How to Read a Mold Lab Report
Reading a mold lab report comes down to one comparison: what is inside versus what is outside on the same day. A spore count on its own means almost nothing, which is why a report with no outdoor sample cannot tell you whether you have a problem.
Most people open a lab report, look for a number that says pass or fail, and do not find one. That is not the lab being unhelpful. There is no legal or federal threshold for mold spores in indoor air, so the report gives you data and leaves the interpretation to whoever is reading it. Here is how to read it yourself.
Start with the outdoor sample
Outdoor air always contains mold spores. In the valley the count changes with wind, season and whether anything has been disturbed nearby. So the outdoor sample taken the same day is the baseline your indoor rooms are measured against.
Broadly, indoor counts meaningfully lower than outdoor, with a similar mix of types, suggests normal conditions. Indoor counts substantially higher than outdoor, or a type present indoors that is absent outdoors, points to an indoor source.
If your report has no outdoor sample, it cannot support either conclusion. That is the single most common defect in a report handed to a homeowner, and it is worth pushing back on before you accept any conclusion drawn from it.
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What the columns mean
Raw count is how many spores the analyst actually counted on the slide or plate. Spores per cubic metre is that count scaled by the volume of air sampled, and it is the figure to compare between rooms. A low raw count converted from a very short sample can produce a large per-cubic-metre number, so glance at both.
Genus is the type of mold. Reports rarely identify species from air samples, and that is normal rather than a shortcoming.
Background debris or loading describes how much other material was on the sample. Very heavy loading can obscure spores and make a count less reliable, which the lab will usually flag.
Detection limit matters more than people expect. If a type was not detected, that means not detected at the sensitivity of that sample, not proven absent.
Which types actually tell you something
What a lab report cannot tell you
It cannot tell you whether you will get sick. Sampling measures what was in the air during a short window; it does not assess health, and no reputable report will make that leap. Symptoms are a medical question for a doctor.
It cannot tell you where the growth is. Air sampling suggests that something is elevated somewhere. Finding it is an inspection job involving moisture readings and, where warranted, opening the assembly.
It cannot tell you why. The number never explains the water source, and the water source is the entire problem. A report that leads to a cleanup without identifying the moisture is a report that will be repeated next year.
And it cannot, by itself, tell you a job is finished. That requires sampling done under the right conditions, which is set out on our page about how to know your mold was actually removed. If you are deciding what sampling to commission in the first place, see mold testing.
Common questions
- What is a normal mold spore count indoors?
- There is no established normal figure, and no federal or Nevada standard sets one. Interpretation is relative: indoor counts are compared against an outdoor sample taken the same day, looking at both the totals and the mix of types.
- My report has no outdoor sample. Is it still useful?
- Only in a limited way. It can confirm which types are present, but without the outdoor baseline it cannot establish whether indoor levels are elevated. If a conclusion was drawn from it, ask what that conclusion was based on.
- Does finding Stachybotrys mean my house is dangerous?
- It means sustained moisture has been present, because that is what it needs to grow. Health effects are a medical question and vary between people. Treat it as strong evidence of a water problem that needs finding.
- Why does my report say Penicillium/Aspergillus together?
- Their spores are visually very similar under a microscope, so air-sample analysis reports them as a combined category. Distinguishing them requires culturing, which is a different and slower test.
- Can I compare two reports from different companies?
- Carefully. Different sampling methods, volumes and conditions produce numbers that are not directly comparable. Compare methodology first, and only then the figures.
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