Air Quality Tests for Mold: What the Numbers Mean
A mold air quality test pulls a measured amount of air across a collection slide or plate, and a lab reports the spores it finds by type, usually as a count per cubic metre of air. The numbers only mean something when compared with an outdoor sample taken the same day. Canada has no pass or fail limit for mold in house air, so the report is a matter of interpretation.
How a mold air quality test works
The most common method is the spore trap. A small pump draws air through a cassette for a set number of minutes, and airborne particles stick to a coated slide inside. A lab analyst counts and sorts the spores under a microscope, then scales the count up to a figure per cubic metre. Results usually come back within a few days.
The other method is a culture sample. Air is drawn over a plate of growth medium, the plate is incubated, and the lab counts the colonies that grow. This can identify molds down to the species, which a spore trap often cannot. It takes a week or more, and it only counts spores that are alive and willing to grow on that medium, so it misses dead spores and fragments that still matter for allergies.
A proper sampling visit includes at least one outdoor sample and one or more indoor samples, all taken the same day with the same equipment.
What the numbers on a mold air test report mean
The report is a table. Each row is a mold type and each column is a sample location. Three things matter when you read it.
- Indoor total against outdoor total. Outdoor air always carries mold spores. In a house without a mold problem, indoor counts are generally similar to or lower than outdoors.
- The mix of types. Indoor air should look like a thinned-out version of the outdoor air, with the same types in roughly the same order. A type that dominates indoors but is minor or absent outdoors suggests something is growing inside.
- Water-damage indicators. Some molds, such as Stachybotrys and Chaetomium, need very wet building materials and are rarely found in outdoor air in any quantity. Even a low count indoors is worth following up.
You will often see a row labelled Aspergillus/Penicillium-like. Those two groups have small round spores that cannot be told apart on a spore trap, so labs report them together. A raised count in that row, well above outdoors, is one of the more common signs of indoor growth.
Is there a safe mold spore count?
No. Health Canada has not set a numerical exposure limit for mold in indoor air, and neither has Ontario. People react too differently for one number to work, and counts change too much from hour to hour. Any report that stamps a house as safe or unsafe based only on a spore count is going beyond what the science supports. Some labs and consultants use their own rule-of-thumb ranges. Those are opinions, and you should ask what they are based on.
Why air test results swing from day to day
An air sample is a few minutes of one room on one day. Many things move the result.
- Season. Outdoor counts in Southwestern Ontario are high in late summer and fall and very low in winter when the ground is frozen or snow-covered. A winter comparison can make an ordinary house look elevated.
- Weather. Rain, wind and humidity all change outdoor counts within hours.
- Activity. Vacuuming, walking on carpet, a running furnace fan or a pet shaking itself stirs settled spores into the air.
- Open windows. Outdoor spores come straight in and blur the comparison.
- Where the mold is. Growth sealed inside a wall cavity or above a ceiling may release very little into the room on the day of the test.
That last point matters most. A clean air test does not prove a house has no mold. It shows that not much was airborne in that room during the sample. We have no trouble believing a house can have a wet, moldy wall cavity and an unremarkable air result, because the physics allows it.
When an air test is useful
Air sampling is a reasonable tool in a few situations: tracking down a musty odour when a physical inspection has found nothing, documenting conditions for a dispute or a transaction, or giving independent confirmation after a large remediation job. It is a weak tool for deciding whether visible mold needs to be removed, since Health Canada's answer to that is always yes.
If you are paying for sampling, hire an independent consultant who does not sell removal, and make sure an outdoor sample is included. Ask for a written interpretation along with the lab table.
What to do with your results
If indoor counts look like outdoors and there is no smell, staining or dampness, you likely have nothing to chase. If the indoor mix is different or indicator molds show up, the next step is finding the wet material with a moisture meter and thermal camera, because the air result tells you something is growing and says nothing about where. Keep the report. If remediation follows, it is a useful baseline to compare against afterwards.
Common questions
How many air samples does a house need?
At minimum one outdoors and one in the area of concern. Consultants often add a sample from an unaffected room for comparison, and more for larger homes or several problem areas.
Should windows be closed before a mold air test?
Yes. Consultants generally ask that windows and exterior doors stay closed for a period before sampling so the indoor air reflects the house and not the weather.
Can an air purifier affect mold test results?
It can. A HEPA purifier running in the room lowers airborne counts during the test, so tell the consultant and follow their instructions on whether to turn it off beforehand.
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