It is well known that the outdoor level of fungal propagules is extremely variable over time, sometimes changing from minute to minute. There are also huge seasonal variations (in temperate climate) adding to the overall variability. Using spore traps it has been shown that even sampling side-by-side during the same time frame can give results that differ by orders of magnitude. A study presented at a recent AIHce conference showed that under the best weather conditions at least three outdoor samples were needed to have a statistically safe determination of the outdoor level of mould spores. Under the worst weather conditions hundreds of samples were necessary. This means that for practical purposes it is not realistic to be able to establish a statistically safe determination of the outdoor level and therefore not useful as a reference to the indoor air level. In addition, what is immediately present outdoors will not in that same instant be present indoors. The delay of impact from outdoors to indoors is also influenced by presence or absence of a mechanical ventilations system, building envelope tightness, air exchange rate with in the building, air filtration effectiveness and open windows/doors. A more useful control or reference sample would be one collected from an area in the building not suspected to be contaminated or having complaints.

Basically to get useful data that can be reproduced and give a much more reliable measure of potential risk of exposure in a room.

An average spherical fungal spore with a diameter of 3 µm settles at an approximate rate of 1 meter per hour in still air. If there has been no activity in a room for many hours prior to air sampling, almost all fungal particles will have settled on surfaces with very little left in the air. Passive sampling in this situation can significantly underestimate what is actually present in a room and which will be aerosolized with normally occurring activity. That is why it is not uncommon to have false negative results in rooms with visual mould growth when using passive sampling.

In a pilot experiment, passive air samples were taken in eight rooms early in the morning before the rooms were occupied and then again late in the afternoon after the rooms had been occupied. The results showed that the airborne fungal levels found in morning sampling were on average only 9% of that found in the afternoon, and in some cases as little as 1%. The same experiment was conducted using aggressive sampling. The results of aggressive sampling found the airborne fungal level early in the morning (where there had been no activity for many hours) on average did not differ from the values obtained in the afternoon where maximum activity had occurred. The significance of this experiment is that it demonstrated that using aggressive sampling eliminated the variability of occupant activity and provides a more reliabily predictive result for the presence of fungal material.

The idea is not to create absolutely the worst case scenario in a room but rather to mimic a high activity level that will sometimes occur, such as if a vacuum cleaner is in use, or a ventilation fan is turned on, or if children are running around. The Makita Blower is used as the standard instrument and has a maximum air flow of 43 l/s, which is close to that of common vacuum cleaners. The protocol prescribes that one blows on surfaces from a distance of approximately 2 meters (air speed 3,3 m/s), and avoid blowing into a radiator, or behind closets, or other places outside the normal cleaning areas. The particles that should be aerosolized, are the ones that are naturally aerosolized, when there is activity, as described above, in a room.