus and contaminant of agricultural commodities As a contaminant can cause neurotoxic, immunosuppressive, genotoxic, carcinogenic and teratogenic effects None Ochratoxin A, citrinin, mellein None
Aspergillus penicillioides Airborne contaminant, common on dry materials Associated with allergic rhinitis, facilitates the growth of house dust mites None None Potential for treatment of petrochemical effluents
Aspergillus restrictus Low water requirement, common colonizer of stored commodities Non-pathogenic, possibly and allergen None None None
Aspergillus sclerotiorum Soil and stored goods None None None Potential in termite and plant pathogen control
Aspergillus sydowii Saprophyte found in soil and plant matter Aspergillosis, onychomycosis, and keratomycosis in humans; Coral reef pathogen None Indole alkaloids None
Aspergillus unguis Soil and decomposing plant matter and moist substrates such as building materials Frequently found the homes of asthmatic children, can cause superficial mycosis None Sterigmatocystin None
Aspergillus versicolor Soil, plant debris, marine environments, indoor, food contaminant Opportunistic pathogen, can cause aspergillosis and onychomycosis None Sterigmatocystin and cyclopiaxonic acid Potential for heavy metal removal, enzyme purification
Aureobasidium pullulans Generalist species (soil, air), epiphyte and endophyte on many plant species Contaminant in humidifiers or air conditioners, causes humidifiers or air conditioners (“humidifier lung”). None None Potential for enzyme production (amylase, proteinase, lipase, cellulase, xylanase, manganese, transferases)
Chaetomium globosum Saprophytic on plants, soil, straw, and dung. Also indoors and on wooden items As an indoor contaminant can cause non-atopic asthma, sinusitis, and respiratory problems. Also causes onychomycosis and cutaneous infections. None Emodins, chrysophanol, chaetoglobosins A and C, chetomin None
Cladosporium sphaerospermum Saprophytic species, opportunistic plant pathogen Allergen in humans with respiratory problems; subcutaneous phaeohyphomycosis Secondary parasite on weakened and dying plants, biofertilizer Radiotrophic fungus (ability to use radiation as an energy source to stimulate growth) None
Eurotium chevalieri Saprophytic, xerophilic species Opportunistic pathogen that causes human cutaneous aspergillosis None Gliotoxin, Sterigmatocystin None
Paecilomyces variotii Thermophilic, Common saprophyte (composts, soils, food), Indoor mold Generally non-threatening; Occasional reports of respiratory problems in immunocompromised individuals None None None
Penicillium brevicompactum Saprophytic species, frequent fruit inhabitant, occasional spoilage of stored goods None None None Source of Mycophenolic acid (immunosuppressant drug)
Penicillium corylophilum Common soil species, psychrophilic and xerophilic Potential respiratory irritant None Citrinin, epoxyagroclavine Pathogenic to mosquitoes
Penicillium crustosum Spoilage of protein-rich foods Consumation may cause transient neurological problems (convulsion, tremors, ataxia, tachycardia) None Penitrems A through G, thomitrems A and E, roquefortine C None
Penicillium purpurogenum Saprophytic species, soil and plant matter Rarely causing respiratory issues None None Produces lovastatin
Penicillium spinulosum Cosmopolitan saprophyte, thrives in low temperatures and low water availability Unlikely to cause human infection as it can grow above 37 °C None None None
Penicillium variabile Cosmopolitan endophyte, saprophyte None None None Potential use in biocontrol of vegetable pathogens
Scopulariopsis brevicaulis Saprotroph, various organic matter Drug resistant, opportunistic pathogen that causes skin-related diseases None Satratoxin H, atranones A trough G None
Stachybotrys chartarum Saprophytic, often found on cellulose-rich building materials in damp environment Respiratory, dermatological, eye and constitutional problems None None None
Trichoderma viride Saprophytic and parasitic on other fungi None None None Useful for biocontrol of plant pathogens
Wallemia sebi Indoor mold, xerophilic and often found on highly sugared or salted products Respiratory inflammations, allergenic None Wallimidione, walleminol, walleminone None

ERMI Molds Table – Second Category (Not Common in Water-Damaged Homes)

