Mould sampling is often bought in the expectation that it will produce a verdict. It rarely does. Sampling produces a measurement of a defined thing at a defined moment, and its usefulness depends almost entirely on whether the question was framed before the sample was taken. This page sets out the main sampling methods, what each one physically collects, how laboratories analyse them, and why a raw spore count on its own answers considerably less than people expect it to.
The first discipline in sampling is deciding what result would change the decision. If visible growth has already been found, a sample confirming that it is fungal changes nothing: the growth is removed because it is present. If a musty odour is reported and no source has been found, a comparative air sampling exercise may indicate whether a concealed source exists, which does change what happens next. If a remediation has been completed, a defined sampling comparison forms part of the evidence that it worked, which changes whether the area is released.
Sampling without a question produces numbers that then have to be interpreted retrospectively, and retrospective interpretation is where most misreading occurs. It is also where a result is most likely to be used to justify a conclusion that was already preferred.
The second discipline is deciding the comparison in advance. Almost no fungal measurement is interpretable in isolation. The comparisons that carry weight are indoor against simultaneous outdoor, affected area against unaffected area in the same building, and post-remediation against pre-remediation in the same location using the same method. Those comparisons must be designed into the sampling plan, not assembled afterwards from whatever happens to be available.
Spore trap sampling draws a measured volume of air through a narrow slit and impacts particles onto an adhesive-coated surface, which is analysed under a microscope. It captures both viable and non-viable material, which matters because dead spores and hyphal fragments remain allergenic and remain evidence of past or present growth. Results are reported as spores per cubic metre and grouped by spore type — the analyst identifies what morphology allows, which is usually to genus or to a group of genera with indistinguishable spores rather than to species.
Culturable air sampling uses an impactor to deposit particles directly onto a nutrient agar plate, which is then incubated. Only organisms that are alive and willing to grow on that particular medium at that particular temperature will form colonies, so the result is reported as colony forming units per cubic metre. This method allows identification to species for many organisms, because a grown colony offers far more morphological detail than a single spore, but it systematically under-represents the total fungal burden. Some genera grow readily and dominate a plate; others are slow, fastidious or simply do not survive impaction.
Neither method is superior in the abstract. Spore trap sampling gives a better picture of total particle burden and gives it within a day or two. Culturable sampling gives species-level identification and produces the CFU/m³ units used by the published Dubai Municipality figures. Where a project is being compared against those figures, culturable sampling by an appropriate impaction method is the technique that produces a comparable number; a spore trap result is not the same measurement and should not be compared against a CFU value.
Volume and duration matter. A sample volume too small for the environment produces a result dominated by chance; a volume too large overloads the collection surface and makes the deposit uncountable. In an occupied space, whether occupants were moving during collection changes the result substantially, because settled spores are readily re-entrained by activity. A defensible sampling record states the volume, the flow rate, the duration, the height, the location, and what was happening in the room.
A tape lift presses a clear adhesive strip against a surface, lifting whatever is on it for direct microscopic examination. It is the fastest and least expensive way to answer the most frequently asked question in a mould investigation: is this discolouration actually fungal growth, or is it soot, mineral staining, dust ghosting or an adhesive failure? The analyst can also distinguish growth in situ — hyphae attached to and penetrating the substrate, with structures characteristic of active colonisation — from spores that have merely settled from the air, and that distinction is often the whole point of the exercise.
Swab sampling wipes a defined area with a moistened swab, which can then be cultured or analysed directly. It reaches into textures and crevices that tape cannot follow and is useful on irregular surfaces, but the collection efficiency is variable and quantitative comparison between swabs should be treated cautiously.
Surface sampling is qualitative or semi-quantitative in nature. It establishes what is present on a surface. It does not establish how much is in the air, how far the contamination extends behind the surface, or whether the material can be salvaged. Those are separate questions answered by air sampling, by opening up and by material assessment respectively.
Bulk sampling submits a physical piece of the material itself — a section of plasterboard, a fragment of insulation, a piece of carpet underlay, a plug of settled dust. The laboratory can then examine the material in cross-section and report not only what is present but whether growth has penetrated the substrate or is confined to its surface, which is directly relevant to whether the material can be cleaned or must be removed.
Bulk sampling of settled dust is sometimes used as a longer-term indicator, on the reasoning that dust integrates deposition over weeks rather than capturing a few minutes of air. That is a genuine advantage, but the trade-off is that dust also integrates historic events, cleaning regimes and the effect of foot traffic, and it cannot be tied to a moment in time. It is a poor choice for post-remediation verification for exactly that reason.
Bulk samples are destructive and should be taken in a way that does not itself spread contamination. Cutting or breaking a colonised material releases spores, so collection is performed with appropriate control, particularly in an occupied building.
