Mould Spore Testing: Counts, Species and Interpretation

Spore testing is most useful where a measurable exposure question needs an answer. Typical scenarios include post-flood or leak investigations, recurring musty odours without obvious source, disputed damp and mould complaints between landlords and tenants, pre-purchase due diligence on properties with damp history, clearance testing after remediation, and occupant health complaints where a clinician has requested exposure data.

It is less useful where the building has obvious, large-scale visible growth and an obvious moisture source — in that case the priority is to stop the moisture and remove the colonised material, not to quantify what is already evident.

A typical visit lasts one to three hours depending on building size. The technician calibrates the pump on site, then takes a paired outdoor reference sample upwind of the building. Indoor samples follow a defined hierarchy: the suspect room first, then adjacent rooms, then supply air from any central HVAC system if relevant.

Each cassette is labelled with location, start and end time, flow rate and total sampled volume. A short building walkthrough records visible growth, moisture meter readings, ventilation provision and any complaints. This contextual record is essential for the laboratory and for interpretation.

Interpretation is comparative. Three checks dominate. Indoor versus outdoor totals: indoor counts at or below outdoor counts are usually unremarkable; indoor counts substantially above outdoor counts indicate an indoor source. Species composition: outdoor air is normally dominated by Cladosporium and Alternaria. A strong indoor presence of Aspergillus, Penicillium or moisture-marker genera such as Stachybotrys or Chaetomium with little outdoor counterpart points to indoor amplification.

Finally, room-to-room patterns help localise a source. A sharp drop in spore counts on moving away from a single room suggests that room is the reservoir. A uniform pattern across many rooms may indicate distribution through ventilation.

Spore testing has real limits and we are clear about them. It is a short snapshot — spore release fluctuates with humidity, activity and air movement. Non-viable cassettes count both living and dead spores; they cannot, on their own, tell you whether the source is currently active. Species identification by microscopy resolves genera, not individual species — further methods such as viable culture or qPCR are needed where finer resolution matters.

A spore count is not a medical diagnosis and does not, on its own, prove that an occupant's symptoms are caused by mould exposure.

You receive the laboratory's analytical report with raw counts and genera identified, alongside an interpretive report from MouldAirTesting.uk. The interpretive report includes the indoor–outdoor comparison, comments on species of concern, the building observations made during sampling, and clear recommendations on whether further investigation, remediation or re-testing is appropriate.