The best strategy to determine if building occupants are exposed to hazardous mould is to take air samples. It is important to note that even in rooms with visible mould growth air sampling may give very low spore counts. Two methods are widely used in sampling air for mould.
- Impacting air on some growth media. This method is used when one is interested in determining the concentration of viable mould spores/fragments in the air.
- Impacting air on some inert sticky surface. In this case the mould spores and other particulate are directly counted under a microscope regardless of whether the spores are viable or not.
Both methods have limitations. Therefore, whether to use the first, second or both methods depends on the type of data required, which in turn depends on the objective of the investigation.
The problem of using a single type of media is that some molds may not grow well (or may not grow at all) in the selected media. Hence, although such molds may be the dominant contaminants in the air, they may end up being missed or underestimated. The solution, therefore, is to use more than one type of mold sampling media or select one that is known to support a wide range of environmental molds. A good example is Malt Extract Agar (MEA). The problem with this media is that it also supports the growth of bacteria to some extent. 
If the environment sampled is contaminated with bacteria, the bacteria grow faster than molds and interfere with mold growth. This problem can be overcome by incorporating a suitable antibiotic or other suitable compounds (e.g., Rose Bengal) into MEA to suppress bacterial growth. Rose Bengal not only suppresses the growth of bacteria but also restricts the spread of fast growing molds thus making it easy for colony counting.
Direct microscopy allows identification of the dominant contaminant (at least to genus level) regardless of whether the mold is dead or cannot grow on media used.
After plating onto DG18 and MEA and incubation (see petridishes marked “B”), Stachybotrys appeared on MEA but not on DG18. These observations clearly indicate how wrong conclusions can be made if the right type of media is not used either in air sampling or culturing of bulk samples.
The mold training course started as a half-day course covering topics such as:
Viable air samples are often collected on agar media either in strips (if using Reuter Centrifugal Sampler) or in Petri-dishes for Andersen sampler. Unlike non-viable air sampling, detection and subsequent enumeration and identification of airborne fungal particulates collected on growth media depends on whether the spores and hyphal fragments are viable and whether the media used can support their growth into colonies. For this reason, colony counts are usually lower than spore counts. Even if all the fungal structures were viable, colony counts are likely to be lower than the spore/hyphal fragment counts because what is counted as a single colony could have developed from more than a single spore or hyphal fragment. In one study it was found that the ratios between the total fungal spores collected by the Burkard sampler and the viable fungi collected by the Andersen sampler ranged between 0.29 and 7.61.
Is Non-viable Fungal Air Sampling Alone Adequate? In most cases viable air sampling is only used in situations where identification of the moulds to species level is required. However, our observation in the lab seems to suggest use of spore traps alone may not be adequate for airborne fungal sampling. On many occasions we have recovered moulds in viable samples that were not observed in non-viable samples even when viable and non-viable samples were taken side by side. For example Chaetomium and Stachybotrys spores, which are fairly easy to identify from spore traps have appeared in viable samples, yet, they were not detected from the non-viable samples. We have also observed that although non-viable sampling gives higher counts than viable sampling in most cases, this is not always the case. There are many factors that can contribute to these “unexpected” results.
Since both non-viable and viable air sampling have limitations, using either method singly is not adequate. To obtain conclusive information on the level of contamination and the diversity of airborne fungi in a building, taking both viable and non-viable air samples is preferable. We recommend the Calgary Health Region’s protocol, “Fungal Air Testing, Investigation and Reporting Requirements for Residential Marihuana Grow Operations (Revised May 2006)”. With few exceptions, the protocol requires that fungal air sampling consist of both viable samples (e.g. RCS or similar) and non-viable samples (e.g., Air-O-Cell) taken side by side.