Name That Mould

Moulds are micro fungi which can grow on building materials. About 200 species are of concern to us in indoor situations.

When a building is suspected of having mould, the client or the building professional and investigation team collect samples and bring them to a laboratory for analysis. Confirming the growth of mould and its extent is essential to decide on the remediation steps necessary, and it may help us to understand the health issues present. The types and nature of mould growth may also allow the investigator to judge the age of the water intrusion responsible for the mould growth.

Whether the samples are bulk, surface or air samples, they represent only a snapshot or window of the area sampled at a particular point in time and space. This factor is important to note, as results of the analysis only relate to the sample.

Bulk samples will be common household mwaterials such as drywall board, wallpaper, wood, insulation material, carpet etc. Most bulk samples will have visible signs of mould growth such as some discoloration, a patch or uneven texture. In general a 5 cm2 sample is recommended for the analyst to make a good assessment.

Surface samples are considered as non-destructive methods and include tape-lifts and swabs. The tape-lift is a very easy and convenient method. The technique involves the use of clear cello tape (5-6 cm long) to lift the mould from its substrate by gently tapping on the tape and placing it back on a piece of wax paper. Swabs are ideal for wet and uneven surfaces.

Air samples are the most popular methods for indoor air quality assessments. These fall under two main categories: viable ones target moulds that are still alive and will grow on specific agar media, and non-viable ones assess all the mould particles in air.

In both, a certain volume of air is impacted on a surface for a given time. The impacted surface is an agar medium for viable sampling (Andersen, N6, RCS) or an adhesive on glass slide within a cassette for non-viable sampling (Air-O-Cell, AllergencoD, Micro5).

While technological progress has been made in the analytical fields, especially at the molecular level, the microscope has remained the tool par excellence in the analysis. New DNA-based technologies are making slow inroads but their costs are prohibitive and they cannot always be applied. Research in this field is under-funded and the long awaited barcode technology to identify moulds in building samples is looming far in the horizon. Under these conditions, Aspergillus, Cladosporium, Chaetomium, Penicillium, Stachybotrys and consorts will for some time more be studied through the microscope, much to the delight of the mycologist.

Direct microscope examination and culturing

Two techniques are used to analyse the samples: direct microscope examination (DME) and culturing. Bulk and surface samples are most commonly analysed by DME. The sample is scanned under a stereomicroscope and slide preparations are made to identify the mould to the genus level and to assess the extent of mould growth.

Non-viable spore trap air samples are obtained by impacting airborne particles in a volume of air on to a sticky area of a glass slide inside a cassette.

Mould spores are identified to the genus or assigned to groups based on their morphological features such as size, shape, surface texture and colour. The concentration of each type or group is calculated from direct counts made from the sample.

Viable air samples are analysed by the culturing method. Colonies recovered from the sample after 5-7 days’ incubation are counted and a representative of each group is then isolated on two different agar media to allow a definitive identification. This technique allows the analyst to identify and quantify the moulds recovered from the sample to the genus after 5-7 days, or to the species after 10-14 days.

Both DME and culturing answer the two main questions from an analytical point of view: identification and quantification of moulds. However, they both have benefits and limitations. DME can be performed within a couple of hours, but the identification can be done only to the genus or group level and growth assessment is subjective. In contrast, culturing allows for the identification to the species level and the determination of concentrations based on real counts. The drawback is that it takes longer to get the results, and only 10-20% of moulds present in air are recovered by this method.

Selecting the right sampling and testing methods is therefore important, and most building professionals will need guidance. The type of sample and the analytical method requested are linked; bulk and tape-lifts are indicated for confirming/identifying suspected mould by DME in quick turn around times. DME is the standard method used by investigators in the assessment of suspect mould growth in occupied commercial and residential buildings where the immediate turnaround is necessary to make timely decisions on occupancy and remediation measures.

Non-viable spore traps are recommended as clearance samples after a remediation has been carried out, or for identifying some hidden mould problems.

Viable air sampling is essential in hospital environments and in long term health care facilities where the risk of fungal infection is of most concern. Other circumstances might warrant a combination of sample types and analytical methods. As far as laboratories are concerned, accredited ones will meet the required ISO standards and are monitored by relevant institutions such as the American Industrial Hygiene Association (AIHA).

Each sample submitted to a lab for analysis should be well documented, labelled for later reference, adequately packed and accompanied by a chain of custody. Samples intended for culturing work and wet samples should reach the lab within 24 hours and be shipped under cool conditions.

Rafic Dulymamode, Ph. D., is Director of the Pinchin Environmental Microbiology Laboratory in Mississauga, Ontario.