UBA Mould Guideline 2024: Sampling and Assessment in Practice

The mould guideline of the German Environment Agency (Umweltbundesamt, UBA) is the reference in Germany for detecting and assessing mould in buildings. Courts, insurers and clients ask for it – so anyone taking air samples should know what the guideline actually says. An updated edition has been available since April 2024.

This article condenses its requirements for measurement strategy, air sampling, outdoor reference, assessment and documentation. It ends with an honest comparison of what the AirTrap system covers – and what it does not.

In short: the 2024 edition changes nothing in the measurement chapters; the update concerns chapter 6.1 on usage classes. Air samples serve to find hidden sources, not to estimate exposure. Assessment is by comparison with the outdoor air and by spore type, using the assessment aids in Table 9 (culture) and Table 10 (total spores). There are no limit values.

What the 2024 edition changes compared with 2017

The “Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden” (guideline on the prevention, detection and remediation of mould in buildings) appeared in December 2017 and replaced the guidelines of 2002 and 2005. The UBA’s Indoor Air Hygiene Commission (IRK) updated it in 2024; the current version is dated April 2024.

According to the UBA, the update concerns chapter 6.1 “Nutzungsklassen” (usage classes):

• Definition of a room added. A room is a self-contained part of a building formed by floor, ceiling and walls. The recommendations explicitly extend to all adjoining building structures, including cavities that are permanently or temporarily connected to the room air or from which harmful substances can reach the room.
• Usage class IV deleted. The former class IV for cavities and building elements has been removed without replacement. What remains is class I (special hygiene requirements), class II (regularly used rooms with their ancillary rooms) and class III (cellars, garages, stairwells and other rooms outside the dwelling that are not in permanent use).

The UBA gives the reason for both changes as preventing misinterpretation and making the guideline’s hygiene-based, preventive approach clearer: cavities connected to the room air fall under the recommendations for the adjoining room – not under a separate, milder class of their own.

For measurement practice this means chapter 5 – measurement strategy, assessment tables and the reference values in Annexes 7 and 8 – is unchanged; the 2024 edition should still be the one cited. One detail on the side: for remediation control the guideline still refers to WTA guideline 4-12-16/D; the current edition is 4-12-21/D (May 2021).

Measurement strategy: the question first, then the method

The guideline is explicit (our translation): “There is no sampling and detection method for moulds and bacteria that is applicable to every question.” Before any further investigation the objective has to be defined and a matching investigation strategy presented. The strategy itself is described in DIN EN ISO 16000-19, which grew out of the earlier VDI 4300 Part 10.

Where visible growth has a known cause, further measurements are usually unnecessary – the area is remediated. And: air samples serve above all as an indication of hidden growth; a quantitative estimate of exposure is not possible with them.

Which method fits which question follows from chapter 5.1.2:

• Is the discolouration mould? Tape lift and microscopy; mycelium and spore-bearing structures prove growth.
• How deep and how far does the growth reach? Material sample to DIN ISO 16000-21: culture (CFU/g) plus direct microscopy, compared with background values – this separates growth from contamination.
• Is there a hidden source? Air sample compared with outdoor air or a reference room – as a total spore count to DIN ISO 16000-20 and/or culturable to DIN ISO 16000-16 to -18.
• Which species, and is it viable? Only culture delivers species and CFU; it misses poorly culturable moisture indicators and dead spores, though.
• Was the cleaning after strip-out successful? Total spore measurement after mobilising settled dust, to WTA guideline 4-12 – see Clearance testing to WTA 4-12.

Infobox 12 of the guideline ranks the methods: air measurements to DIN ISO 16000-16 to -18 and -20 and material samples to -21 are generally accepted rules of technology; direct microscopy with tape lifts and MVOC measurements count as state of the art. Not recommended are contact plates on building materials, settle plates, house-dust analysis and total bacteria in air. The difference between the two air methods is explained in Total spore count vs. culture.

Air sampling: conditions, volume, outdoor air, number of samples

The guideline’s assessment aids apply to air samples taken, in its words, under use or use-like conditions in normal living rooms without dust agitation. That is the most important boundary condition: whoever ventilates, vacuums or stirs up dust before sampling measures something other than what the guideline assesses.

