The science of mold in buildings

Mold spores are already in every building. They arrive on clothing, on pets, through open doors and through the ventilation system, and they are present in settled dust in houses with no moisture problem at all. Their presence is not the variable that changes. Water is.

What decides whether a spore germinates and forms a colony is whether the surface it lands on holds enough water that the organism can extract it. That quantity has a name in building science and in food science alike: water activity, written aw, measured from 0 to 1. It is not the same as the relative humidity of the room, and the difference is why a house can feel comfortable and still grow mold behind a wall.

This is the reason a risk assessment can be done at all without entering a property. Presence of mold cannot be inferred from outside a building. The conditions that permit mold — the climate load, the drainage, the construction era, the water events a property has been through — are recorded, and they are what a moisture risk model works from.

Building mycologists describe indoor colonisation in stages, and the staging is a moisture scale rather than a timeline. The organisms that can start first on a marginally damp surface are the ones that tolerate the least water. The organisms that show up on saturated drywall are the ones that need the most and cannot begin without it.

At the dry end sit the xerophiles — species that grow at water activities below 0.80, which for most fungi is simply too dry. Aspergillus penicillioides and Wallemia sebi live here, largely in settled dust and on dry goods, and their presence is a statement about dust and chronic mild damp rather than about a leak.

In the middle, around 0.78 to 0.90, sit species like Aspergillus versicolor: common on damp gypsum board, in carpet and mattress dust, and on walls with a persistent condensation problem.

At the wet end, above roughly 0.90, sit the cellulose decomposers and the late colonisers — Chaetomium globosum, then Stachybotrys chartarum, which needs sustained free water. These do not appear because a room was humid. They appear because something stayed wet.

Knowing which organism is growing tells you about the building. It does not, on its own, tell you about a person's health, and the two questions are routinely collapsed in consumer material in a way the evidence does not support.

A mycotoxin is a compound some fungi can produce under some conditions. Capability is not production: a species known to be capable of producing a toxin is not evidence that it produced any, in that building, in quantities anyone was exposed to. The major evidence reviews tie health effects to dampness itself more consistently than to any named organism.

So the useful reading of a species list is backwards from how it is usually sold. It is not a threat ranking. It is a moisture record, and the moisture is both the thing that caused the growth and the thing that has to be fixed.

A laboratory can identify what is present in a sample. That is a real capability and this pillar does not argue otherwise. What a sample cannot do is tell you where the water came from, how long it has been there, or whether the conditions that produced the growth are still in place.

A modelled estimate answers a different question — how likely it is that a property has conditions favourable to growth — and it answers it for an address rather than for a square inch of one wall. Neither replaces the other. Reading either as if it were the other is the most common expensive mistake in this subject.