Where moulds grow

The simple answer is that if the micro environmental conditions are right mould can grow literally anywhere in the house and on almost any material. Moulds are ubiquitous in the environment and we are exposed to them on a daily bases. There are certain limiting factors or conditions that have to be met for mould spores to germinate. The basic factors limiting mould growths in building are:

Mould spores

Mould spread through gene production of unique reproductive structures call spores. These are small capsules or “seeds” of mould that are usually carried by the air currents. They range in size from about 2-3 microns (1cm = 10 000 micrometers) to about 40 microns.  Mould spores are present in outside air throughout the year and in recent study concentrations in the city centre at Cardiff have reached as high a 24-h mean as nearly 85 000 m−3 air. We inhale mould spore with every breath we take.

Nutrients

Almost any organic substance can provide nutrients for the mould growth. Materials such as  wood, paper, textiles, plaster, adhesives  and  all sorts of microscopic debris will provide all the required nutrients for mould to grow. Eliminating mould from homes by limiting the amount of nutrients available is impossible. Although the presence of sugars is not a requirement some moulds can quickly develop on high sugar content substrate. The present of high concentration of salts may not be limiting as long as there is sufficient amount of moisture available to counteract the desiccating effects of salts.

Temperature

Moulds can grow across the whole spectrum of temperatures we humans are able to tolerate. The very low temperatures such as those in the fringe only slow down mould growth but do not stop it. On the other hand high temperatures such as those in tropics even accelerate mould growth. Different moulds have evolved to prefer certain temperatures but given the fact that there are approximately 100 000 fungal species no matter what temperature in our home it will always attract some sort of mould.

Moisture

Availability of moisture or free water is possibly one of the most important factors limiting the mould growth in our homes. It is also the only one we have the ability and means to limit or reduce to a level that is not sufficient to sustain mould growth.  As always different moulds have evolved to tolerate or require different quantities of moisture but broadly speaking vast majority of mould require relative humidity to be around 70% for growth. Generally the indoor relative humidity in homes is much lower than 70% but under certain situation it can be consistently higher. Small microenvironment in our homes such as those in the corner of external walls or around the windows can have localised relative humidity well above the required minimum even though the ambient humidity in the building is significantly lower. The relationship between moisture and temperature is described in detail in section about condensation.

Light

Moulds as any fungi do not require light to grow but in same cases day light may be required in certain stages of the life circle. Amount of light may be a trigger factor for release of spores.

Aeration

All moulds require certain amount of air to grow. Metabolic activities of the mould mycelium can be radically decreased in oxygen poor atmospheres.

pH.

Moulds differ in their pH requirements. Ph range is an important factor but most mould will tolerate relatively wide range of Ph, sometimes between 3-7. Some such as Aspergillus niger and Penicillium funiculosum can grow at pH 2 and below.