Mold Testing Explained: What Air Samples, Tape Lifts, ERMI, HERTSMI and VIA-CELL Tests Can—and Can't—Tell You

When homeowners discover mold—or suspect it—they often ask the same question:

"Can we test for it?"

The answer is yes. In fact, there are several different types of mold tests available today. But understanding what each test is designed to tell you is just as important as the results themselves.

At All Clear Mold & Pathogen Solutions Inc., we use laboratory testing when it helps answer a specific question. However, we also spend a great deal of time explaining what testing cannot tell us. Unfortunately, many people believe that a single laboratory report provides a complete picture of a home's health. It doesn't.

No single mold test can diagnose the condition of an entire building.

Like any diagnostic tool, each test has strengths, limitations, and an appropriate purpose.

Air Sampling: A Snapshot in Time

The most common mold test performed during a home inspection is the Air-O-Cell air sample.

During this test, a calibrated air pump draws a measured volume of air through a collection cassette. The laboratory then examines the captured particles under a microscope and estimates the concentration and types of airborne mold spores present during the sampling period.

Notice those last four words: During the sampling period.

An air sample is very much like taking a photograph. It captures what was happening during a brief period of time, but it cannot tell us what occurred yesterday, what will occur tomorrow, or what may be hidden behind walls, beneath flooring, or inside an attic.

Mold Doesn't Always Produce Spores

One of the greatest limitations of air sampling is that mold is not constantly releasing spores.

Many molds spend much of their life actively growing without producing significant airborne spores.

That means an air sample may show relatively low counts even though substantial mold growth exists somewhere within the building.

For example, an air sample may report:

300 spores per cubic metre.

Does that mean the home has only a minor mold problem?

Not necessarily.

There may be active mold growth hidden inside wall cavities, beneath flooring, inside crawlspaces, or within attics that simply isn't releasing spores at the moment the sample is collected.

The mold didn't disappear.

It simply wasn't "blooming."

When High Air Counts Are Helpful

Ironically, very high airborne spore counts can actually make diagnosis easier.

If the sampling occurs during an active sporulation event, the elevated counts clearly demonstrate that mold is affecting the indoor environment.

Certain molds deserve particular attention.

The presence of Stachybotrys or Chaetomium spores—even in relatively small numbers—is often significant because these molds typically indicate chronic moisture intrusion or prolonged water damage.

Likewise, laboratories sometimes identify hyphal fragments.

Hyphae are the thread-like structures that make up the growing body of mold. Finding hyphal fragments in an air or surface sample often suggests active fungal growth, even when airborne spore counts themselves are relatively low.

Why Mold Counts Can Be Misleading

People naturally compare numbers.

One report may show:

• 800 spores

• 1,800 spores

• 18,000 spores

Those numbers appear dramatically different.

But consider this:

The head of an ordinary pin can hold approximately one million mold spores.

Whether a laboratory reports 800 spores or 18,000 spores, both represent only a tiny fraction of what may exist within an actively growing colony.

The laboratory accurately reports what it captured.

It cannot report spores that weren't airborne during the sampling period.

This is why experienced inspectors never rely solely on laboratory numbers.

Air sampling is most likely to capture elevated spore concentrations when mold is actively sporulating. During periods when mold is growing but releasing relatively few spores, air samples may underestimate the extent of contamination.

Tape Lift Testing

Tape lift samples are collected directly from visible staining or suspected mold growth.

The sample is examined microscopically to determine:

• The type of mold present

• General characteristics of the fungal structures

• Whether mold is visible on that particular surface

Tape lifts are excellent for answering one question:

"What is growing here?"

However, they cannot determine:

• Whether the mold is alive or dead

• Whether it is releasing spores

• Whether it is affecting air quality elsewhere in the building

• The overall extent of contamination

Like every laboratory method, tape lift testing answers one specific question—it does not diagnose the health of the building as a whole.

Air-O-Cell vs. VIA-CELL Testing

These two tests are often confused, but they answer very different questions.

Air-O-Cell Testing

Air-O-Cell samples are examined under a microscope.

The laboratory identifies mold spores based on their appearance and estimates how many are present.

Air-O-Cell testing is excellent for:

• Comparing indoor and outdoor air

• Identifying airborne mold groups

• Assessing airborne spore concentrations

• Supporting pre-remediation investigations

Its biggest limitation?

