Mould and ventilation management

Sampling mould

Why sample mould?

To log the intensity of surface mould colonisation – by culturing spores on a growing medium, or by measuring the 'biomass'.
To measure mould intensity in a room, a cavity or void – by extracting air and measuring CFUs (colony forming units) through culturing viable spores. Microbiological air sampling systems are used.
To confirm mould is present.
To confirm the type of mould present – genus/species.
To confirm the amount of mould contamination in the room air – usually by culture of viable spores in a given volume of air obtained by an air sampler.
In the context of a legal claim, when presence of mould could be linked to illness in occupants.
In the context of building performance management.
In the context of property management – when mould could present a health and safety risk.
To check if cleaning has been successful – by sampling before and after a cleaning operation.

The sampling has 2 aims:

to identify mould species or at least genus, or the mould family; and
to assess the amount of mould on sample areas, each the size of the sampling template, i.e. 30 X 30mm.

In cases where mould has been cleaned, substrates can be further tested to make sure mould is no longer present. Sampling can also be carried out over the following months and years to monitor effectiveness of remedies by confirming that mould is at least no longer significantly present on room surfaces.

There are sophisticated techniques to sample the air in the room, or air even in an enclosed duct, carried out by experienced mycologists and specialist remediation companies. Holes can even be made in voids so air may be sucked out for sampling.

It is also common to sample dust, which can contain mould fragments and spores among other unwanted organic and inorganic matter.

When air is sampled you can culture all the viable mould spores, yeasts and bacteria in it, and derive a total spore count broken down into mould and bacteria colonies. Within the sampler are plates of agar on which, after a few days, the viable cells and spores form colonies. Spores, yeast or bacteria cells that have colonised the agar test plates of the air sampler are called colony forming units (CFUs), and the number of CFUs can be expressed by area or volume, e.g. X CFUs per m2 or m3. An air sampler will suck in a known amount of air and you can therefore calculate the number of CFUs per m3 of the air sampled. Spores in the air can also be determined by just leaving a petri dish of agar in the mouldy room for a set time and culturing any mould spores that have settled on the medium.

There are several ways of determining the amount of mould on a surface.

Photograph an area and compare the intensity of mould per area to another area – divide surfaces into low, medium or high mould density.
Press a pad of agar onto a surface and culture the moulds sampled. The petri dish agar will have a known area, so the number of colonies can be counted and computed to CFU per m2.

There are many issues that cause confusion and differences of opinion among expert mycologists. Some suggest that just assessing CFUs misses the point. It is not just the viable spores in the air that matter. Some moulds do not produce many spores or indeed any; some moulds produce toxins, and dead mould can be a health risk.

In the test example that follows, a sample of mould was swabbed from a defined area, within a template, and instead of the mould being cultured to determine CFUs per m2, the biomass of hyphae, spores and fragments, etc. is given a measurement.

How many CFUs can be in indoor air for humans to still be safe from their potentially harmful effects? Jorgen Bech-Andersen advises that levels of 3,000 CFU per m3 for moulds and 5,000 CFU per m3 for bacteria are 'realistic', using an RCS air sampler. For surface samples he advises that 'a maximum number of 10 colonies per petri dish (25cm2)' is allowed, using malt extract agar after incubation at 28°C for 4–5 days. He states clearly that Stachybotrys chartarum and Aspergillus versicolor should not occur at all in an air sample due to their known production of mycotoxins, and that other moulds will also fall into this category once their health impact is confirmed.

There are as yet no official guidelines, standards or regulations for fungal or bacterial limits. According to Dr Jagjit Singh, the World Health Organisation (WHO) suggests that:

the presence of certain fungal pathogens such as Aspergillus fumigatus and certain toxigenic fungi such as Stachybotrus atra should be considered unacceptable;
more than 50 colony-forming units (CFU) per m3 of fungi from indoor sources should prompt investigation if there is only 1 species present;
up to 150 CFU per m3 of fungi from indoor sources should be considered acceptable if there is a mixture of species; and
up to 500 CFU per m3 of fungi from indoor sources should be considered acceptable if the species present are primarily Cladosporium or other common phylloplane fungi – higher counts should be investigated to ensure there is no indoor source.

There is a lack of conformity here, when one mould expert's perception of mould risk seems at odds with the WHO limit on acceptable levels. The apparently significant difference of opinion in how much mould is likely to be a health riskmight lead us to err on the side of caution and accept the WHO guideline as the most sensible trigger level to use.

