Non-viable Air Sampling

Indoor air sampling for airborne fungi is frequently conducted to assess the levels of fungal contamination and subsequently the potential risk to building occupants. It is also used to determine if there was hidden mould growth in the building or to determine the effectiveness of remediation procedures. One of the most cited advantages of non-viable air sampling is that detection of fungal structures (spores, hyphal fragments, etc) is not dependent on their viability or the suitability of agar media. Non-viable air samples are collected with samplers such as Air-O-Cell, Allergenco, VersaTrap, Burkard, Cyclex, Cyclex-d and Micro-5 among others. The spores (whether viable or dead) and other particulates are trapped on the sticky surface of the spore trap and can then be directly enumerated and identified under a microscope. Since both viable and nonviable spores can be enumerated, an efficient non-viable air sampler is expected to give a better estimate of the level of airborne fungal contamination than a viable air sampler.

Viable Air Sampling

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.

Non-viable Air Sample

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.

Conclusion

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.

References

Adhikari A., Sen M.M., Gupta-Bhattacharya S., Chanda S. (2004). Airborne viable, non-viable, and allergenic fungi in a rural agricultural area of India: A 2-year study at five outdoor sampling stations. Science of the Total Environment, 326 (1-3), pp. 123-141.

Calgary Health Region (2006). “Fungal Air Testing, Investigation and Reporting Requirements for Residential Marihuana Grow Operations (Revised May 2006)”.