The short answer
Compare every indoor sample with the outdoor control taken at the same time. Indoor totals well above outdoor, or spore types indoors that are rare outside — especially water-damage indicators such as Stachybotrys or Chaetomium — point to a mould source inside. A report that gives indoor numbers with no outdoor comparison is hard to interpret.
The parts of a spore trap report
| Section | What to look for |
|---|---|
| Sampling details | Date, time, weather, who sampled, pump flow rate and sample duration |
| Locations | Each room sampled, plus the outdoor control |
| Spore types | Counts for each genus or group, usually per cubic metre (spores/m³) |
| Totals | Total spores per sample |
| Debris rating | How much dust and particulate was on the slide, which affects accuracy |
| Laboratory certificate | Attached unaltered, with the laboratory’s name |
| Interpretation | A plain-English explanation of what the comparison shows |
Reading the comparison
Totals. An indoor total similar to or lower than outdoors is the usual pattern in a building without a mould problem. An indoor total several times the outdoor figure is a strong signal.
Rank order. Outdoor air is usually dominated by spore types such as Cladosporium. If indoor air is dominated by a different type — Aspergillus/Penicillium-like spores, for example — something inside may be producing them, even if the total is not high.
Indicator types. Stachybotrys and Chaetomium grow on wet, cellulose-rich materials such as plasterboard paper and timber. Their spores are heavy and don’t travel far in air, so even low counts indoors are worth investigating.
Hyphal fragments. Pieces of the mould body itself, rather than spores. Elevated fragments can indicate disturbed or deteriorating growth.
Why there’s no pass mark
The World Health Organization found no health-based numeric threshold for mould could be recommended, and Australia has not legislated one. Some reports compare results with industry rules of thumb; if yours does, it should say where those numbers come from. The ANSI/IICRC S520 standard frames the goal after any works as a return to normal fungal ecology — essentially, indoor air that looks like the outdoors.
Red flags in a report
- No outdoor control sample.
- No record of whether windows, doors and air-conditioning were open, closed or running.
- A laboratory result without the laboratory’s name or certificate.
- Results from a tester who also quoted for the removal work — not wrong in itself, but the report should be read with that in mind.
Questions people ask
What is a normal indoor spore count?
There is no fixed normal number. In a healthy building, indoor counts are usually lower than outdoors and show a similar mix of spore types. The comparison matters more than the absolute figure.
My report says Aspergillus/Penicillium. Is that bad?
These spores look alike under the microscope and are reported together. They are common outdoors too. They become a concern when indoor counts are clearly higher than outdoor.
References
- [1]WHO guidelines for indoor air quality: dampness and mould (2009) — World Health Organization Regional Office for Europe
- [2]ANSI/IICRC S520 Standard for Professional Mold Remediation (4th ed., 2024) — Institute of Inspection, Cleaning and Restoration Certification
Checked against these references on 2 November 2026.