Clearance Testing After Mould Remediation: The WTA 4-12 Procedure

The remediation is finished, the fine cleaning is done, the dust screen is still up. Now comes the question the client, the property manager and the insurer will ask at the end: is the mould really gone? The eye cannot answer that. A clearance test provides the evidence – air samples from the remediated area, compared with the outdoor air and with an unaffected reference room.

This guide walks through when a clearance test makes sense, what state the room has to be in, how to take the samples, how the results are assessed and what belongs in the record. It follows the German WTA guideline 4-12 “Objectives and control of mould remediation in indoor spaces” (2021 edition) and the mould guideline of the German Environment Agency (UBA).

In short: a clearance test is the documented verification at the end of a remediation – visual inspection, moisture readings and air samples. It takes place after the fine cleaning, while the containment is still in place. Assessment is comparative: indoor air against outdoor air and against an unaffected room. There is no legal limit value.

Why a clearance test?

The remediation objective is met when the growth has been removed, the cause of the moisture has been fixed and the area can be used again without restriction. Whether that has been achieved cannot be seen on a remediated wall: spores are microscopic, and dead spores and fungal fragments are allergenic too. WTA 4-12 therefore describes a verification at the end of every remediation – for small damage a visual inspection is usually enough; for medium and large damage, measurements are part of it.

For contractors and site managers the clearance test has three practical functions:

• Acceptance. A lab report with an outdoor comparison is an objective basis for the acceptance of the works and the final invoice – instead of a discussion about the smell in the room.
• Liability. If complaints arrive months later, the documented clearance test shows the condition at handover. Without a measurement it is one word against another.
• Evidence. Insurers, tenants, property managers and, if it comes to it, courts can follow a measurement record with a lab report. They cannot assess a photograph of a repainted wall.

A clearance test matters most where sensitive people use the room – children, older people, allergy sufferers, people with a weakened immune system – and in nurseries, schools, medical practices and care homes.

Timing and preconditions

The most common cause of a conspicuous clearance test is not residual growth but dust. So before sampling, check that the area is actually ready to be measured:

• Remediation complete: affected materials have been removed or properly treated, and the cause of the moisture has been fixed.
• Fine cleaning done: every surface in the work area has been HEPA-vacuumed and damp-wiped – including pipes, window sills, door frames and the containment itself. Debris and waste are out.
• Building elements dry: moisture readings on the remediated elements are unremarkable and documented. Drying equipment is off.
• Air scrubbers and negative pressure off: running HEPA units distort the result. Switch them off a few hours before the measurement and note the time.
• Room condition: windows and doors have been closed for several hours, nobody is working, and nothing has been cleaned immediately beforehand. The room is in the state in which it will later be used.
• Containment still up: measure before the dust screen comes down. If the clearance test is conspicuous, the area stays contained and re-cleaning is easy.

Visual inspection and moisture readings first, air samples second: if growth or dust is still visible on the surfaces, or the building elements are still damp, the measurement is a waste of money – the result is predictable.

Method: air samples, outdoor air, surfaces

Which method?

For clearance testing the total spore count to DIN ISO 16000-20 has proven itself: a defined volume of air is drawn through a sampling cassette, the particles impact on a coated slide, and the lab counts and identifies the spores under the microscope. All spores are captured – including dead ones, which culture methods do not show. The result is available within days rather than after a week of incubation. Culture methods (viable sampling to DIN ISO 16000-17/-18) are useful when viable spores and identification to species level matter – for the question “did the remediation work?”, the total spore count is usually the more direct answer.

Outdoor reference

Without an outdoor sample no assessment is possible. Outdoor air always carries spores, and its composition changes with the season and the weather. So take at least one outdoor sample on every measurement day – same method, same duration, close to the building, protected from rain and not right next to compost or piles of leaves.

Reference room

If possible, take an additional sample in an unaffected room of the same building. It shows what the “normal” indoor air in the house looks like and makes the assessment of the remediated area considerably more robust.

Number and position of samples

• At least one air sample per remediated room; more for large rooms or several work areas.
• Sample in the middle of the room at breathing height (about 1 to 1.5 m) – not right at the remediated surface, not at the window, not in a draught.
• Some measurement concepts add a sample under agitation (with a fan, for instance) to capture settled dust. If you do this, document it – otherwise the values are not comparable.
• Surface samples (tape lift, contact plate or swab) complement the air sample when individual remediated surfaces or the quality of the fine cleaning are in question. They show whether growth structures or a lot of settled spore dust remain on the surface.

The basics of the measurement strategy – room conditions, number of samples, comparison samples – are described in DIN ISO 16000-19. More on the method: Indoor air measurement.