Preparing the room

• Windows and doors closed for several hours, usually overnight; ventilate normally before that.
• No cleaning and no vacuuming immediately before the measurement; after building work, damp-clean beforehand.
• Potted plants, organic waste and other known spore sources temporarily out of the room – note it in the record.
• Device in the middle of the room, about 1 m from the walls, inlet at 1 to 1.5 m height; keep still during the measurement.

Volume and repeat measurements

The guideline sets no volume; its footnotes assume 100 or 200 litres per sample. Because spores are unevenly distributed, it recommends multiple measurements, for instance two volumes in duplicate. The smaller the volume, the higher the detection limit.

Outdoor reference and reference room

Whether a source is indoors can only be said by comparison. The outdoor air is sampled close in time to the indoor measurement – same method, same volume, near the building, not next to compost or organic-waste bins. Where the outdoor air is not a usable reference, for instance in buildings with mechanical ventilation, unaffected rooms in the same building serve as reference rooms.

The guideline warns against a purely numerical comparison: in summer the spore-rich outdoor air masks indoor sources; in winter and with snow it is at times extremely low in spores – which is why the reference values in Annexes 7 and 8 belong in the assessment.

How many samples?

• At least one sample per suspect room; two in the rooms that matter most.
• At least one outdoor sample per measurement day, more when the weather changes.
• If possible, an unaffected reference room in the same building. For a three-room flat that is usually five to six samples.

The practical procedure with pump and cassette is described in Indoor air measurement.

Assessment: indoor-outdoor comparison and moisture indicators

Three ranges instead of limit values

There are no health-based limit or guide values for moulds in indoor air, and the guideline does not expect any in the near future either. Its sentence on the matter (our translation): “The aim of the measurements is therefore not a quantitative estimate of exposure but the detection of mould sources indoors.” Assessment works in three ranges: background level, a transition range with a possible indoor source, and concentrations that point to an indoor source with high probability.

Moisture indicators and dispersal

Two criteria carry the assessment: the difference between indoor and outdoor air per spore type – and the type itself. Moisture indicators listed in Table 2 of the guideline, among them Acremonium, Aspergillus versicolor, Chaetomium, Phialophora, Scopulariopsis, Stachybotrys chartarum and Trichoderma, hardly occur in outdoor air and point to moisture damage even in small numbers.

Dispersal matters too. Penicillium and Aspergillus form many small, dry spores and raise the airborne concentration even with small damage. Stachybotrys, Chaetomium or Acremonium release their spores poorly into the air – a few spores in the room can mean extensive damage.

Table 10 for the total spore count

For the total spore count, Table 10 of the guideline summarises the assessment aid (difference indoor air minus outdoor air in spores/m³, simplified):

The guideline attaches three conditions:

• The rows are not stand-alone criteria; they are to be considered together.
• Very low concentrations are statistically uncertain; a slide can only be evaluated quantitatively from ten spores. Single spores of a moisture indicator can still be a first indication.
• Applying the tables, in the guideline’s words, requires a high level of expertise – in late autumn, for instance, settled outdoor spores can simulate an indoor load.

Outdoor types such as Cladosporium, basidiospores and ascospores serve only as a plausibility check of the sample’s origin; for culturable fungi, Table 9 applies, in CFU/m³. How this logic shows up in the lab report is explained in Understanding your mould lab report.

Documentation: inspection record, sampling record, expert report

The guideline prescribes no format but demands clarity and traceability. Three documents belong together:

• Site inspection record. Location, age and construction of the building, fittings, ventilation and heating, use, number of occupants, heating and ventilation habits, odours; visible growth, previous moisture problems, water damage, building work; possible sources such as organic waste, potted plants, animals, humidifiers, and compost or nurseries nearby. Plus the building-physics readings (temperature, air, material and surface moisture) and photographs.
• Sampling record. All data that allow an unambiguous assignment of the samples: sample number, room, position, height, device, flow rate, duration, volume, time, indoor and outdoor conditions.
• Expert report. To chapter 5.3, with a fixed structure: reason for the investigation, task and aim of the measurements, site visit and findings, measurement strategy and methods, sampling records, results, assessment with its basis, summary with recommendations, annexes (measurement records, lab findings, photographs).

