The potential for mould in buildings and homes is ubiquitous. This is because mould spores are found everywhere naturally. However, mould requires water in order to germinate and grow.

Not all moulds are harmful. In fact the potential for an adverse health reaction by occupants is primarily related to the total number of spores present. Conversely, all moulds present a risk if they are found at high enough concentrations.

Current thought and best practice is that levels or quantity of any mould species, including its spores  (the seed phase) and its “mycelium” (the matted growth phase), is a key indicator of risk. Both mycelia and spores tend to be visible, since mycelium is white and clearly appears to be spreading, while spores are black, green, blue, or a dark hued colour, and live on top of the mycelium, waiting for wind to spread them.

The practice of regarding the growing phase of mould as a risk represents common sense. We have all seen mould, and not all people are sensitive to it. So it’s when it’s growing that it becomes a risk.

The two stages of the growth cycle of any single mould species – mycelium and spores – have been seen as comprising a chicken or egg argument which has confounded any regulation of spore concentrations by the building and property industry. However, there is in fact almost never a high concentration of spores in a premises unless they have grown there from mycelium, so the argument is really moot. Either high spore counts in the air, or visible mould inside a premises, must be seen as problems.

So how much mould is too much for occupants? The cleanliness values of the particular culture we live in are an inappropriate basis for determining this. Rather, the industry has tended to simply regard any visible mould as a potential problem that needs to be nipped in the bud. The flip side of this admirable policy has been that any mould which isn’t visible is treated as if it’s not there. Clearly that’s not good practice.

So where does this leave us with mould spores in the air?  Are the spores a problem after the visible mould is gone and after the drying of the premises has been finished?

The Asthma and Allergy Foundation of America points out that each sensitive individual has their own trigger for allergies. Their specific trigger needs to be determined by a doctor. Mould is one of the triggers that are tested for, but only one, with others including dust mites, cockroaches, pollens, pet fur and rodents. Since mould is on the list, we would have to say it is a potential problem to leave spores in the air at any premises after the visible mould has been removed.

Is there a way to measure the quantity of spores in air? A large industry has built up around this idea. A sample volume of air is taken at a room in the premises, and the air in that sample is passed over either a collector or a detector inside the sampling machine. The various particles present in the sample are eventually counted, either with regard to size or shape, each of which is a reasonable indicator of origin and identity.

So what quantity of mould constitutes a problem at a premises? One way to determine this is to simply ask what quantity of particles is present at premises that have no visible mould, or premises that are near new or have recently been fully refurbished, or have been well cleaned and have no occupants displaying symptoms of allergies. The benefits of this approach are that the identity of the particles is not the focus, and instead only the number of particles is taken into account. This quantity is used as the baseline for testing other premises against.

Outdoor air contains particles too. These particles are a healthy challenge to the immune system, and include mould spores, pollen, minerals and many others.  Healthy or well-ventilated premises contain outdoor air which has been modified by the building engineer to have an agreeable temperature.

Practice in the particle quantification industry has therefore been to use the outdoor air as a guide to the desired indoor air particulate levels. While outdoor air as the control or baseline is fine, it assumes that all premises have good ventilation practice, and that all premises are designed for efficient heating and or cooling, which is not always the case.

The indoor environment in many parts of the world contains a separate air circulation, and so something other than the outdoor air needs to be used as a control or baseline. This is where the approach of using clean indoor premises in that geographic area as a baseline came from.

Particles in the size range 0.3 to 10 micrometres are generally used as the indicators of particulate matter in air. Two types of meter exist to measure these. One uses a light beam to detect passing particles going by the detector inside the machine. This type detects all particles at the assigned size and larger. The other type uses a system that results in particles being weighed, as a set, and this type detects all particles at the assigned size and smaller.  Regardless of which meter type is used, the measurement outcomes are similar.

Yet another approach is to use apparatus which collects particles of 0.3 to 20 micrometres from a volume of air onto an adhesive disc, a filter, or a gel. The apparatus then examines these under a microscope for identity, an approach which assumes that size in itself is a poor indicator of origin and identity.

It turns out that this latter method introduces a lot of uncertainties of its own, not the least being the skill of the operator. The operator optimally looks at shape and to some extent colour, both being subjective and unmeasured, since rather than being measured or imaged, these characteristics are simply “eyeballed” without sufficient records being kept.

After the dust has settled, metaphorically, on choice of technique, or when everything is done and dusted, one ends up with a number, and from it a level of air quality that merges all the data into a line of best fit. Amazingly, all air samples in the world are seen to fit a trend, where there are many tiny (or “fine”) particles and rather fewer of the larger (or “coarse”) particles. Gravity is responsible for the distribution among particles sizes that remain in air after hours and days, the larger particles settling quicker.

The jury is in, that it is the coarse particle fraction (2.5 to 10 micrometres) that contains mould spores and other allergenic particles.

Counting the number of particles in the coarse fraction is a challenge which the industry has stepped up to, thanks to the increasing use of the meters mentioned above. Data is now becoming available on how well these numbers correlate to incidences of visible mould, or to water intrusion history.

With hard facts at hand, it becomes possible to compare remediation techniques against each other. The aim is always to get a building back to the level of particulate indoor air quality it had when it was first built and occupied.

Image from image from media licdn