It cannot determine whether the spores are alive or dead.

A dead mold spore looks essentially identical to a living one under a microscope.

VIA-CELL Testing

VIA-CELL testing takes a completely different approach.

Rather than simply identifying what is present, the laboratory cultures the collected sample under carefully controlled conditions to determine whether viable (living) microorganisms remain.

This makes VIA-CELL particularly valuable after remediation or biodecontamination, when the question isn't simply:

"Are mold particles still present?"

Instead, the question becomes:

"Did the treatment successfully destroy the living microorganisms?"

Because of this, All Clear often recommends VIA-CELL testing following our InstaPURE™ whole-home biodecontamination process when clients request independent laboratory verification of treatment effectiveness.

What About ERMI and HERTSMI Testing?

Over the past several years, ERMI (Environmental Relative Moldiness Index) and HERTSMI-2 testing have become increasingly popular, particularly among individuals with mold sensitivities and some healthcare practitioners.

Unlike air testing, these methods analyze settled household dust using DNA technology (qPCR).

Rather than counting airborne spores, the laboratory identifies mold DNA that has accumulated over weeks or even months.

Because dust represents a longer period of accumulation, ERMI can sometimes provide a broader picture of a home's history than a single air sample.

However, it's important to understand what these tests were designed to measure—and what they were not.

ERMI Does Not Measure Current Air Quality

One of the most common misconceptions is that a high ERMI score automatically means the air inside the home is currently unhealthy.

That is not what the test measures.

ERMI measures mold DNA present in settled dust.

It cannot determine:

• Whether mold is actively growing today

• Whether the mold is alive or dead

• Whether the original moisture problem has already been corrected

• Whether occupants are currently being exposed

A home may have a relatively high ERMI score because contamination occurred years earlier.

Conversely, a home with an active mold problem may have a relatively modest ERMI score if dust hasn't yet accumulated.

HERTSMI-2

HERTSMI-2 is a simplified scoring system derived from ERMI.

It focuses on five mold species commonly associated with water-damaged buildings and is sometimes used by healthcare practitioners when evaluating patients with significant mold sensitivities.

Like ERMI, however, HERTSMI-2 should never be viewed as a stand-alone assessment of a building.

It does not replace:

• A comprehensive inspection

• Moisture investigation

• Building science evaluation

• Air sampling where appropriate

• Professional interpretation of the building's history
  

Every Test Answers a Different Question

Rather than asking:

"Which mold test is the best?"

A better question is:

"What question are we trying to answer?"

Testing Method	Best Used For	Important Limitations
Visual Inspection	Identifying visible damage, moisture issues, and likely mold sources	Cannot detect hidden mold without further investigation
Moisture Mapping	Finding conditions that support mold growth	Does not identify mold species or air quality
Tape Lift	Identifying visible mold on a surface	Cannot determine viability or building-wide impact
Air-O-Cell	Measuring airborne spores during sampling	Snapshot only; cannot determine if spores are alive or detect non-sporing growth
VIA-CELL	Determining whether viable microorganisms remain after treatment	Does not identify hidden moisture sources or quantify all contamination
ERMI / HERTSMI-2	Evaluating mold DNA accumulated in settled dust over time	Does not determine active growth, current air quality, or whether mold is alive

Building Science Still Matters Most

Buildings are complex systems.

No laboratory test—regardless of how sophisticated—can replace a thorough understanding of how a building performs.

An accurate assessment should always consider:

• Moisture intrusion

• Building history

• Ventilation

• Humidity levels

• Thermal imaging (where appropriate)

• Occupant observations

• Construction details

• Laboratory findings

When these pieces are considered together, they provide a far more complete understanding than any single laboratory report ever could.

Our Philosophy

At All Clear Mold & Pathogen Solutions Inc., we don't use testing to create unnecessary concern, nor do we rely on a single laboratory result to define the extent of a mold problem.

Laboratory testing is an important diagnostic tool—but it is only one part of the investigation.

Our goal is to understand why mold is present, determine how it is affecting the indoor environment, and recommend the most appropriate solution based on building science, moisture conditions, inspection findings, and laboratory evidence.

Ultimately, the most reliable conclusions come from combining these pieces of information—not from relying on any one test in isolation.

Because restoring a healthy indoor environment isn't about chasing numbers on a laboratory report. It's about understanding the building as a whole and solving the problem at its source.