Fungal Contamination in Public Buildings: Health Effects and Investigation Methods

3. Investigation of Fungal Contamination of the Non-Industrial Workplace (Continued)

3.4.2 Culturable air samples

Air samplers collect fungal propagules either on agar media or in aqueous suspensions. Such samples provide information on culturable or "viable" propagules in air. It is important to consider that existing air-sampling techniques underestimate the true airborne concentrations of fungal spores for several reasons. The number of fungal propagules determined by culture are substantially less (by 1% to 50%) than those determined by direct methods; however, this varies between species. Different species of fungi have different growth requirements so the use of any medium produces different recoveries. The spores of fungi decline in viability with time; the spores of some species remain viable for years and for other species for months. Some species grow very fast or are aggressive in culture and produce antifungal agents that can affect the growth of other species present in culture. The variability of spore clouds in the air in a building with active mold growth is much larger than the precision of available sampling methods. Air sampling is useful for investigating large buildings for mold contamination and must be considered if the investigation was prompted by health complaints.

It is seldom possible to take enough samples to conduct rigorous statistical analyses, but statistical principles need to be considered when determining the number of samples to be taken (ACGIH 1999). Careful consideration must be given as to how and where each sample is to be taken.

Air samples should be taken during normal activity in the building, while the ventilation system is operational. Factors to consider include taking samples in a given space and allowing one or two hours between duplicates (e.g. go around each floor of the building in one direction, go up each level and then down, morning and afternoon, etc.). This technique takes into consideration the variability of airborne spore concentrations over time and with different activities, as well as varying thermal and wind loads. Air samples should not be taken when it is raining. Rain has a transient effect on the microbial populations in outdoor air that can result in a reduction of the sensitivity of the indoor-outdoor comparison. The number of outdoor air samples should in principle be equal to the num ber taken indoors. Since this is seldom practical, there needs to be at least between three and six samples taken outdoors during the period(s) when the indoor sampling is under way. These need to be taken above grade to avoid collecting windblown soil particles containing fungi which can affect the comparison of the indoor-outdoor diversity. It is recommended that outdoor air samples be collected as close to the air intake as possible or facing into the wind on the building roof. Other considerations can be found in the AIHA Field Guide (Dillon et al. 1996) and the ACGIH bioaerosols manual (ACGIH 1999).

The basis of the current methods for interpreting the results of air sampling is a comparison of the diversity of the fungi inside with outdoor air samples, taking into account indicator species and species with poor recoveries on agar media such as Stachybotrys chartarum (CEOH 1995a; Dillon et al. 1996; ACGIH 1999). There is a shifting array of fungal species in outdoor air as the season progresses. Average numbers of total propagules in July range from 20,000 per m³. to peak levels of twice that value. In the absence of snow cover, total Aspergillus/ Penicillium comprise <1% of the total fungal spores present in outdoor air. When there is snow cover, the total number of fungal spores decreases, and the proportion of Aspergillus/Penicillium therefore increases to 10% to 20%.

The advantage of properly collected and analyzed viable air samples is that the data can be used to detect signs of the early stages of a mold problem, as well as growths in wall cavities or ventilation ducts (where dilution by outside air limits the sensitivity of the analysis).

3.4.3 Sticky surface samplers

Sticky surface samplers such as Zefon Air-O-Cell^TM, Allergenco^TM and Burkhard^TM are increasingly used in IAG investigations. There is little published information on their comparative quantitative and qualitative performance (Dillon et al. 1996). However, some studies have provided information on the cut points of these samplers (Aizenberg et al. 2000). A limitation of these methods is the skill of the microscopist in counting fungal propagules in a field containing debris of various kinds.

Advantages of data from properly collected and analyzed sticky surface samples include the fact that the results are available within a day and in situations when there is a high percentage of non-viable spores in the air, the data are more reliable.