Understanding Different Types of Mould and What Your Testing Results Mean

When you receive a mould test report, it will name the mould genera found in your property, such as Aspergillus, Penicillium or Cladosporium. Laboratories identify mould to genus level from the shape and structure of the spores under the microscope, which is the standard for assessment reports. But what do these names mean? Each genus needs a different amount of moisture, has different health effects, and tells you something different about what is happening in your building. This guide explains the common types of mould found in New Zealand homes and workplaces, and what their presence tells you. For how to read the numbers and condition ratings on your report, what we see in New Zealand buildings, and what to do about a positive result, see the related guides at the end.

Why Mould Type Matters

Not all mould is equal. Different genera require different moisture levels to grow, produce different allergens and toxins, and indicate different underlying problems in a building. A report showing Cladosporium on a bathroom ceiling tells a very different story from one showing Stachybotrys behind a wall lining. The genera identified determine:

• The likely moisture source and severity of the underlying problem
• The health risk to occupants, particularly vulnerable individuals
• The appropriate remediation approach and urgency
• Whether the mould is actively growing or settled from elsewhere

One point many people miss: dead mould spores can still affect your health. The allergens and other compounds in a spore's wall stay active after the spore dies. That's why dry, apparently dead mould can still set off allergies and breathing symptoms when it is disturbed.

Mould and Building Materials

In nature, moulds break down dead plant material, and many modern building materials make an ideal food source. Engineered wood products such as MDF, plywood, OSB and chipboard are particularly vulnerable, because their processed wood fibres soak up moisture and are easy for mould to digest. The paper facing on plasterboard, timber framing, carpet backing and even some glues can all grow mould once they get damp.

That's why mould damage to porous building materials is permanent: the mould digests the material itself, and cleaning the surface can't undo that. The fungi that do the most damage to timber are a different group again, the wood-rot fungi, covered in our guide to wood rot in New Zealand buildings.

Water Activity: Why Different Moulds Grow in Different Conditions

The biggest factor in which moulds can grow on a surface is water activity (a_w): how much of the water in a material is free for mould to use, on a scale from 0 (completely dry) to 1.0 (pure water). Different moulds need different amounts, so the moulds named on your report tell you a lot about how damp the material has been.

Mycologists sort indoor moulds into three broad groups by how much water they need:

Xerophilic Fungi: Primary Colonisers (a_w below 0.80)

These moulds can grow on materials that don't even feel damp. Known as primary colonisers, they are the first to appear when moisture rises, and they can start growing within 48 to 72 hours. They are the most common indoor moulds worldwide.

Aspergillus conidiophore and spores under microscopy

Penicillium conidiophore under phase contrast microscopy

Chrysosporium spores on a textile fibre, a pattern we see in New Zealand samples after flooding

• Aspergillus — One of the moulds we identify most often indoors. Its "dry spores" are very small (under 5 μm), float easily and can be breathed deep into the lungs. Aspergillus fumigatus and A. flavus are the most common causes of serious mould infection worldwide, mainly in people with weakened immune systems. Some species produce aflatoxins, which are linked to liver cancer. A compound in its cell wall, (1→3)-β-D-glucan, stimulates the immune system, so even dead spores can cause inflammation with repeated exposure
• Penicillium — Just as common indoors, and often found with Aspergillus. It also produces small "dry spores" that float easily, often in large numbers, and it grows in soil, on damp building materials and on spoiled food. Some species produce mycophenolic acid, a strong immune suppressant that is also used as a medicine. It rarely causes infection, but breathing it in over a long period can contribute to hypersensitivity pneumonitis (HP), an inflammation of the lungs
• Wallemia — Less often reported, but distinctive: it thrives on very dry materials, such as stored goods and dust. Finding it can point to long-term, low-level humidity
• Chrysosporium — Copes with dry conditions and grows readily on organic materials such as dust, wool fibres, feathers and paper (it can break down keratin, the protein in wool and feathers). We find it more often in New Zealand samples than in Australian ones, probably because New Zealand household dust contains more organic material. We have also recorded it growing on the sarking paper of New Zealand homes after floods. That's unusual for a mould that normally prefers dry conditions, and a useful sign that organic materials have stayed wet for a long time

What this means for your report: if Aspergillus and Penicillium dominate indoors, especially at levels above those outdoors, humidity or condensation is likely to be the cause, even when there is no visible water damage. As primary colonisers, they are the early-warning moulds that can appear within days of a moisture problem.

