After professional mould remediation, you want to know the job is done. Post-remediation verification (PRV) testing is the way to confirm this with objective data rather than a visual check alone. But a laboratory report full of spore counts and species names can be difficult to interpret without context. This guide explains the testing methods used, what the numbers in a clearance report mean, what passes and what does not, and what happens if a test reveals that remediation has not fully succeeded. For a broader explanation of the PRV process, see our post-remediation verification guide.
Quick Answer (BLUF)
Clearance is achieved when post-remediation indoor air spore counts are comparable to (or lower than) the outdoor reference sample collected at the same time, and no elevated indicator species are present. There is no single universal pass/fail number, clearance is a relative comparison between the indoor result and the outdoor control. Independent testing by someone other than the remediator is best practice.
Why Clearance Testing Matters
Visual inspection alone cannot confirm successful remediation. Mould spores are microscopic, a surface can appear clean and still carry significant spore concentrations. More importantly, remediation activity itself (cutting, brushing, HEPA vacuuming) can temporarily elevate spore counts in areas adjacent to the work zone. Without testing, neither the homeowner nor the remediator can objectively confirm that the building’s air quality has returned to an acceptable baseline.
For insurance claims, landlord-tenant disputes, and pre-sale disclosure, a laboratory clearance certificate from an independent assessor is often a requirement. In the Illawarra, where mould is a well-documented complaint in both rental and owned properties, having documented clearance has practical value beyond peace of mind.
Testing Methods
Air Sampling
Air sampling involves drawing a measured volume of air through a collection cassette that captures airborne particles (including mould spores). The cassette is sent to an accredited laboratory for analysis under a microscope.
Viable air sampling: Samples are grown on agar media to identify living mould colonies. Gives species identification and colony-forming unit (CFU) counts. Takes 5-7 days for results.
Non-viable (spore trap) air sampling: Captures both living and dead spores for direct microscopic count. Results in 24-48 hours. This is the most commonly used method in residential PRV testing in NSW.
For each indoor sample, a paired outdoor (ambient) sample should be taken at the same time under the same conditions. This is the control, the baseline against which the indoor result is compared.
Surface Sampling
Surface samples (swab or tape lift) are taken from specific locations, treated surfaces, containment barrier edges, HVAC registers. They answer the question: is this specific surface clean? Surface samples are useful as a supplement to air sampling but do not capture overall room air quality.
Moisture Readings
Not a microbiological test, but moisture content readings taken after drying confirm that the building materials are at levels where mould cannot re-establish. Plasterboard readings below 14-16% and timber framing below 15-19% are the target range for the Illawarra, though some professional standards specify tighter tolerances.
Interpreting Air Sampling Results
Laboratory results for spore trap sampling typically include:
- Total spore count, total mould spores per cubic metre of air (spores/m³)
- Species breakdown, the proportion of each genus or species identified in the sample
- Outdoor control, the simultaneous outdoor sample at the same address
What to look for in the indoor vs outdoor comparison:
| Result | Interpretation |
|---|---|
| Indoor total count ≤ outdoor total count | Generally passes, indoor air is at or below ambient outdoor levels |
| Indoor count 1.5-2x outdoor count | Borderline, may indicate residual contamination; review specific species |
| Indoor count >2x outdoor count | Elevated, requires investigation and potentially further remediation |
Species composition matters as much as total count. The presence of elevated Stachybotrys, Chaetomium, or certain Aspergillus/Penicillium species indoors (not reflected in the outdoor control) indicates building-specific mould, not just outdoor spores drifting in, and is a clearance failure regardless of total count.
Cladosporium is extremely common outdoors and is normally present indoors at similar or lower levels. Elevated Cladosporium indoors in proportion to outdoor counts may just reflect an open window, not a building mould problem.
What Constitutes a Pass?
In Australia, there is no single legislated pass/fail threshold for residential mould testing. Industry guidelines (AIOH, IICRC S520, NORMI) use an “outdoor equivalence” standard: the indoor environment post-remediation should reflect outdoor conditions rather than an elevated building-specific load.
Practical clearance criteria used by accredited assessors in the Illawarra:
- Indoor total spore count within the range of the outdoor control (within 1.5x)
- No indicator species (Stachybotrys, Chaetomium, heavily elevated Aspergillus/Penicillium) present at significantly higher counts indoors than outdoors
- All surface samples from treated areas return negative or comparable to clean control surfaces
- All moisture readings in treated materials within acceptable range
What Happens if a Test Fails?
A failed clearance test means some aspect of remediation is incomplete. This is the value of independent testing, a contractor who also does their own clearance testing has an obvious conflict of interest.
If a post-remediation sample fails:
- The remediator reviews the result and the affected area, identifying whether the elevated count is from a specific zone or distributed throughout the building
- Additional targeted treatment is applied to the area identified as the source
- Re-testing is performed after re-treatment, typically after another drying period
- The process repeats until clearance is achieved
Most reputable remediation contractors in the Illawarra will include a re-treatment guarantee in their scope, if post-remediation testing fails, they will return and retreat at no additional cost. Confirm this before work begins.
FAQs
Who should do the post-remediation testing?
Ideally, the clearance testing should be performed by an independent assessor, someone who was not involved in the remediation work. This avoids the conflict of interest of a remediator assessing their own work. Ask your remediator if they use an independent assessor or if the testing is conducted in-house.
How long after remediation should testing be done?
Testing should be done after containment has been removed and the treated area has had adequate air exchange with the rest of the building, typically 24-48 hours after remediation is complete. Testing immediately after remediation, while the area is still sealed, does not reflect normal occupancy conditions.
Do I need post-remediation testing for a small bathroom job?
For a minor single-room job in an otherwise clean building, formal clearance testing may not be necessary, a professional visual inspection and moisture check may be sufficient. For jobs involving suspected systemic contamination, insurance claims, or properties with vulnerable occupants, formal air sampling is recommended. Contact us to discuss your specific situation.
How much does post-remediation clearance testing cost?
A basic clearance test (2-3 indoor air samples plus an outdoor control) through an accredited laboratory typically costs $250 to $500 for the testing, plus the assessor’s time for sampling and report writing. For larger jobs requiring multiple locations, allow $500 to $1,000 for a comprehensive clearance assessment.