"Can you test the air for mould?" is one of the most common questions our laboratory is asked, and the answer is yes, with one condition: the number on its own is close to meaningless, and the value is entirely in what it is compared with. This article explains how a spore-trap air test is taken, what the laboratory does with it, why the outdoor sample is the whole point, and the situations in a home where air testing is the right tool.
How the sample is taken
A spore-trap cassette is a small sealed unit with a narrow inlet and a sticky glass slide inside. It is connected to a pump calibrated to draw air at 15 litres per minute, and run for a set number of minutes, so the volume of air that passed through is known exactly. Air entering the inlet is accelerated and turned sharply; spores and fungal fragments, being heavier than air, cannot make the turn and are thrown onto the adhesive. The cassette is then sealed, labelled and sent to the laboratory. The sample is a few minutes of one room's air, captured on a slide, and that is exactly what it should be understood as: a snapshot.
What the laboratory does with it
The slide is examined under the microscope and read to ASTM D7391-20, the standard test method for categorising and quantifying airborne fungal structures. The analyst counts spores and other fungal structures across a defined area of the slide, identifies the genera present from their shape, size, colour and arrangement, and scales the count to the volume of air sampled. The result is reported as fungal structures per cubic metre of air, by genus, alongside notes on anything else the slide shows: hyphal fragments, pollen, skin cells, fibres, soot. A slide is read by a person, which is why the training of the analyst matters as much as the equipment.
Why the outdoor sample is the whole point
Outdoor air in New Zealand always carries mould spores, in every season and every region, and the mix changes with the weather, the time of year and what is growing nearby. Indoors, most of what you breathe is that same outdoor air after it has come in through the windows and doors. So an indoor count of, say, a thousand structures per cubic metre is not high or low in itself. It is high if outdoor air that day was carrying two hundred, and unremarkable if outdoor air was carrying three thousand because it is a warm, damp afternoon in late summer.
That is why every air test we take is paired with an outdoor reference sample from the same property on the same day, and why the comparison is made on two axes at once. Count: is the indoor total well above outdoors? Composition: is the indoor mix dominated by genera that are scarce outside? Outdoor air in this country is typically led by Cladosporium. An indoor sample led instead by Aspergillus and Penicillium, or showing Chaetomium or Stachybotrys that are absent from the outdoor slide, is telling you there is a source inside the building, even when the total count is not dramatic. A count that matches outdoors in both size and mix is what a normal building looks like.
What an air test can tell you, and what it cannot
It can tell you whether the air in a room carries a mould burden above what the outdoor air explains, and whether the genera point to water damage. It is the only practical way to test for mould you cannot see: behind a lining, under a floor, inside a ceiling cavity. It is also the standard way to confirm, after remediation, that a building is back to a normal fungal ecology.
It cannot, on its own, tell you where the source is. It cannot prove there is no mould, because a single sample is a snapshot and some genera are poor at becoming airborne at all: Stachybotrys spores are sticky and heavy, so a room with a Stachybotrys colony on the wall can return a modest air count, which is exactly why visible growth is sampled from the surface rather than from the air. And an air test does not find the water. That is why, in an assessment, air samples sit alongside surface samples and moisture readings rather than replacing them. A surface sample says what is growing; the moisture meter says why; the air sample says who is breathing it.
When air testing is the right tool in a home
- A musty smell with nothing visible. The classic case. The growth is concealed, and the air is the only place it shows.
- Symptoms with no obvious cause. An elevated, water-damage-dominated indoor sample turns a suspicion into a measurement that a doctor or a landlord can act on.
- After a leak, a flood or a slow plumbing failure, especially where the materials have dried and look fine.
- Before and after remediation. A baseline, then a post-remediation verification that the work achieved a normal indoor ecology.
- Rental properties. When a tenant and a landlord disagree about whether a house is mouldy, an independent measurement against an outdoor reference replaces two opinions with one number.
- Buying a house where the vendor has "just repainted" a room that used to smell.
Air testing in a workplace is a different service with a different reference framework. If the building is an office, a workshop or a plant, and the question involves exposure standards, VOCs or ventilation, that is our workplace indoor air quality page rather than this one.
Reading the numbers
New Zealand has no regulatory limit for indoor mould spore concentrations, and no responsible laboratory will give you a single number that separates safe from unsafe. What a good report gives you instead is the indoor result read against the outdoor reference, the genera named, and each area rated against the ANSI/IICRC S520 conditions: normal fungal ecology, settled spores from a source elsewhere, or actual growth. Our guide to reading your mould test report walks through those fields one at a time. If you are weighing up whether you need an air test at all, the mould testing page sets out how air sampling fits into an assessment and what it costs.