Direct microscopy of a spore trap or tape lift involves counting and characterising particles across a defined portion of the deposit and extrapolating to the whole. Precision is limited by the counting statistics: at low concentrations a handful of spores on a slide translates to a wide confidence interval on the reported value, and a difference between two low results may not be a real difference at all. Analysts also apply detection limits, and a result reported as "none detected" means none were found in the fraction examined, not that none were present.
Culture involves incubating plates for a defined period at a defined temperature and identifying colonies by macroscopic and microscopic characteristics, sometimes with additional media to resolve difficult genera. The incubation conditions bias the outcome: a medium and temperature chosen for common indoor genera will under-recover organisms adapted to other conditions.
DNA-based approaches, including quantitative PCR against panels of specific organisms, detect target sequences whether the organism is alive or dead. They are precise for the organisms on the panel and blind to everything else, and their results are not interchangeable with either spore counts or colony counts.
In all cases the laboratory should be accredited for the method it is performing, the chain of custody should be documented, and the report should state the method, the analysed fraction, the detection limit and the reporting units. A result without its method is not a result.
Fungal spores are a normal component of outdoor air, and indoor air in a naturally or mechanically ventilated building necessarily reflects what is outdoors. A concentration indoors that appears high in isolation may simply be tracking a high outdoor background, and a concentration that appears low may still indicate an indoor source if the outdoor background at that moment was very low.
The comparison is therefore made on two axes. The first is concentration: indoor total concentration is compared against the simultaneous outdoor concentration, with an expectation that a building without an indoor source will sit at or below outdoors. The second, and the more informative of the two, is profile: the relative proportions of the spore types found indoors are compared against those found outdoors. Where the indoor mixture broadly mirrors the outdoor mixture, the indoor air is most likely reflecting infiltration. Where a genus is markedly more prominent indoors than outdoors, that is the signature of an indoor amplification source and it is a more robust finding than any absolute number.
For the comparison to hold, the outdoor sample must be collected on the same day, with the same method, at the same flow rate and volume, at a location representative of the air entering the building and away from an obvious local source such as landscaping waste or standing water. An outdoor sample taken a week earlier, or by a different method, does not support the comparison.
Filtration complicates the picture in a useful way. A building with effective filtration and a positive pressure regime should sit clearly below outdoors. A result at parity with outdoors in such a building is worth a second look, because it suggests either that the filtration is not performing or that something indoors is contributing.
Airborne spore concentration is not a stable property of a room. It varies by orders of magnitude within a single day depending on whether people are moving, whether doors have been opened, whether the air conditioning is running, whether cleaning has just taken place, and what the outdoor concentration is doing. A sample of ten minutes characterises those ten minutes.
There is also no established dose–response relationship that allows an individual's health outcome to be predicted from a count. The published literature associates dampness and mould in buildings with increased respiratory symptoms and asthma exacerbation, but it does not provide a threshold below which a person is safe or above which a person will become unwell. A number cannot therefore be used to reassure an occupant or to alarm one, and it should not be presented as if it could.
Nor does a count define extent or locate a source. A high indoor result tells nobody where the growth is; a low result does not exclude substantial growth inside a sealed cavity that is not communicating with the room air at that moment. Concealed growth behind a sealed lining can produce entirely unremarkable air results right up until the lining is opened.
The practical consequence is that a count belongs inside an argument rather than at the end of one. Combined with an inspection, a moisture map, an outdoor comparison and a profile analysis, it contributes. On its own, printed on a certificate with a threshold beside it, it invites a conclusion that the measurement cannot support.
There is no enforceable UAE exposure limit for mould, and no published Abu Dhabi reference value for it either. Abu Dhabi's Occupational Standards and Guideline Values document (Version 3.0, 2016) carries the statement on the Abu Dhabi Public Health Centre's own page that "This document is currently suspended. Entities are to comply with relevant local or federal standards in force.", and its Schedule A is a chemical-agent table containing no fungal, mould or bioaerosol value.
The published local figures come from Dubai Municipality's Technical Guidelines for Indoor Air Quality for Healthy Life (Doc Ref DM-HSD-GU119-IAQ, Version 4, issue date 11 December 2024). Table 1, for new buildings, gives total fungal counts of 500 CFU/m³ and total bacterial counts of 500 CFU/m³. Table 2, for existing buildings, gives total fungal counts of 500 CFU/m³ and total bacterial counts of 1,000 CFU/m³. The bacterial count is the only difference between the two tables for these parameters; the fungal figure is the same in both. Table 3, for thermal comfort in all new and existing buildings, gives relative humidity of 20–60%, with the qualification "Never to exceed 56%RH at 80F (27C)", and dry bulb temperature of 22.5–25.5 °C. The guideline states these microbiological limits without specifying a sampled air volume or a sampling duration for them — the Average Time column is left blank for total bacterial and total fungal counts, whereas every chemical and particulate limit in the same table carries one. Section 9-8-7 sets only a general sampling period, that measurements should be made on an 8-hour basis except where otherwise specified with an accepted surrogate of four averaged half-hour measurements, and does not assign that basis to culture-based sampling; no sampler type, flow rate, culture medium or incubation regime is given. The guideline requires only that testing be carried out by a laboratory accredited under the Emirates International Accreditation Centre (EIAC) scheme on calibrated equipment. A CFU/m³ figure is therefore not comparable between two surveys unless the reporting laboratory states the air volume it sampled and over what period.