Name Ecology & habitat Humans and Animals Pathogenicity Plant Pathogenicity Toxins Importance
Acremonium strictum Saprotroph found in soil, plant debris, and rotting mushrooms Causes rashs and flu-like problems. Respiratory infections in immunocompromised individuals. Plant pathogen and plant endophyte None Potential in biocontrol of Silver Scurf potato disease
Alternaria alternata Plant pathogen, indoor mold Associated with active asthma symptoms Causal agent of leaf spot in a high number of plants None None
Aspergillus ustus Indoor and soil mold Isolated cases of human onychoses None None None
Cladosporium cladosporioides Xerophilic, xerotolerant, and a psychrophilic plant facultative parasite Rarely and predominantly superficial infections. Phaeohyphomycosis and respiratory problems in immunocompromised individuals Causes fruit rot and blossom blight of berries None None
Cladosporium herbarum Common saprophyte in soil and organic debris, psychrotrophic Allergen, causes asthma and hay fever None None Frequent contaminant of refregerated meats and dairy products
Epicoccum nigrum Pathogen of plants and other fungi, endophyte Associated with allergic asthma, rhinitis, hypersensitivity pneumonitis, and allergic fungal sinusitis Leaf Spot Disease None Pigment and nanoparticles production, bio-control of crop pathogens
Mucor racemosus Dimorphic plant saprophyte, opportunistic human pathogen Pulmonary, cutaneous, and gastrointestinal infections in immunocompromised individuals None None Soy fermentation, production of industrial enzymes, potential in production of anticancer compounds
Penicillium chrysogenum Often indoor (damp or water-damaged) mold Skin, ear, sinus infections, pneumonia in immunocompromised individuals None None The original source of penicillin
Rhizopus stolonifer Common saprophyte, stored foods decomposer Opportunistic agent, causes Mucormycosis in immunocompromised individuals Damages ripe fruit None None

How to do an ERMI test?

The practicality of the ERMI test lies in that you can conduct the sampling of your home. The samples are collected by vacuuming a standard size area of the floor in several rooms of your home. This is done with a special vacuum cleaner extension nozzle and a dust collector, provided with the ordered ERMI mold test kit. Vacuuming is done on a 1m x 2m (~39 x 79 inches) rectangular sampling area for 5 minutes. The dust collector needs to be placed into a re-sealable bag and sent to an expert laboratory via mail. If your home doesn’t have carpeting, the samples can be collected via special Swiffer cloths.

Interpreting ERMI test results

Based on the results found in your home, mold species are selected and grouped into those with high concentrations in moldy homes (Group 1) and those with low concentrations (Group 2). To calculate your ERMI index, the individual concentrations of the mold species detected in your home are statistically transformed into index values, and the Group 2 value is subtracted from Group 1. This is your ERMI value, which can range between -10 and +20, and is interpreted accordingly (Fig. 2):

Environmental relative moldiness index (ERMI) scale
Figure 2. Environmental relative moldiness index (ERMI) scale. The black curved line (at arrow) represents the ERMI values of the 1096 homes assembled from lowest to highest. (Original results: Vesper et al., 2007)
  • Level 1: ERMI value ≤-4 represent a low relative mold index, and mold contamination is unlikely;
  • Level 2 to Level 3: ERMI value >-4 to 0 and > 0 to ≤ +5 represent a moderately high index, and further investigation should be conducted to establish if your home has a mold contamination problem;
  • Level 4: ERMI value >+5 to +20 represents a high relative moldiness, and further investigation is needed to determine the source of mold contamination.


  1. Vesper, S., McKinstry, C., Haugland, R., Wymer, L., Bradham, K., Ashley, P., … & Friedman, W. (2007). Development of an environmental relative moldiness index for U.S. homes. Journal of Occupational and Environmental Medicine49(8), 829-833.
  2. Vesper, S., Wakefield, J., Ashley, P., Cox, D., Dewalt, G., & Friedman, W. (2011). Geographic distribution of Environmental Relative Moldiness Index molds in USA homes. Journal of environmental and public health2011.
  3. Vesper, S. J., Wymer, L., Kennedy, S., & Grimsley, L. F. (2013). Decreased pulmonary function measured in children exposed to high environmental relative moldiness index homes. The open respiratory medicine journal7, 83.
  4. Vesper, S., & Wymer, L. (2016). The relationship between environmental relative moldiness index values and asthma. International journal of hygiene and environmental health219(3), 233-238.
  5. Täubel, M., Karvonen, A. M., Reponen, T., Hyvärinen, A., Vesper, S., & Pekkanen, J. (2016). Application of the environmental relative moldiness index in Finland. Applied and environmental microbiology82(2), 578-584.

Published: August 16, 2021 Updated: September 9, 2021

Dusan Sadikovic

Written by:

Mycologist - MSc, PhD
Mold Busters

Fact checked by:

Mold Busters

Charles Leduc