When you receive a mould testing report, the results will identify specific mould genera found in your property — groups such as Aspergillus, Penicillium, or Cladosporium. Laboratory microscopy identifies mould to genus level based on spore morphology and structural characteristics, which is the standard for assessment reporting. But what do these names actually mean? Each genus has different moisture requirements, different health implications, and tells a different story about what is happening in your building. This guide explains the most common mould genera found in New Zealand homes and workplaces, why they grow, and how to interpret what your laboratory results are telling you.
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
An important point many people miss: mould spores can be viable (living) or non-viable (dead) — and both can cause adverse health effects. The allergenic proteins and immune-stimulating compounds in mould cell walls remain active regardless of whether the spore is alive. This is why even dry, apparently inactive mould can still trigger respiratory symptoms and allergic reactions when disturbed.
Mould and Building Materials
Moulds evolved to break down organic matter in nature, and many modern building materials provide ideal food sources. MDF, plywood, OSB, and chipboard are particularly susceptible — their low lignin content, high cellulose composition, and permeable structure make them easily digestible. Plasterboard paper facing, timber framing, carpet backing, and even some adhesives can all support mould colonisation once moisture is present.
This is why mould damage to building materials is permanent — the fungal enzymes break down the substrate itself. Cleaning the surface does not reverse structural degradation that has already occurred in porous materials.
Water Activity: Why Different Moulds Grow in Different Conditions
The single most important factor determining which mould genera can grow on a surface is water activity (aw) — a measure of the free water within a substrate that an organism can use to support growth, measured as a fraction from 0 (completely dry) to 1.0 (pure water). Different mould genera have evolved to exploit different moisture niches, which is why the genera identified in your report are a direct indicator of the moisture conditions in your property.
Mould scientists classify fungi into three broad groups based on their water requirements:
Xerophilic Fungi — Primary Colonisers (aw below 0.80)
These moulds can colonise materials at relatively low moisture levels — even conditions that might not feel damp to the touch. Known as primary colonisers, they are the first to appear when moisture rises and can establish growth within 48 to 72 hours of conditions becoming favourable. They are the most common indoor moulds worldwide.
Aspergillus conidiophore and spores under microscopy
Penicillium conidiophore under phase contrast microscopy
- Aspergillus — One of the most frequently identified genera in indoor environments. Produces extremely small "dry spores" (under 5 μm) that are easily airborne and respirable, penetrating deep into the lungs. Aspergillus fumigatus and A. flavus are the most common cause of invasive mould infections worldwide and carry a high mortality rate. Some species produce aflatoxins, which are associated with liver cancer. The (1→3)-β-D-glucan polymer in its cell wall is a potent immune stimulant, meaning even dead spores trigger inflammatory responses with cumulative exposure
- Penicillium — Equally common indoors, often found alongside Aspergillus. Also produces small, easily airborne "dry spores" that can exist in large numbers. Found in soil, damp building materials, and spoiled food. Some species produce mycophenolic acid — a potent immunosuppressant also used pharmaceutically. Rarely causes direct infection, but chronic inhalation exposure contributes to Hypersensitivity Pneumonitis (HP)
- Wallemia — A less commonly reported but distinctive xerophilic genus that thrives on very dry substrates. Often found on stored materials and dust. Its presence can indicate chronic low-level humidity issues
What this means for your report: Finding Aspergillus and Penicillium dominant indoors — particularly at concentrations above outdoor levels — suggests elevated humidity or condensation conditions, even if no visible water damage is present. As primary colonisers, these genera are the "early warning" moulds that can establish within days of a moisture event.
Mesophilic Fungi — Secondary Colonisers (aw 0.80–0.90)
These genera require more moisture than the primary colonisers and are secondary colonisers — they appear after moisture conditions have persisted long enough for the environment to become more established. They are often associated with intermittent dampness, condensation cycles, or moderate water ingress.
Alternaria (Ulocladium) spores under microscopy
- Cladosporium — One of the most abundant outdoor and indoor moulds globally, commonly found on plant material, window frames, bathrooms, and poorly ventilated spaces. Has a small spore size that disperses easily. Generally considered lower risk than Aspergillus, but can cause infection in rare cases — affecting lungs (55% of reported cases), superficial sites (28%), and deep tissues (15%). Commonly found in both air samples and on damp building materials, and often the dominant genus in outdoor air, making it an important reference point when comparing indoor versus outdoor results
- Ulocladium (Alternaria) — A significant allergen trigger, particularly associated with IgE-mediated hypersensitivity reactions including allergic rhinitis and asthma exacerbation in children. Produces larger spores (10–40 μm) that deposit in the upper airways, though fragments can be respirable. Some species produce mycotoxins including alternariol and tenuazonic acid. A strong indicator of water damage when found growing indoors
- Bipolaris — Less commonly identified but associated with moderate moisture. Can cause allergic fungal sinusitis in sensitised individuals
What this means for your report: Secondary colonisers indicate more sustained moisture — not just elevated humidity, but likely condensation accumulation, slow leaks, or poor drainage. Ulocladium in particular is a reliable water-damage indicator genus. Their presence suggests the moisture problem has been ongoing long enough for conditions to progress beyond what primary colonisers alone would indicate.
Hydrophilic Fungi — Tertiary Colonisers (aw above 0.90)
These moulds require near-saturated conditions and are tertiary colonisers — they need continuously wet materials over an extended period to establish. Stachybotrys, for example, typically takes 7 to 12 days of sustained saturation to colonise a surface. Their presence on building materials is a strong indicator of prolonged water damage, active leaks, or ongoing moisture intrusion — not merely elevated humidity.