Assessment to WTA 4-12 and the UBA guideline

Germany has no legal limit value for mould spores in indoor air. Assessment is comparative – WTA 4-12 and the UBA guideline follow the same logic:

• Indoor against outdoor. After a successful remediation the spore spectrum in the room essentially matches that of the outdoor air, and the concentrations are not clearly above it. A room with markedly more spores than outside has a source inside – or is not clean.
• Indicator species. The type matters more than the total. Genera that grow on damp building materials and are rare in outdoor air – Stachybotrys, Chaetomium or clearly elevated Aspergillus/Penicillium types, for instance – point to residual growth or carried-over spore dust even at low counts. Common outdoor genera such as Cladosporium at a similar level to outside are unremarkable.
• Comparison with the reference room. The remediated room should not be clearly above the unaffected room – neither in concentration nor in spectrum.
• Surfaces. No growth structures (hyphae, mycelium) may remain detectable on remediated surfaces; individual settled spores are normal.
• Plausibility. Mind the season and the weather: in winter or with snow the outdoor air carries few spores, so indoor-to-outdoor ratios quickly look high. A freshly mown lawn outside the window changes the reference too.

The assessment itself belongs in the hands of a competent person. For larger damage, in public buildings or where a dispute is foreseeable, verification by an independent expert is advisable. How to read the numbers in the lab report is explained in Understanding your mould lab report.

Documentation: what belongs in the record

A clearance test is only worth as much as its documentation. Every measurement record should contain:

• Property, client, date, time, name of the person sampling
• Remediated rooms and surfaces; brief description of the damage and the remediation measures
• Result of the visual inspection and the moisture readings with measurement points
• Room conditions: windows and doors closed since, temperature, humidity, air scrubbers and drying switched off since
• Outdoor conditions: weather, temperature, anything unusual (rain, snow, mowing)
• Per sample: sample number, room, position and height, device, flow rate, duration, air volume, time
• Photographs: overview of the rooms, remediated surfaces, sampling points with the pump set up
• Lab report with concentrations per genus, outdoor comparison and images from the slide
• Assessment, conclusion and signature

The record, lab report and photographs become part of the handover documents. File them so you can still find them years later – complaints rarely come in the first week.

In practice: clearance testing with AirTrap XL and Sporecyte

This is how a clearance test runs with the AirTrap system – from set-up to report:

1. Prepare. Document the visual inspection and moisture readings, switch off air scrubbers and drying, close windows and doors, take photographs.
2. Sample. Set the Sporecyte pump on a tripod or a raised surface in the middle of the room, insert the AirTrap XL cassette, remove the caps, start the timer: 15 l/min for 5 minutes, i.e. 75 litres per sample. Then close the caps and label the cassette. One sample per remediated room, plus outdoor air and, if possible, a reference room. Sample surfaces with AirTrap swabs where needed.
3. Send. Register the samples in the online return form – with room and sampling point per cassette – and post them to the lab in the prepaid return envelope from the pack.
4. Receive the report. The slides are analysed in the ISO/IEC 17025-accredited Sporecyte laboratory. The report arrives by e-mail within 48 hours of the samples reaching the lab: spores/m³ per genus, every indoor sample compared with the outdoor reference, an elevation rating and images from your slide.
5. Assess and hand over. Combine the report with your record, enter the assessment, remove the containment, hand over the area.

Allow around ten minutes per room for sampling including set-up; transit to the lab usually takes a few days. Every cassette includes the lab analysis, so the cost of a clearance test is known before the appointment. For firms that test regularly, the refill packs with 24 cassettes are the economical option; the pump and calibration kit are under Pumps & accessories. The whole process is on How it works.

Frequently asked questions about clearance testing

What if the clearance test is conspicuous?
Find the cause first, then fix it. In most cases it is dust from the works: repeat the fine cleaning – especially on pipes, lintels and in corners – and measure again after a few hours of rest. If the samples show indicator species, check for hidden residual growth (behind skirting, in cavities, under floor coverings) and for moisture. Carry-over from neighbouring rooms or an unfavourable outdoor reference also happens. The containment stays up until the repeat measurement is unremarkable.

Who may carry out a clearance test?
A competent person from the remediation firm can take the samples, provided the procedure is documented; the analysis is done in the lab. For evidence towards third parties, or where a dispute is foreseeable, an independent expert should measure and assess.

Is a visual inspection enough?
For small, superficial damage usually yes – the UBA guideline takes the same view. For medium and large damage, growth in cavities or sensitive occupants, air samples should be added.

Are there limit values?
No. Assessment is always by comparison with the outdoor air and with unaffected rooms, and by the genera found.

How many samples do I need?
At least one air sample per remediated room, one outdoor sample per measurement day and, where possible, a reference room. A pack of eight cassettes therefore covers a typical flat remediation with up to six rooms.

How long until the result?
Sampling five minutes per sample, transit a few days, report within 48 hours of arrival at the lab.

See refill packs   Pumps & accessories

Written by the AirTrap team. Sources: WTA guideline 4-12-21/D “Ziele und Kontrolle von Schimmelpilzschadensanierungen in Innenräumen” (May 2021), UBA guideline on the prevention, detection and remediation of mould in buildings (2017, updated 2024), DIN ISO 16000-20. This article is practical guidance and does not replace the assessment of a competent person.