The guideline draws two clear lines. Health diagnoses for individual occupants are the doctor’s job, not the technical expert’s; blanket statements about toxic or infectious effects of the fungi found do not belong in the report. Statements on the urgency and scope of a remediation by damage category and usage class are permitted.

For quality assurance the guideline recommends laboratories accredited to DIN EN ISO/IEC 17025, or with comparable quality assurance, that take part in proficiency tests. What the lab does with the sample is described in Lab analysis.

AirTrap XL and Sporecyte measured against the guideline

The AirTrap system uses the total spore count: the AirTrap XL cassette deposits spores by slit impaction on a coated slide – the principle of DIN ISO 16000-20 and thus, per Infobox 12, a generally accepted rule of technology. The standard protocol is 15 l/min for 5 minutes, i.e. 75 litres per sample. Analysis takes place in the ISO/IEC 17025-accredited Sporecyte laboratory; every AI-assisted result is checked by a specialist before release.

What matches the guideline

• The outdoor comparison is mandatory. The report compares every indoor sample with the outdoor reference and rates the elevation; without an outdoor sample there is no interpretation. That is the logic of Table 10.
• Dead and non-culturable spores are captured. The guideline names exactly this as the method’s strength – for Stachybotrys chartarum, for instance – and as a tool for remediation control, especially after biocide use.
• Fast. The report arrives within 48 hours of the samples reaching the lab.

What the system does not do

• No culture. There are no CFU/m³ and no identification to species level. Table 9 therefore does not apply, and moisture indicators that can only be told apart at species level (such as Aspergillus versicolor) appear as the Aspergillus/Penicillium type. Where species or viability is decisive, add a culture-based air sample to DIN ISO 16000-17/-18.
• Genus or type level. The report gives spores/m³ per genus or spore type – the same way Table 10 groups them. The system does not do material samples to DIN ISO 16000-21; for surfaces there are tape lifts and swabs.
• 75 litres instead of 100 or 200. The standard volume is smaller than the one in the guideline’s footnotes, so the detection limit is correspondingly higher. In decisive rooms a second cassette is therefore worth it.
• The assessment stays with you. The report is a basis for assessment, not an expert report. Inspection, building physics and the assessment to chapter 5.2 are yours.

The pump and calibration kit are under Pumps & accessories.

Frequently asked questions

Is there a limit value for mould spores in indoor air?
No. The guideline gives no health-based limit or guide values. Tables 9 and 10 help with the question of an indoor source, not with the question of a health risk.

Is the total spore count enough, or do I also need culture?
For the question of a hidden source and for remediation control, the total spore count is a suitable method according to the guideline. Where species, viability or specific health questions (such as Aspergillus fumigatus) are at stake, culture is added.

Can I assess without an outdoor sample?
Usually not. The guideline requires comparison with the outdoor air or a reference room; it names remediation control as an exception.

The outdoor air is almost spore-free in winter – what then?
Then indoor-outdoor differences quickly look conspicuous, even without a source. Bring in the winter values from Annex 8 and a reference room, and assess the spore types above all, not the total.

Usage class IV has been deleted – do cavities no longer count?
The opposite. The new definition of a room explicitly includes adjoining structures and cavities where they are connected to the room air. The recommendations for the room they adjoin apply to them.

Indoor air measurement   Pumps & accessories

Written by the AirTrap team. Sources: Umweltbundesamt (ed.), “Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden”, prepared by the Indoor Air Hygiene Commission, December 2017, updated edition April 2024 (chapters 5.1–5.3 and 6.1, Annexes 7 and 8); Bundesgesundheitsblatt 2024, “Aktualisierung zu: Leitfaden … (2017) – Kapitel 6.1 ‚Nutzungsklassen‘”; DIN EN ISO 16000-19; DIN ISO 16000-20; WTA guideline 4-12-21/D (May 2021). This article is practical guidance and does not replace the assessment of a competent person.