Six easy steps to mould sampling

How to take an informal sample

For an informal sampling, take a cotton bud, moisten it with a few drops of distilled water and take a swab of mould. The (usually blackened) cotton bud can then be placed in a brand new sealable bag (e.g. a freezer bag) or you could snip the bud in half and seal the sample in a sterile container. Alternatively, you could tear off a section of mouldy wall or ceiling paper and carefully encapsulate this sample in a resealable bag. Label the sample with a self-adhesive label and note sample location, etc. Careful record keeping is absolutely essential. Noting full details of the sample location, date, name of property, etc., post it to an analytical laboratory as soon as possible after taking the sample.

When posting off samples for analysis, keep duplicate samples (in case samples are lost in transit, or problems arise in their administration and logging in a lab). Samples can also be accidentally spoiled. In some legal contexts it may be necessary to obtain test results from more than 1 laboratory.

A more informative report on a mould problem

Step 1: A mould swab sampling kit is not expensive, and can help you produce a more informative report on a mould problem. Some moulds are known to be particularly harmful to health, so you should consider carrying out sampling – especially if occupiers are sensitive to mould.

Step 2: Peel off the backing on the paper template and place the adhesive sampling template on the area to be tested. Write a number or letter on the frame to log the sample. Make sure you log the exact position sampled.

Step 3: The cotton swab is twisted and pulled out of its plastic tube holder, and then dipped into a sterile solution. Ideally, wear surgical gloves for the sampling, to prevent cross-contamination.

Step 4: The swab is used to clean off and collect the mould within the template area.

Step 5: The swab, now blackened by mould, is pushed into the carriage tube. Fill in the details of the sampling – your company, date, sample location and address, etc. – on the tube’s identification label.

Take a sample where the mould is most concentrated, and another towards the fringes of the heavily contaminated area. Also take a control sample away from the visibly contaminated zone.

Specialist advice may be needed to sample, assess and remediate mould contamination. This will depend on the circumstances of each individual case. Consider the extent, or possible extent of the mould and its likely effect on the building and building users.

Step 6: The sample is carefully parcelled up and posted to a testing laboratory for analysis.

Mould cleaning in pictures

Figure 1: A surveyor checking mould. A non-penetrative moisture meter helps locate the epicentre of dampness, as part of the diagnostic process. The surveyor is wearing a protective mask. He should also check behind the skirting board – remember that much mould in a property lingers unseen.

Figure 2: A second surveyor (wearing a respirator) looks in the ceiling void, where mould is expected. Taking off the light diffuser allows a convenient access position for a borescope – perhaps even through an existing hole. Once refitted there will be no sign whatsoever of the investigation. Experts are able to suck air from such a ceiling void for sampling. Mould was discovered using the borescope.

Figure 3: Wearing protective gloves, a template is applied to a mouldy surface for taking a swab sample.

Figure 4: A notice is placed at the perimeter of the mould zone to warn building users to keep clear during the mould-cleaning process.

Figure 5: Professional mould remediation equipment. To the left, a professional heavy-duty vacuum cleaner with a HEPA (high efficiency particulate air) filter. To the right are 2 steam cleaning machines, powered from 380V and 240V supplies respectively.

Figure 6: Standard tap water is poured into the machine to produce a reservoir of steam. The operative wears protective gloves, overalls and respirator. Skirting boards have already been removed. The machine heats up the water to 6 bar pressure (typical house water pressure may be 3 bars).

Figure 7: Steam jets to the surface, to kill mould, which the operative scrubs and loosens.

Figure 8: Working towards the room corner using a lance to power steam onto the mouldy surface.

Figure 9: A cotton or microfibre cloth swabs mould off the surface.

Figure 10: A lancette in this case cleans mould along the top of a retained skirting board (specialist cleaners usually request that skirting boards be removed).

Figure 11: Brush head loosens mould and cleans the surface.

Figure 12: The mould has gone and the air is now fresh and safe, so the surveyor taking a post-clean-up mould sample has no need for personal protective equipment (PPE).

Figure 13: Swabs of mould are tested for their biomass. Biomass includes fungal fragments, spores and hyphae. The amount of biomass present on the sampled area will be reflected in a mycometer reading. Post-cleaning, a low reading should be logged.

The testing is now complete, results will form part of the final clean-up report.