Mold Testing Methods Explained

“Mold testing” can mean an air pump and cassette, tape pressed on a wall, a piece of drywall sent to a lab, or a store-bought dish left open on a counter. Each method answers a different question. This guide explains what each can and can’t tell you.

Start with the question, not the test

Mold spores are a normal part of indoor and outdoor air. The Minnesota Department of Health points out that if testing is done properly, finding some mold is expected. So “Is there mold?” is rarely a useful question on its own. Better questions: Is growth hidden behind this wall? Is this stain mold or soot? Did the cleanup work?

EPA recommends sampling only after building a plan around a testable theory of where the mold is and how people are exposed. It also says that when mold growth is plainly visible, sampling is usually unnecessary. AIHA likewise treats air sampling as a supplement to a visual inspection.

Air sampling: spore traps

How it works: A pump draws a measured volume of air through a cassette. Particles in the air stick to a small coated glass slide. A lab analyst counts and identifies spores by microscope. AIHA calls these “nonviable,” “spore trap,” or “total spore count” samples because they count spores whether or not they are alive.

What it can tell you:

• Roughly how the spore types and relative amounts indoors compare with outdoor air or with a comparison room.
• Whether hyphal (mycelial) fragments are present. AIHA notes that mold-damaged buildings tend to show more of these fragments on spore trap samples.
• Results come back faster than culture methods (AIHA cites typical turnaround of 24 to 72 hours).

What it can’t tell you:

• Species. AIHA notes that microscopy alone can’t identify species, and some genera are reported only as groups, such as Penicillium/Aspergillus-like spores.
• Whether you are safe. AIHA states that air sampling does not and cannot evaluate health risks for occupants.
• What the air is like at other times. EPA compares an air sample to a snapshot. AIHA cites research showing large swings within a single day.

Air sampling: culturable (viable) samples

How it works: Air is impacted onto a growth medium such as an agar plate, and the lab grows and identifies whatever colonies develop. Results are reported as colony-forming units.

Strengths: Culture can sometimes identify fungi to the species level, which AIHA notes can help an investigator trace hidden contamination.

Limits: Only spores that survive sampling and grow on the chosen medium show up. The choice of medium strongly affects which fungi appear. AIHA notes that dead spores can still trigger allergic responses. Lab turnaround is typically about one to two weeks, and AIHA reports there is no reliable correlation between culture results and spore trap results.

Surface sampling: tape lifts and swabs

How it works: Clear adhesive tape is pressed onto a suspect surface, or a sterile swab is wiped across it. The sample goes to a lab for direct microscopic examination (or sometimes culture).

What it can tell you: Whether a discoloration contains fungal growth structures. EPA notes that sampling can help tell mold apart from soot or dirt. EPA also notes surface sampling can help judge whether an area was adequately cleaned.

What it can’t tell you: Anything about the air, or about areas you didn’t sample. The Minnesota Department of Health cautions that growth may not be uniform across a surface and can change over time.

Bulk sampling

How it works: A physical piece of material, such as drywall, carpet, or insulation, is removed and sent to the lab. EPA lists bulk samples among the standard sample types.

Useful when: The question is about a specific material. Because it is destructive, it is used selectively.

ERMI dust testing

The Environmental Relative Moldiness Index (ERMI) was developed by EPA scientists. It analyzes mold DNA in a household dust sample and compares the result to a national scale built from 36 indicator molds. EPA says plainly that ERMI is a research tool and is not recommended except for research. It has not been validated for routine use in homes.

DIY settle-plate kits

Most store kits are a dish of growth medium that you leave open so airborne spores can settle on it. University of Connecticut Environmental Health and Safety explains the problem: because spores are everywhere, it would be rare for the dish not to grow mold. There are no accepted standards for running or interpreting these tests, and the dishes can give the false impression of a mold problem. Nor can a plate tell you where the spores came from.

Why outdoor baseline samples matter

Because spores are always present, indoor air results mean little without a reference. AIHA says spore trap interpretation should be limited to judging whether indoor spore types are materially different from those outdoors or in a control area. EPA lists omitting outdoor control samples as a common pitfall of sampling by inexperienced people. After remediation, EPA says indoor mold types and concentrations should resemble local outdoor air. AIHA lists outdoor conditions such as season, precipitation, and wind among the factors that affect results, which is why the outdoor sample should be collected at the same visit as the indoor ones.

Why the lab matters

EPA recommends following analytical methods from AIHA or ACGIH. AIHA’s Environmental Microbiology Laboratory Accreditation Program (EMLAP) covers labs that analyze air (such as spore traps), surface (tape lifts, swabs, wipes), and bulk samples. AIHA guidance notes that accredited labs must meet quality requirements beyond proficiency testing, including duplicate analyses and on-site assessments. Vegas Mold Pro sends samples to an independent AIHA-accredited laboratory.

Putting it together

For example, a hypothetical homeowner with a musty hallway closet and no visible growth might get a visual inspection with moisture readings, one or more air samples in the closet area, and an outdoor baseline. Someone with obvious growth on a bathroom ceiling might need no sampling at all. For next steps, see how to read a mold lab report and what an inspection report should contain. For our services, see mold testing in Las Vegas.

Testing results are not a medical diagnosis. If you have symptoms, consult a physician.

Sources

• Mold Remediation in Schools and Commercial Buildings, Chapter 3 (Sampling) — U.S. EPA
• Mold Testing or Sampling — U.S. EPA
• The Environmental Relative Moldiness Index (ERMI) fact sheet — U.S. EPA
• Should I test for mold using ERMI? — U.S. EPA
• FAQs About Spore Trap Air Sampling for Mold — AIHA
• Environmental Microbiology Laboratory Accreditation Program — AIHA LAP
• Testing for Mold — Minnesota Department of Health
• Facts About DIY Mold Test Kits — University of Connecticut EHS