Mould glossary: mould terms explained

Spore

A microscopic seed-like cell that mould releases to reproduce. Spores drift on indoor air and stay dormant until they land somewhere damp with something to feed on.

Hyphae

The fine, thread-like filaments a mould grows once a spore germinates. They spread through and across a surface as the colony feeds.

Mycelium

The network of hyphae that makes up the visible body of a mould colony.

Germination

The moment a dormant spore starts to grow, which happens when it has enough moisture, a food source and a workable temperature.

Substrate

The material a mould grows on and feeds from, such as plaster, timber, grout, dust or the sizing in fabric.

Biofilm

A film of microbial growth that establishes in and on a surface, such as inside old shower silicone, where wiping the top does not reach it.

Water activity

A measure, written as aw, of how much moisture in a material is available for mould to use. Most household moulds need a surface water activity above roughly 0.80 to grow.

Relative humidity

The amount of water vapour in the air compared with the most it could hold at that temperature. Higher relative humidity means damper air and more surface moisture.

Surface relative humidity

The relative humidity right at a surface, which is what actually governs mould growth. It climbs as a surface gets colder than the room air.

Dew point

The temperature at which air becomes so saturated that water condenses out of it. When a surface falls below the dew point, condensation forms on it.

Condensation

Liquid water that forms when warm, moist air meets a surface cold enough to reach its dew point. It is the most common source of the moisture mould needs indoors.

Thermal bridge

A part of a building, often a metal window frame or a concrete edge, that conducts heat out quickly and runs colder than the surfaces around it, so it condenses first.

Ventilation

Moving air through a space to carry moisture out. Good ventilation lowers the surface moisture mould needs and is one of the few levers a renter fully controls.

Wet season

The northern Australian rainy season, roughly November to April, when high humidity and heavy rain drive the tropical mould peak.

MVOC

Microbial volatile organic compound. The gases mould gives off as it grows, and the source of the musty smell. Common examples are geosmin and 1-octen-3-ol.

Geosmin

An MVOC with a strong earthy odour that humans can detect at extremely low concentrations. It is a large part of the classic musty mould smell.

1-octen-3-ol

An MVOC often described as a mushroom or fungal smell, released by active mould growth and part of the musty signature indoors.

Ergosterol

The main sterol in a fungal cell membrane, the fungal counterpart of cholesterol. It is absent from human cells, which is why it is the target of most antifungal agents.

Eugenol

The dominant compound in clove oil. In laboratory studies it disrupts the fungal cell membrane and raises its permeability, which is the basis of clove oil's activity against mould.

Citral

The active pair of compounds in Litsea cubeba and lemongrass oils. In laboratory studies it interferes with the fungus building ergosterol for its membrane.

Terpene

A class of aromatic, fat-soluble plant compounds, many of which show antifungal activity in the laboratory by partitioning into and disrupting the fungal membrane.

Vapour phase

The airborne, gaseous form of an essential-oil compound. Because the active terpenes are volatile, they can act on mould through the air, not only on direct contact.

Aspergillus

A very common indoor mould genus, including the black-spored Aspergillus niger. It grows on a wide range of damp materials.

Cladosporium

One of the most common household moulds worldwide, often olive-green to brown to black, found on damp walls, window frames and fabrics.

Stachybotrys

A dark, slimy mould, Stachybotrys chartarum, that needs very wet conditions and is the species people usually mean by the loose term toxic black mould.

Penicillium

A common indoor mould, usually blue-green, that grows on damp building materials and water-damaged goods.

Black mould

An everyday term, not a single species. Most dark household mould is actually Cladosporium or Aspergillus; colour alone is unreliable, and only a laboratory can identify the species.

Mildew

A common word for early-stage or flat, powdery surface mould. It is not a separate organism from mould in the everyday sense.

Remediation

The professional process of fixing a mould problem: finding and correcting the moisture source, containing the area, and physically removing the mould.

Source removal

The governing principle of professional remediation. The mould material is physically taken out, because killing it in place still leaves it behind.

IICRC S520

The widely used industry standard for professional mould remediation, which sets out moisture correction, containment and physical removal.

HEPA

High-efficiency particulate air filtration, used in remediation vacuums and air scrubbers to capture fine mould spores from the air.

ULV fogging

Ultra-low-volume fogging, which shears a solution into a fine mist. In remediation it is an optional step used after moisture correction and physical removal, not a replacement for them.

Containment

Sealing off a work area during remediation so disturbed spores do not spread to the rest of the building.