Mould Air Testing: Detecting Airborne Spores Reliably

The standard method uses a calibrated air pump drawing 15 litres per minute through a sticky cassette — typically an Air-O-Cell, Allergenco-D or equivalent. Spores impact onto the trace, the cassette is sealed and shipped to a UKAS-accredited laboratory.

Direct microscopy at 600–1000× magnification identifies spores by morphology and counts them per traverse, with results extrapolated to spores per cubic metre. Where viability or finer species ID is needed, culture-based methods on malt extract agar or DNA-based qPCR are used in parallel.

Sampling locations follow a defined hierarchy: outdoor reference, suspect room, adjacent rooms, central air-handling supply, and any specific zone identified by visible growth or occupant complaint.

A raw spore count is meaningless without context. Three comparisons make it diagnostic.

Indoor versus outdoor. Indoor total counts that exceed outdoor counts indicate an indoor source. Indoor counts well below outdoor are typical of well-filtered HVAC.

Species composition. Outdoor air is dominated by Cladosporium and Alternaria. A heavy indoor presence of Aspergillus, Penicillium or Stachybotrys with little outdoor counterpart is a strong amplification signal.

Room-to-room patterns. A spore profile that drops sharply on moving away from a single room localises the source. A uniform pattern across multiple rooms suggests an HVAC distribution issue.

Stachybotrys chartarum and Chaetomium have particular weight — both require sustained wetness and indicate ongoing or recent water damage that warrants intrusive investigation regardless of count.

Mould exposure produces a spectrum of effects, not a single disease. Three patterns dominate.

Allergic response. IgE-mediated rhinitis, conjunctivitis and asthma in sensitised individuals. Roughly one in five UK adults reacts to common mould allergens.

Irritant and toxic response. Non-allergic upper-airway irritation, cough and headache linked to fungal volatiles and mycotoxins. Stachybotrys-produced trichothecenes are the most studied example.

Invasive infection. Aspergillosis and related infections in immunocompromised patients. Rare in the general population but life-threatening in transplant and oncology patients.

Children, the elderly and people with existing respiratory disease respond at lower exposure levels than healthy adults. Air testing provides the dose-side data that clinicians need to weigh against patient symptoms.

Air testing is most powerful as part of an integrated investigation rather than a standalone exercise.

Moisture mapping. Pin-type and capacitance moisture meters across suspect surfaces identify wet zones before destructive opening.

Thermal imaging. Surface temperature differentials locate hidden cold bridges and trapped moisture without contact.

Surface sampling. Tape lifts on visible growth confirm species and viability, complementing the airborne picture.

HVAC inspection. Cooling coils, drip trays and duct interiors are frequent reservoirs. Internal swabs and cassette samples within the AHU pinpoint these. Pair with a ventilation assessment when system-borne distribution is suspected.

Sustainable mould control follows the moisture, not the spore. Killing visible growth without removing the water source guarantees recurrence.

Stop the source. Repair the leak, address rising damp, fix the failed flashing, eliminate the cold-bridge condensation. Humidity control →

Dry the structure. Drying times are governed by material — plaster days, timber weeks, masonry months. Re-test only when moisture readings have stabilised.

Remove contaminated material. Porous materials with established growth (insulation, plasterboard, carpet) are replaced, not cleaned. Non-porous surfaces are decontaminated under containment.

Verify with re-testing. Clearance air samples confirm spore counts have returned to outdoor-reference levels before re-occupation.