The science of mould: how it grows, spreads and comes back

Mould is a fungus. Not a plant and not an animal, but a separate kind of life that feeds by breaking down the material it sits on. It begins as a spore, a single cell so small that thousands would fit across a full stop. When a spore lands somewhere damp it swells and pushes out a fine thread called a hypha. Those threads branch and weave into a network called mycelium, which is the living body of the mould. The stain or fuzzy patch you notice on a wall is mostly this mycelium, along with the fresh spores it has started to release back into the air.

If any of these terms are new, the plain-language mould glossary defines them one by one, so the rest of this page reads easily whether or not you have met the words before.

A dormant spore is patient. It can sit on a wall or rest in household dust for months and do nothing at all. To wake up and grow it needs four things at the same time: moisture, a food source, a workable temperature and time. Take away any one of them and growth stalls. Food is almost never the limiting factor indoors, because mould feeds on ordinary house materials such as plasterboard, timber, paint, dust, cardboard, the paper facing on gyprock, and even the traces left by a hand on a wall. Temperature is rarely limiting either, since mould is comfortable across much the same range that a home usually sits at.

That leaves moisture as the one variable you can genuinely manage, which is why the pillar guide to how mould works and why it comes back keeps returning to water. Time matters too: a surface that dries within a day or two rarely grows anything, while a surface that stays damp for a week gives spores all the opening they need.

It helps to picture those four factors as a chain. Break any single link and growth cannot continue, which is the whole basis of practical mould control. Since you cannot realistically strip a home of food sources or hold it at an uncomfortable temperature, the two links you can actually break are moisture and time. Dry a surface quickly and keep it dry, and even a wall thick with resting spores stays clean.

Mould does not need a puddle. It reads the moisture in the air and in the surface itself. Relative humidity is the share of moisture the air is holding compared with the most it could hold at that temperature. Once the air against a surface sits above roughly 70 to 80 per cent relative humidity for long enough, many common moulds can grow with no liquid water present at all. Warm air holds more moisture than cold air, so when humid indoor air meets a cold surface, a window, an external wall, or the back of a wardrobe, it cools and can no longer carry what it held.

The temperature at which the air lets go of that moisture is the dew point, and the water it sheds onto the cold surface is condensation. This is the everyday mechanism behind most bedroom and bathroom mould, and it is set out in detail in the guide to condensation, dew point and cold surfaces. It also explains why mould so often starts in the coldest corner of a room rather than the middle of a warm wall.

Surfaces carry their own measure too, called water activity, which describes how much of the water in a material is actually available to a fungus rather than locked away. Different species have different thresholds. Some need a genuinely wet surface, while others get going on material that only feels faintly damp to the touch. This is why a room can pass a quick hand check and still grow mould where the surface stays cool and slow to dry.

Air movement feeds into all of this. Still, trapped air in a closed wardrobe, behind heavy furniture or in a rarely opened room lets humidity build against a cold surface and stay there. Moving air evens out temperature and carries damp away before it can settle, which is why the mustiest spots in a home are so often the ones with the least airflow.

Here is the part most people miss: the spores are already in the room. They drift in on the air, on clothing, on the dog, and through every open door and window, then settle into the dust across every home in the country. You cannot remove them from the environment, and there would be little point trying, because more arrive the moment you finish. The way they move and settle is covered in the guide to how mould spores spread through your home.

So wiping a patch of mould only ever addresses what is visible today. If the surface is still cold and damp, the resident spores simply start to grow again and the stain returns in the same spot, often within weeks. This is the single most common reason a wall is cleaned and the mould reappears: the surface was treated and the moisture was not. No treatment keeps mould away on its own, because the spores never leave and the conditions keep inviting them back. Honest mould control is about removing the invitation, not chasing a stain.

That damp, earthy, cupboard smell is not the spores themselves. As mould feeds and grows it releases microbial volatile organic compounds, a group of gases usually shortened to MVOCs. Two of the most recognisable are geosmin, which carries the earthy note people link with turned soil, and 1-octen-3-ol, which reads as the classic mushroomy, musty edge. Because these are gases, you often notice a mould problem before you can see it, and the smell can travel from a wall cavity or a subfloor you cannot reach. The full picture is in the guide to what causes the musty mould smell.

The smell can also linger after a surface looks clean, because these compounds soak into porous materials and keep releasing slowly. A faint musty note in a room that appears spotless usually means growth is still active somewhere out of sight, or that a material stayed damp long enough to hold the odour after the visible mould was removed.

Some plant oils are naturally antifungal, which is really the plant's own defence against moulds put to work in the home. The active molecules matter more than the oil names. Eugenol, the main compound in clove, and citral, drawn from litsea cubeba, both act on the fungal cell membrane. That membrane depends on a fungus-specific building block called ergosterol, and these compounds interfere with it, breaking down the structure the cell relies on to hold itself together. Because the compounds are volatile they also work as a vapour, reaching into the porous surface and the surrounding air rather than only the spot you wipe. The chemistry is unpacked further in the guide to how essential oils work on mould.

Only a handful of groups account for most of what grows on Australian walls, ceilings and window frames:

• Cladosporium is the olive-green to brown speckling you often see along window seals and in bathrooms, and it tolerates cooler, drier conditions than most.
• Aspergillus shows up as powdery green, grey or yellowish patches on damp plaster and stored goods, and it is one of the most common moulds in house dust.
• Penicillium forms blue-green to grey growth on damp materials and food, spreads quickly, and shares many of the same indoor conditions.

What people loosely call black mould is usually a darker, slimier growth that favours surfaces which have stayed wet for a long time, such as chronically damp plasterboard or grout. The name gets applied to several dark species, so it describes colour more than a precise identification, as the guide to what black mould actually is explains.

For a homeowner, the exact name usually matters less than the pattern: where it grows, what colour it is, and how long the surface has stayed wet. Those clues point back to the moisture behind the growth far more reliably than the species does, and the moisture is the part you can act on.

Everything on this page comes back to a single idea: mould is a moisture problem wearing a stain. Manage the water in the air, the temperature of your surfaces and the time they stay damp, and you take away the opening. Treat only what you can see and the conditions quietly rebuild it. That is why the real work sits in ventilation, warmth on cold surfaces and drying, with surface treatment as the finishing step rather than the whole plan.

For the surface itself, First Light is a six-oil blend made in Australia, built on the mechanism described above. Lab tested: 99.2% reduction in black mould on direct contact, measured by an independent lab, with the full method set out in the independent lab results.

From here you can see the blend and how it is used across the range, or follow any of the guides above to go deeper on the part of the science you need.