Mesophilic Fungi: Secondary Colonisers (a_w 0.80–0.90)

These moulds need more moisture than the primary colonisers. Known as secondary colonisers, they appear once damp conditions have lasted a while, and they are often linked to on-and-off dampness, repeated condensation or moderate leaks.

Cladosporium sphaerospermum: spores budding in branching chains, the classic sign of this genus

Alternaria (Ulocladium) spores under microscopy

Scopulariopsis conidial chains under microscopy

• Cladosporium — One of the most common moulds in the world, indoors and out, found on plants, window frames, in bathrooms and in poorly ventilated spaces. Its small spores spread easily. It is generally lower risk than Aspergillus and only rarely causes infection. It is often the main mould in outdoor air, which makes it a useful reference point when comparing indoor and outdoor results
• Ulocladium (Alternaria) — A major allergy trigger, strongly linked to hay fever and to asthma flare-ups in children. Its spores are larger (10 to 40 μm) and mostly lodge in the nose and throat, although fragments can reach the lungs. Some species produce mycotoxins, including alternariol and tenuazonic acid. Growth indoors is a strong sign of water damage. In our lab work, Ulocladium is a stronger sign of water damage in Australian samples than in New Zealand ones, where Chrysosporium tends to play a bigger role on organic materials
• Scopulariopsis — Produces distinctive chains of lemon-shaped, thick-walled spores on brush-like structures. We find it comparatively rarely in New Zealand, where Aspergillus and Penicillium tend to occupy the same indoor niches, and it is easy to mistake for one of them unless the analyst looks closely at the spore shape and wall texture. We have recorded it where humidity stays above 75% and after indoor floods, most often as a fine, white, powdery growth on fabrics, which is where we most often confirm it. S. brevicaulis can cause nail infections (onychomycosis) and can infect people with weakened immune systems
• Bipolaris — Less often identified, and linked to moderate moisture. It can cause allergic fungal sinusitis in people who are sensitised to it

What this means for your report: secondary colonisers point to longer-lasting moisture, not just high humidity. The likely causes are a build-up of condensation, a slow leak or poor drainage. Ulocladium in particular is a reliable sign of water damage. Finding these moulds means the moisture problem has gone on long enough to move past the early-warning stage.

Hydrophilic Fungi: Tertiary Colonisers (a_w above 0.90)

These moulds need materials that are almost saturated, and stay that way. Known as tertiary colonisers, they only grow on materials that have been wet for a long time: Stachybotrys, for example, typically needs 7 to 12 days of constant wetness to take hold. Finding them on building materials strongly suggests long-term water damage, an active leak or water that keeps getting in, not just high humidity.

Stachybotrys chartarum (black mould) spores under microscopy

• Stachybotrys chartarum — The well-known "black mould". It needs cellulose-rich materials, such as plasterboard, timber and paper, that stay wet for weeks. It produces trichothecene mycotoxins, which interfere with how cells make proteins, DNA and RNA. Its spores are sticky and heavy, so it doesn't get into the air easily and often doesn't show up in air samples. That's why we recommend a surface sample from any visible growth if Stachybotrys is suspected (how to take a tape lift). Finding it always means serious, long-term water damage
• Chaetomium — Another tertiary coloniser, often found with Stachybotrys on badly water-damaged materials. It needs long-standing water. It produces mycotoxins (chaetoglobosins and chetomin) that have been linked to asthma and can affect the fine hairs (cilia) that clear the airways. A clear sign of major water damage

Chaetomium perithecia under microscopy

Acremonium under the microscope: the red arrows point to the spore-bearing structures used to identify it

• Fusarium — Linked to very wet conditions. It can cause infections (fusariosis) in people with weakened immune systems, and some species produce trichothecene mycotoxins
• Trichoderma — Needs high moisture. Its heavy "wet spores" rarely show up in air samples unless the material has dried out or been disturbed. It gives off microbial volatile organic compounds (MVOCs), which contribute to the musty smell of water-damaged buildings
• Acremonium — A slow-growing tertiary coloniser, often found in constantly wet areas such as showers and kitchens. It rarely becomes airborne. Finding it points to long-standing moisture problems that haven't been dealt with

What this means for your report: tertiary colonisers point to serious, long-lasting moisture. They take days to weeks to establish, so the water problem isn't recent or minor. If Stachybotrys or Chaetomium are found, the building has had significant water damage, and both professional remediation and fixing the source should be a priority.

Beyond identification

Knowing which mould is growing is only the first step. For the full picture, you also need to understand the numbers and condition ratings on your report, the patterns we see in New Zealand buildings, and what to do (and what not to do) about a positive result. The guides below cover each of these.