You sent samples to a lab. The report came back with numbers and species names you've never seen. Here's how to read it without a microbiology degree.
The two pieces of information that matter
For each sample, the lab returns:
- Species (or species group) detected
- Spore concentration - measured in spores per cubic meter of air (spores/m³)
Everything else on the report is supporting detail. These two numbers are what you act on.
Step 1: Compare inside vs. outside
The most important comparison in the entire report is the indoor sample against the outdoor control sample (which the lab tests at the same time, from the same air).
- Indoor lower than outdoor: good. Indoor air is "cleaner" than the local outdoor air.
- Indoor similar to outdoor: generally fine. The home isn't a mold source.
- Indoor significantly higher than outdoor: flag. Likely an indoor source.
- Indoor has different species than outdoor: strong flag. Even at low counts, indoor-specific species suggest indoor growth.
Skip this comparison and the spore-count numbers in isolation mean very little.
Step 2: Look at the total spore count
As a rough reference for indoor residential air:
| Indoor count | What it usually means |
|---|---|
| Under 1,500 spores/m³ | Typically within normal range |
| 1,500-3,000 spores/m³ | Elevated - depends on species + outdoor baseline |
| 3,000+ spores/m³ | Likely indoor source; investigate |
These are general references, not hard cutoffs. The species composition matters as much as the raw count.
Step 3: Look at the species
Some species are common in outdoor air and benign indoors. Others are "water-damage indicators" - their presence indoors usually signals active moisture.
Common outdoor (usually OK indoors at similar-to-outdoor levels):
- Cladosporium
- Basidiospores
- Rusts
- Smuts
- Nigrospora
Water-damage indicators (concerning indoors):
- Aspergillus / Penicillium group - by far the most common indoor problem indicator. Often grouped on the report because the spores look similar under a microscope. A high indoor count compared to outdoor strongly suggests active growth somewhere.
- Stachybotrys - the "black mold" species. Found in air = something is actively producing it indoors. Almost always means cellulose materials are wet for an extended time.
- Chaetomium - like Stachybotrys, signals long-term water damage.
- Fusarium - wet conditions, often in carpet or behind appliances.
A few water-damage-indicator spores in air is not a crisis - they can drift in from outside or stir up from old, dried colonies. A high concentration in air is.
Step 4: Raw count vs. spores/m³
Reports sometimes show two numbers:
- Raw count - what the lab physically saw on the slide
- Spores/m³ - the adjusted concentration, normalized for sample volume
Always make decisions on spores/m³. Raw count alone doesn't account for how much air was tested.
Step 5: Don't over-interpret a single sample
One air sample is a snapshot. Mold spore counts vary by:
- Time of day
- Indoor vs. outdoor activity (vacuuming, opening doors)
- Season (outdoor counts spike in fall when leaves decay)
- Whether the HVAC just ran
A single elevated sample doesn't prove an indoor source. A consistent pattern across multiple samples (or before/after comparisons) does.
What we do with your report
We give you the raw lab data plus our written interpretation - what the numbers mean, whether they suggest active growth, and what to do next. The report itself is yours to keep - useful for insurance, real estate transactions, or your doctor.
Book free inspection → - including independent testing without remediation.