Mould testing in Canadian homes: what the results actually tell you

Mould testing is one of the most frequently misunderstood services a laboratory offers. Clients often expect a simple pass/fail result, but mould spore counts require context to be meaningful. This article explains what the numbers represent and what to do with them.

Why outdoor baseline samples are not optional ¶

Fungal spores are present in outdoor air year-round in Canada, with counts that vary significantly by season. In late summer and early autumn, outdoor spore counts in Ontario can be quite high due to decomposing leaf litter and agricultural activity. An indoor spore count that looks alarming in isolation may be entirely consistent with outdoor conditions.

This is why a properly conducted mould assessment always includes at least one outdoor air sample collected at the same time as the indoor samples. The comparison between indoor and outdoor counts. And between the genera present indoors versus outdoors. Is what makes the indoor result interpretable. A laboratory report that presents only indoor counts without an outdoor reference is missing the context needed to draw a conclusion.

Genera that warrant attention regardless of count ¶

Most fungal genera found in indoor air are also common outdoors and are not inherently problematic at moderate counts. However, certain genera are rarely found in outdoor air and their presence indoors at elevated concentrations suggests an indoor moisture source. Stachybotrys chartarum (often called black mould) is the most well-known example. It requires very high moisture levels to grow and is not typically found in outdoor air samples.

Chaetomium, Fusarium, and Trichoderma are similarly associated with water-damaged building materials. Their presence in an indoor sample, even at relatively low counts, is worth investigating further. The laboratory report will identify genera to the level that spore morphology allows and will flag any that are atypical for outdoor air.

What to do after receiving a mould report ¶

If the report shows elevated indoor counts of water-damage-associated genera, the next step is a visual inspection and moisture investigation. Not immediate remediation. The source of moisture needs to be identified and corrected before any remediation is effective; otherwise, regrowth is likely.

For remediation guidance, Health Canada's publication on fungal contamination in indoor environments is a useful starting point. For large-scale remediation (more than one square metre of visible mould), a qualified indoor environmental professional should be involved in the planning.

Mould reports are most useful when they are part of a broader investigation that includes a visual inspection and a moisture assessment. If you have questions about what your report results mean for your specific building, contact us directly.