Home Mold Testing: ERMI, Air Sampling, and Dust Analysis Explained

At a Glance

• Mold testing methods measure different things depending on the technique used

• Air samples reflect a moment in time, while dust samples reflect longer-term accumulation

• ERMI and HERTSMI scores are based on DNA analysis, not visual inspection

• A single number rarely tells the full story

• Visible moisture problems often matter more than laboratory results alone

• Testing can be helpful in specific situations but is not always necessary

Why Mold Testing Feels So Confusing

Homeowners researching mold testing quickly encounter conflicting advice. Some sources insist that air testing is unreliable. Others promote ERMI scores as definitive proof of contamination. Remediation companies may recommend testing, while some inspectors dismiss it entirely.

Part of the confusion comes from the fact that mold testing does not measure a single, universal variable. Different methods measure different aspects of mold presence. Without understanding what each test is actually analyzing, it becomes easy to misinterpret results.

In residential settings, mold testing is best understood as a tool. It is not a diagnosis on its own. It provides data points that must be interpreted alongside building history, moisture patterns, visible damage, and occupant symptoms.

Air Sampling: What It Measures

Air sampling is one of the most common residential mold testing methods. It involves drawing a measured volume of air through a cassette that traps airborne spores. The sample is then analyzed under a microscope to identify and count spore types.

Air sampling answers a specific question:

What is floating in the air at the time of testing?

This method is useful for:

• Comparing indoor air to outdoor air

• Identifying elevated spore counts

• Detecting certain mold species in circulation

• Post-remediation clearance testing

However, air sampling has limitations.

Air conditions fluctuate throughout the day. Opening windows, running HVAC systems, recent cleaning, weather conditions, and even movement inside the home can influence results. A sample taken in the morning may differ from one taken in the afternoon.

Air sampling may also miss hidden mold growth inside walls or beneath flooring if spores are not actively circulating.

It is best viewed as a snapshot, not a long-term record.

Surface and Bulk Sampling

Surface sampling involves swabbing or taping visible mold growth and sending it to a lab for identification. Bulk sampling involves sending a piece of contaminated material for analysis.

These methods confirm what a visible patch contains, but they do not measure overall airborne load or hidden contamination. They are most helpful when identifying species present in obvious growth areas.

In residential testing, these methods are usually supplemental rather than primary.

Dust Sampling and ERMI

Dust sampling approaches mold testing differently.

Instead of measuring spores floating in the air, dust sampling collects settled dust from floors or surfaces. This dust is analyzed using quantitative PCR, a DNA-based method that identifies mold species at a molecular level.

One widely known dust-based scoring method is ERMI, which stands for Environmental Relative Moldiness Index. ERMI testing measures the DNA of specific mold species associated with water damage and compares them to species commonly found in normal indoor environments.

The result is a numerical score.

Unlike air testing, which captures a moment in time, dust testing reflects accumulated mold fragments over weeks or months. This can provide a broader picture of historical contamination.

Because ERMI relies on DNA detection, it does not require visible spores or active growth to register mold presence.

However, ERMI also has limitations.

The scoring system was originally developed for research purposes, not as a clinical diagnostic tool. Homes with similar ERMI scores may have very different moisture histories. Interpretation requires context.

Dust sampling also cannot distinguish between old contamination and active growth without additional investigation.

What Is HERTSMI-2?

HERTSMI-2 is a modified scoring system derived from ERMI data. It focuses on a smaller subset of mold species commonly associated with water-damaged buildings.

Some practitioners use HERTSMI-2 scores to assess risk levels, particularly for individuals who report heightened sensitivity to mold exposure.

Like ERMI, HERTSMI-2 relies on DNA-based analysis of settled dust. It is not universally adopted in mainstream building science but is frequently discussed in environmental health communities.

The key point is that both ERMI and HERTSMI measure genetic material in dust, not airborne concentration at the time of sampling.

When Mold Testing Makes Sense in a Home

Testing is most helpful when there is a clear question that needs answering.

Situations where mold testing may be useful include:

• After water damage or flooding

• Before purchasing a home with a history of leaks

• After remediation to confirm reduction

• When occupants report persistent symptoms without visible mold

• When there is a musty odor but no obvious source

In these cases, testing can help identify patterns that are not immediately visible.

It can also provide documentation for real estate transactions or remediation follow-up.

When Testing Is Not Necessary

There are situations where testing adds little value.

If visible mold growth is present alongside active moisture intrusion, the priority is moisture correction and remediation. Testing does not change the need for repair.

Similarly, if a known leak is ongoing, lab results will not resolve the underlying problem.

In some homes, repeated testing without a plan for moisture control can create unnecessary anxiety without improving conditions.

Testing is most productive when paired with a practical response strategy.

How to Think About Mold Test Results

Interpreting mold test results requires proportion.

A high spore count indoors compared to outdoors suggests active growth. But equal indoor and outdoor counts may not rule out hidden moisture damage.

An elevated ERMI score may indicate historical contamination, but it does not automatically mean current airborne exposure is high.

Patterns matter more than isolated numbers.

Questions to consider include:

• Is there a history of leaks or flooding?

• Are specific water-damage species elevated?

• Are results consistent with building conditions?

• Has ventilation been assessed?

Comparisons between rooms can also be helpful. A single elevated room may indicate localized moisture, whereas widespread elevation may suggest systemic issues.

Results should guide investigation, not replace it.

Common Misunderstandings About Mold Testing

Several misconceptions contribute to confusion.

One is that a “clean” air test guarantees no mold problem. Because air testing reflects a moment in time, it can miss hidden growth.

Another is that a high ERMI score always indicates severe contamination. Scores must be interpreted relative to building age, geography, and maintenance history.

There is also an assumption that laboratory testing alone determines health impact. In reality, exposure depends on time spent in the environment, ventilation, individual susceptibility, and overall building condition.

Testing measures presence. It does not measure personal response.

Mold exposure in buildings is different from the kind of exposure that can occur through certain stored foods. In homes, testing focuses on airborne spores and dust accumulation. In food systems, moisture and storage conditions influence contamination patterns in products like grains and nuts.

Laboratories and Analysis Methods

Residential mold samples are typically analyzed by environmental laboratories that specialize in microbial testing. Some labs focus on microscopy-based air sampling, while others specialize in DNA-based dust analysis.

The method used is often more important than the specific laboratory name. Air samples rely on microscopic spore identification. ERMI and HERTSMI rely on quantitative PCR.

Understanding which method was used is essential for interpreting results accurately.

Mold Testing Within a Broader Exposure Picture

Mold testing addresses one component of indoor environmental quality.

Moisture control, ventilation, HVAC maintenance, and building materials all influence long-term exposure. Mold exposure may also overlap with other indoor factors such as volatile organic compounds from furnishings or household materials.

Food-based mycotoxin exposure differs from airborne mold exposure in buildings, which we explain in more detail in our overview of mold and mycotoxins.

Looking at the home as a system often provides more clarity than focusing on a single laboratory value.

Conclusion

Mold testing in residential environments can provide useful information when used appropriately. Air sampling, dust sampling, ERMI, and HERTSMI each measure different aspects of mold presence.

No single method captures the full picture.

Understanding what a test measures, and what it does not measure, allows homeowners to make more grounded decisions. Moisture history, building condition, and ventilation patterns often matter as much as laboratory data.