How to Read a Mold Lab Report

What each part of a typical spore trap and surface sample report means, why there is no pass/fail line, and how to read results alongside what the inspection found.

First, what a lab report is and isn’t

A laboratory report describes what an analyst saw on the samples sent in. It does not describe the whole house or say whether anyone’s symptoms were caused by mold. EPA notes that no federal limits have been set for mold or mold spores, so a report cannot show that a home “complies” with a federal standard. The CDC likewise says there are no set standards for an acceptable quantity of mold. Interpretation comes from comparing samples with one another, with the outdoor reference, and with the findings on site.

Vegas Mold Pro sends samples to an independent laboratory accredited by the American Industrial Hygiene Association (AIHA). AIHA’s Environmental Microbiology Laboratory Accreditation Program covers labs that identify microorganisms in air, fluid, and bulk samples collected during indoor air quality studies.

The header: check it before the numbers

• Sample IDs and locations, which should match the field notes and the inspector’s report.
• Sample type: air (spore trap), tape lift, swab, or bulk material.
• Air volume for each air sample, usually in liters. It drives every concentration on the page.
• Chain of custody reference, the record of who handled the samples and when.
• Analysis method and the lab’s accreditation information.

Anatomy of a spore trap (air) result

A spore trap cassette catches airborne particles on a sticky surface as a pump pulls a measured volume of air through it. The lab counts and categorizes fungal structures under a microscope. ASTM D7391, a standard method for this type of analysis, reports results as fungal structures per sample and per cubic meter, and states that “the detector in this test method is the analyst,” meaning results depend on the analyst’s training and experience.

Raw count

The number of spores of each type the analyst actually counted. A raw count of 2 means two spores were seen, a very small basis for conclusions.

Count per cubic meter

The raw count converted to a concentration using the air volume and the portion of the sample examined. Always read it alongside the raw count.

Analytical sensitivity and detection limit

The concentration that a single counted spore represents for that sample. If 75 liters of air were collected and the whole deposit was read, one spore corresponds to about 13 per cubic meter. Labs sometimes read only part of the deposit for common spore types, which raises this value. A result reported as “ND” (none detected) means nothing was seen in the portion examined, not that the air contained zero spores. Some labs label this value “detection limit”; terminology varies.

Background debris rating

A rating, often on a numbered scale, of how much non-fungal particulate was on the sample. ASTM D7391 notes that particulate matter affects what can be counted; heavy debris can hide small spores, so treat low counts on a high-debris sample with caution.

Hyphal fragments

Pieces of the thread-like body of mold, reported separately from spores.

Outdoor reference

An outdoor sample collected the same day is the benchmark. EPA’s remediation guidance says that, after remediation, indoor samples should be similar in types and concentrations to local outdoor air. When indoor totals are higher than outdoors, or when one type makes up a much larger share indoors than out, that pattern points toward an indoor source.

Surface samples: tape lifts and swabs

Surface samples show what is growing on a specific spot. Results are usually qualitative: the lab lists the types seen and a descriptive amount such as rare, few, moderate, or many, and may note whether spores were accompanied by hyphae or other growth structures, which suggests growth on that surface rather than settled dust. These scales vary by lab, so read the key on the report itself. Surface results tell you about that spot, not the air in the room.

Common genera and what they suggest

• Cladosporium: among the most commonly reported spore types in outdoor air studies and often found indoors as well. Indoor levels that track outdoor levels are typical; indoor levels well above outdoors can suggest an indoor source.
• Penicillium/Aspergillus-type: the spores of these two groups look alike under the microscope, so spore trap reports usually combine them. A published study of more than 5,300 samples from water-damaged buildings found Penicillium chrysogenum and Aspergillus versicolor to be the most common species. An indoor count well above outdoors is a frequent flag for further investigation.
• Stachybotrys: the CDC says it can grow on high-cellulose materials such as gypsum board and paper when there is moisture from water damage, leaks, condensation, or flooding. Because it is associated with wet materials, finding it indoors when it is absent outdoors is usually worth a closer look for a moisture source, even at a low count.
• Chaetomium: the same study reported Chaetomium as very common on damp building materials.

Types like Stachybotrys and Chaetomium are often called “water-indicator” organisms because they are associated with materials that have stayed wet. Their presence is a clue about moisture history, not a health verdict. The CDC advises that all molds be treated the same with respect to potential health risks and removal. More on this is on our black mold page.

Hypothetical example

How this invented example might be read: the living room looks broadly like the outdoor sample, with lower totals and a similar mix. The hall bath is different: its total is about twice the outdoor total, it is dominated by Penicillium/Aspergillus-type spores, which were a small share outdoors, and it contains low counts of Stachybotrys and Chaetomium, which were not detected outside. That pattern would point the inspector back to the bathroom to look for a moisture source and hidden growth. It would not, by itself, say how large the problem is or whether anyone would be affected.

Typical sections of an inspection report

The lab’s pages are attached to the inspector’s report, which generally includes the scope, conditions observed, moisture readings, sample locations, a lab summary, interpretation, limitations, and recommendations. See a sample mold report and our overview of inspection reports. For how samples are collected, see mold testing methods.

If you have health concerns, share the report with your physician; it is not a medical finding.

Need a term defined? See the mold inspection glossary.

Sources

• Mold Testing or Sampling — U.S. EPA
• Mold Remediation in Schools and Commercial Buildings, Chapter 3 — U.S. EPA
• About Mold and Health — CDC
• Facts About Stachybotrys chartarum — CDC
• Environmental Microbiology Laboratory Accreditation Program — AIHA Laboratory Accreditation Programs
• ASTM D7391-20, Airborne Fungal Structures by Optical Microscopy — ASTM International
• Associations between Fungal Species and Water-Damaged Building Materials (Andersen et al., 2011) — Technical University of Denmark
• A systematic review of outdoor airborne fungal spore seasonality across Europe (Anees-Hill et al., 2022) — Science of the Total Environment via PMC