The short answer
Mould air testing draws a measured volume of air through a spore trap cassette in each room of concern and outdoors at the same time. A laboratory counts and identifies the spores. If indoor air has more spores, or different types, than outdoor air, there is probably a mould source inside the building.
Who books this
- Musty smells with no visible growth
- Occupants with symptoms who want to know whether the building is a factor
- Insurance, tenancy and workplace matters that need documented results
Methods used
- Spore trap cassettes
- Calibrated air pump
- Outdoor control sample
- Laboratory microscopy
Why the outdoor sample matters
Mould spores are always in outdoor air, and in a warm, humid climate the background can be high and changes with weather and season. An indoor count means little on its own. The comparison that matters is indoor against outdoor, taken at the same time: the total count, and the mix of spore types.
Some genera that grow on wet building materials — Stachybotrys and Chaetomium are the usual examples — are rarely abundant in outdoor air. Finding them indoors is commonly treated as a sign of water-damaged materials, even in small numbers.
There is no legal “safe level” of spores
Australia has no legislated limit for mould spores in indoor air, and the World Health Organization concluded in its 2009 guidelines that no quantitative health-based guideline values or thresholds could be recommended. What the WHO does say is that persistent dampness and microbial growth indoors should be avoided or minimised. A report that tells you a room “passed” against a number should explain where that number came from.
Getting a sample that means something
- Windows and doors are kept closed for a period before sampling so the indoor air is representative.
- Fans, air purifiers and air-conditioning are noted, and set the same way they normally run.
- An outdoor control is taken upwind of the building at the same time.
- Each cassette is labelled, logged on a chain-of-custody form and sent to the laboratory.
An air sample is a snapshot. Disturbance, cooking, rain and wind all shift the result, so sampling conditions are recorded in the report alongside the numbers.
Example air sample
Spores per m³ · bedroom vs outdoor control
Questions people ask
What do spore counts on an air test mean?
Counts are reported as spores per cubic metre of air for each spore type. They are interpreted by comparison with the outdoor sample taken at the same time, not against a fixed legal limit, because none exists.
Can an air test miss mould?
Yes. Growth that is sealed inside a wall cavity may release few spores into the room. That is why air sampling is paired with moisture mapping, which can find the wet area even when the air result is low.
How long do air sample results take?
It depends on the laboratory’s turnaround at the time. Expected timing is confirmed when the inspection is booked.
References
- [1]WHO guidelines for indoor air quality: dampness and mould (2009) — World Health Organization Regional Office for Europe
- [2]Preventing mould exposure — Australian Centre for Disease Control
Checked against these references on 2 November 2026.