Household Mold Types: Complete ERMI Guide (2025)

Mold in homes is a serious concern for homeowners, especially those living in humid climates or houses with poor ventilation. The Environmental Relative Moldiness Index (ERMI) is a research tool developed by EPA scientists to quantify mold contamination in residential environments.

According to the EPA, ERMI uses DNA-based technology to identify and measure 36 specific mold species found in settled dust samples. This index divides these molds into two distinct groups: 26 species associated with water-damaged homes (Group 1) and 10 species commonly found in normal indoor environments (Group 2). Understanding these mold categories can help homeowners identify potential health risks and take appropriate preventive measures.

Group 1: Water Damage Molds

Molds in Group 1 are typically associated with water-damaged homes. According to research published in the Journal of Occupational and Environmental Medicine, these 26 species thrive in environments with excessive moisture due to leaks, flooding, or high humidity.

These tertiary colonizers require specific conditions to grow. Research on Stachybotrys chartarum published in the journal Mycopathologia found that water damage molds like Chaetomium globosum and Stachybotrys chartarum require water activity (aw) greater than 0.9 and relative humidity above 90% to establish colonies—significantly higher moisture levels than common indoor molds.

Key Water Damage Mold Species

Aspergillus flavus/oryzae

Known for producing aflatoxins, these molds pose serious health risks. According to recent research, aflatoxins are classified as Group 1 carcinogens by the International Agency for Research on Cancer, meaning they are known to cause cancer in humans. These mycotoxins can contaminate various crops and building materials.

Aspergillus fumigatus

Commonly found in soil and decaying organic matter, A. fumigatus is recognized as the most common airborne fungal pathogen in developed countries. It can cause respiratory infections in susceptible individuals, particularly those with compromised immune systems.

Stachybotrys chartarum

Also known as "black mold," S. chartarum is one of the world's 10 most feared fungi. This species is notorious for producing macrocyclic trichothecene mycotoxins, including satratoxins, which can cause severe health issues. Research has documented cases of pulmonary hemorrhage in infants exposed to environments with heavy Stachybotrys contamination.

Chaetomium globosum

Recognized by its cotton-like texture, Chaetomium globosum often grows on water-damaged drywall and wallpaper. Like Stachybotrys, it requires constant moisture and high relative humidity to proliferate on cellulose-rich building materials.

Aureobasidium pullulans

Frequently found around windows, in bathrooms, and on water-damaged surfaces, this mold can cause allergic reactions in sensitive individuals.

These Group 1 molds require immediate attention. Prolonged exposure can lead to serious health problems, especially in individuals with weakened immune systems, asthma, or respiratory conditions. If you suspect water damage mold in your home, consider testing your environment with MoldCo's HERTSMI-2 home test.

Group 2: Common Indoor Molds

Group 2 consists of 10 mold species commonly found in homes, regardless of water damage. According to the CDC, the most common indoor molds are Cladosporium, Penicillium, and Aspergillus species. These molds are part of the natural environment and can enter homes through open windows, doors, and ventilation systems.

Common Group 2 Species

Acremonium strictum

Often found in humidifiers, cooling coils, and window sealants, this mold thrives in damp environments with moderate moisture levels.

Alternaria alternata

A well-known allergen that can trigger asthma and allergic reactions. Alternaria is present in high amounts in many climate zones, particularly during spring and summer months.

Cladosporium cladosporioides

This versatile mold can grow on various surfaces, including fabrics, wood, and painted walls. It's one of the most frequently detected molds in indoor air samples.

Penicillium chrysogenum

While this species is known for its role in producing the antibiotic penicillin, it can also cause allergies and respiratory issues when found indoors. Some Penicillium species can produce mycotoxins under certain conditions.

While Group 2 molds are more common and may not always pose the same level of health risk as water damage molds, the CDC emphasizes that it's still important to control their growth to maintain good indoor air quality. Any visible mold growth should be addressed promptly.

Understanding ERMI Scoring

The ERMI score is calculated by comparing the concentrations of Group 1 molds (water damage indicators) to Group 2 molds (common indoor molds). According to EPA research, scores range from -10 to +20, with negative scores indicating lower mold levels similar to normal homes, while positive scores suggest elevated mold contamination.

However, it's important to note that the EPA states ERMI "has been peer reviewed for research purposes but has not been validated for non-research purposes." For homeowners concerned about mold exposure, the HERTSMI-2 test (a simplified version focusing on the 5 most problematic species) may be more appropriate for assessing home safety.

Mitigating Mold Growth in Your Home

The key to mold control is moisture management. Federal health authorities recommend addressing both the mold and the underlying moisture problem to prevent recurrence.

Reduce Indoor Humidity

Use dehumidifiers and air conditioners to keep indoor humidity levels below 50%—ideally between 30-50%. The EPA notes that controlling moisture is the most effective way to limit indoor mold growth.

Fix Leaks Promptly

Repair any leaks in your roof, walls, or plumbing immediately. Research shows that mold can begin growing within 24-48 hours after water damage occurs, making rapid response critical.

Improve Ventilation

Ensure proper ventilation in high-moisture areas like kitchens, bathrooms, and laundry rooms. Use exhaust fans that vent to the outside of your home, not into attics or crawl spaces.

Clean and Dry Wet Areas

Clean and dry any damp or wet areas within 24-48 hours to prevent mold colonization. According to building science research, this timeframe is critical—beyond 48 hours, water-damaged porous materials often require replacement rather than cleaning.

Regular Maintenance

Regular cleaning helps prevent mold spores from settling and establishing colonies on surfaces. Pay special attention to areas prone to condensation and moisture accumulation.

When to Seek Professional Help

Understanding household mold types as categorized by the ERMI can empower homeowners to take proactive steps in preventing mold growth and protecting their health.

If you're experiencing unexplained symptoms like fatigue, respiratory issues, or cognitive difficulties after moving into a home or following water damage, mold exposure may be a contributing factor. The CDC reports that in 2004, the Institute of Medicine found sufficient evidence linking indoor mold exposure with upper respiratory tract symptoms, cough, and wheezing in otherwise healthy people.

For comprehensive guidance on identifying mold in your home and understanding potential health impacts, explore MoldCo's complete guide to mold detection and recovery. If you suspect a significant mold problem, consider professional assessment and remediation, particularly for areas larger than 10 square feet or if anyone in your household has respiratory conditions.

Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. If you're experiencing health symptoms you believe may be related to mold exposure, consult qualified healthcare providers. Consider taking MoldCo's symptom assessment or ordering lab testing to evaluate potential mold-related health impacts.