How to Test a Home for Mold: ERMI, Air Testing, and More

When you suspect your home may be affecting your health, the natural question is: “Is this building safe?”

Unfortunately, no single test can answer that. Different tests measure different things, and each has blind spots. The most reliable assessment starts with the building itself, then uses testing to answer specific questions.

Start With Moisture

Mold needs water to grow. That makes the first and most important step a careful search for moisture, including:

• Roof or plumbing leaks
• Water stains, peeling paint, or warped materials
• Condensation on windows or walls
• Damp basements or crawlspaces
• High indoor humidity
• Problems with heating and air conditioning systems
• A musty smell

Experienced inspectors use tools such as:

• Moisture meters to measure water inside drywall, flooring, and framing
• Thermal (infrared) cameras to spot temperature differences that may point to hidden leaks
• Hygrometers to measure indoor humidity. Keeping indoor humidity below about 50% helps discourage mold growth.

Lab testing should support a physical inspection, not replace it.

Common Testing Methods

Air Sampling (Spore Traps)

How it works: A pump pulls a measured amount of air across a sticky surface. A lab then counts and identifies the spores.

What it tells you: Which spores were floating in the air during the sample, often compared with an outdoor sample.

Limitations: It’s a snapshot. Results change with airflow, weather, humidity, cleaning, and activity in the room. Heavy, sticky molds such as Stachybotrys and Chaetomium often don’t become airborne unless disturbed. A normal air sample doesn’t rule out hidden mold.

Some inspectors use “aggressive” sampling, stirring up settled dust before sampling to better reflect normal activity. This may catch contamination a quiet-room test misses, but methods and interpretation vary among professionals.

Settling or Agar Plates

How it works: Dishes containing a nutrient gel are left open for a set time, then watched to see what grows.

What it tells you: Whether living mold spores are settling in a space. Placing plates in several rooms can help compare areas.

Limitations: Only living spores grow, and not all molds grow equally well on every gel. Plates can’t detect dead fragments or mycotoxins, which may still affect sensitive people. Results depend on placement and timing. Plates offer a rough screening clue, not a full assessment.

Dust DNA Testing (ERMI and Similar)

How it works: A lab analyzes settled dust for the DNA of specific molds. ERMI (Environmental Relative Moldiness Index) looks at 36 species. Twenty-six are associated with water damage, and 10 are common indoor molds. These are combined into a single score. Other dust DNA panels, some of which also test for mycotoxins, are available too.

What it tells you: Because dust collects over time, these tests can reflect which molds have been present over months or years, including dead fragments.

Limitations: Dust DNA doesn’t tell you where mold is growing, whether growth is active now, or how much exposure a person is getting. ERMI was originally developed as a research tool, so a single score shouldn’t be treated as a stand-alone verdict. Looking at which species are present, alongside the building’s history, is more useful.

Surface Sampling

How it works: Tape lifts or swabs are taken from a specific surface.

What it tells you: Whether a visible stain or growth on that surface is mold.

Limitations: It only describes that one spot, not the rest of the building.

Dust Mycotoxin Testing

How it works: A lab tests dust for specific mycotoxins, the chemical compounds some molds produce.

What it tells you: Whether those compounds are present in the sample.

Limitations: A positive result doesn’t prove where the compounds came from, whether exposure is ongoing, or whether they’re causing symptoms.

There Is No Universal “Safe” Number

There is no federally established indoor mold level that separates a universally “safe” building from an “unsafe” one. Some practitioners use their own guidelines, for example for how many colonies on a settling plate are acceptable for sensitive patients. These can be useful in practice, but they aren’t regulatory standards.

It’s also important to listen to your body. A “clean” test result doesn’t guarantee a building is fine for you if you consistently feel worse there and better away. Hidden problems inside walls or ductwork can be missed.

Choosing an Inspector

When possible, choose an inspector who doesn’t also do the remediation work. That way, the advice you get isn’t tied to selling you a service. Look for someone who focuses on finding moisture sources, not just collecting samples.

The Bottom Line

Each test answers a different question:

• Air sampling: What’s airborne right now?
• Agar plates: Are living spores settling here?
• Dust DNA: What molds have built up over time?
• Surface sampling: Is this particular spot mold?
• Mycotoxin testing: Are certain compounds present in this dust?

None of them replaces a careful inspection. The best question isn’t “Which mold test is best?” It’s this: “What are we trying to find out, and which combination of inspection and testing can answer it?”