These are the values Dubai Municipality's indoor air quality guideline sets for buildings that opt into its indoor air quality certificate. The guideline describes itself as guidance rather than as a binding standard, and the certificate route is expressly optional. They are a published reference point within a voluntary Dubai Municipality scheme, not an enforceable UAE limit.
Because those figures are expressed in colony forming units per cubic metre, a comparison against them requires culturable sampling by an appropriate method. A spore trap result in spores per cubic metre measures a different quantity and cannot be placed alongside a CFU figure as though the two were equivalent. Method documents from AIHA, ACGIH and the ISO 16000 series, together with the World Health Organization's 2009 guidance on dampness and mould, are properly described as recognised practice; they inform how sampling is designed and reported and they create no duty in the United Arab Emirates.
Impacts a measured air volume onto an adhesive surface for direct microscopy. Counts viable and non-viable spores. Reported in spores per cubic metre, usually grouped by genus.
Impacts air onto nutrient agar for incubation. Reported in colony forming units per cubic metre. Allows species identification but under-recovers organisms that do not grow on the chosen medium.
Adhesive strip applied to a surface for direct microscopy. Answers whether a discolouration is fungal and whether growth is established in situ rather than settled from the air.
Moistened swab over a defined area, cultured or examined directly. Reaches irregular surfaces that tape cannot follow, with variable and hard-to-quantify collection efficiency.
A physical piece of the material submitted for examination, showing whether growth has penetrated the substrate or sits only on its surface.
A simultaneous sample taken by the same method, at the same flow rate and volume, representative of air entering the building. Without it, an indoor result cannot be interpreted.
There is no enforceable UAE exposure limit for mould, and no published Abu Dhabi reference value for it either. Dubai Municipality's Technical Guidelines for Indoor Air Quality for Healthy Life (Doc Ref DM-HSD-GU119-IAQ, Version 4, 11 December 2024) gives total fungal counts of 500 CFU/m³ for new buildings (Table 1) and 500 CFU/m³ for existing buildings (Table 2), with total bacterial counts of 500 CFU/m³ (new) and 1,000 CFU/m³ (existing). Comparison against those figures requires culturable sampling reported in colony forming units; a spore trap result in spores per cubic metre measures a different quantity. The guideline states these microbiological limits without specifying a sampled air volume or a sampling duration for them — the Average Time column is left blank for total bacterial and total fungal counts, whereas every chemical and particulate limit in the same table carries one. Section 9-8-7 sets only a general sampling period, that measurements should be made on an 8-hour basis except where otherwise specified with an accepted surrogate of four averaged half-hour measurements, and does not assign that basis to culture-based sampling; no sampler type, flow rate, culture medium or incubation regime is given. The guideline requires only that testing be carried out by a laboratory accredited under the Emirates International Accreditation Centre (EIAC) scheme on calibrated equipment. A CFU/m³ figure is therefore not comparable between two surveys unless the reporting laboratory states the air volume it sampled and over what period.
These are the values Dubai Municipality's indoor air quality guideline sets for buildings that opt into its indoor air quality certificate. The guideline describes itself as guidance rather than as a binding standard, and the certificate route is expressly optional. They are a published reference point within a voluntary Dubai Municipality scheme, not an enforceable UAE limit.
Usually not. Where growth is visible it is remediated because it is present, and a result confirming it is fungal changes nothing. Air sampling is most valuable where a source is suspected but not found, and as part of verifying that completed work succeeded.
Because indoor air reflects outdoor air. Outdoor spore concentrations change through the day and with the weather, so a reference collected on a different day or by a different method does not support the comparison the indoor result depends on.
Spores per cubic metre counts all collected spores, living or dead, under a microscope. Colony forming units per cubic metre counts only organisms that grew on a nutrient medium after incubation. They measure different things and the numbers are not interchangeable.
No. There is no established relationship between a spore or colony count and an individual's health outcome, so no count supports a statement about safety. Anyone with symptoms they associate with a building should be assessed by a doctor or an occupational health service.
Sometimes, and only indirectly. A markedly elevated indoor proportion of a genus compared with outdoors suggests an indoor amplification source. But growth sealed inside a cavity may not communicate with room air at all, so a normal air result does not exclude concealed growth.
It is a poor choice for that purpose. Dust integrates deposition over a long period, including everything that happened before the work, so it cannot demonstrate the condition of a space at the moment of clearance.