Stachybotrys chartarum (black mould) spores under microscopy
- Stachybotrys chartarum — The notorious "black mould". Requires continuously wet, cellulose-rich substrates (plasterboard, timber, paper) for weeks to establish. Produces Trichothecene mycotoxins that inhibit protein, DNA and RNA synthesis. Critically, Stachybotrys does not aerosolise easily — its spores are sticky and heavy, so it is not often detected in air samples. This is why surface sampling is recommended on all visible mould when Stachybotrys is suspected. Its presence always indicates serious, prolonged water damage
- Chaetomium — Another tertiary coloniser often found alongside Stachybotrys on severely water-damaged materials. Requires long-standing water. Produces mycotoxins (chaetoglobosins, chetomin) that are highly allergenic even at low doses and impair lung cilia function. A definitive indicator of major water intrusion
Chaetomium perithecia under microscopy
- Fusarium — Associated with very wet conditions. Can cause opportunistic infections (fusariosis) in immunocompromised individuals. Some species produce trichothecene mycotoxins
- Trichoderma — Requires high moisture. Produces "wet spores" that are heavy and not commonly found on air samples unless the material has dried out or been disturbed. Produces microbial volatile organic compounds (MVOCs) that contribute to the characteristic musty odour associated with water-damaged buildings
- Acremonium — A slow-growing tertiary coloniser often found in chronically wet areas such as showers and kitchens. Will not typically be airborne. Its presence indicates long-standing, unaddressed moisture problems
What this means for your report: Tertiary colonisers in your results indicate serious, sustained moisture conditions. These genera take days to weeks to establish — their presence means the water problem is not recent or minor. If Stachybotrys or Chaetomium are identified, the building has experienced significant water damage that requires both professional remediation and source repair as a priority.
MVOCs: The Musty Smell
The distinctive musty odour associated with mould-affected buildings comes from microbial volatile organic compounds (MVOCs) — gases produced during active mould growth and metabolism. MVOCs are a useful indicator that mould is actively growing somewhere in the building, even if it is not visible. Some MVOCs are harmful in their own right: for example, certain Aspergillus species produce aflatoxins during growth, which are associated with liver cancer following chronic exposure.
Outdoor vs Indoor Species: The Reference Point
To understand your indoor results, it helps to know which mould genera are normally abundant outdoors in New Zealand. Typical outdoor genera include Alternaria (found on soil and decaying plants), Cladosporium, Epicoccum, rusts, smuts, and basidiospores. These are natural environmental fungi — finding them indoors at similar levels to outdoors is expected and normal.
By contrast, genera associated with water-damaged buildings include Aspergillus, Penicillium, Chaetomium, Stachybotrys, Trichoderma, and Ulocladium. When these genera are dominant indoors — especially at levels exceeding outdoor concentrations — it points to an indoor moisture source driving growth.
Reading Your Air Sampling Results
Air sampling captures airborne mould spores and fungal fragments onto a collection medium, which is then examined under microscopy. Results are reported as fungal structures per cubic metre of air (fs/m³). But a number alone means very little — context is everything.
The average size of mould spores found following water damage ranges from 2 to 20 microns. Aspergillus and Penicillium spores are under 5 microns — small enough to be easily airborne and respirable, reaching deep into the lungs. Larger spores like those of Alternaria (10–40 μm) tend to deposit in the upper airways. This size difference is one reason why the genera identified in your air sample matter as much as the total count.
Indoor vs Outdoor Comparison
Every assessment includes an outdoor reference sample. This is the most important comparison in your report. In a healthy building, the indoor mould profile should broadly reflect the outdoor environment — similar genera at similar or lower concentrations. Warning signs include:
- Indoor concentrations significantly higher than outdoor — Suggests an active indoor source
- Different dominant genera indoors vs outdoors — Indicates indoor-specific growth, not just outdoor spores drifting in
- Presence of water-damage indicator genera indoors — Genera like Stachybotrys, Chaetomium, or Ulocladium found indoors but absent or rare outdoors points to an indoor moisture problem
Your report will contextualise results against normal building ranges and established datasets for indoor environments, providing a clear picture of whether conditions in your property are typical or elevated.
Genera Composition Matters More Than Numbers
A moderate spore count dominated by Cladosporium (common outdoors) is very different from the same count dominated by Aspergillus or Penicillium (which may indicate an indoor humidity issue). And even a low count of Stachybotrys is significant — because it does not aerosolise easily, any detection in air suggests nearby active growth or recent disturbance of contaminated material. Similarly, Trichoderma produces heavy "wet spores" that are not commonly airborne unless the source has dried out or been disturbed.
Reading Your Surface Sampling Results
Surface samples (tape lifts) capture mould directly from building materials. Results are reported as spores per square centimetre (spores/cm²) along with the genera identified. Critically, microscopy can distinguish between:
- Settled spores — Individual spores that have landed on the surface from the air. These may not indicate local growth
- Active growth — Spores accompanied by hyphae (filaments) and propagules, confirming that mould is actively colonising the material
This distinction directly determines the IICRC S520 condition rating assigned to each sampled area.
IICRC S520 Condition Ratings: What They Mean
Your report classifies each sampled area according to the ANSI/IICRC S520 (2025) standard — the internationally recognised framework for assessing mould contamination in buildings. Understanding these ratings is key to knowing what action is required.
Condition 1 — Normal Fungal Ecology
Mould levels are typical of a healthy indoor environment without water damage or humidity issues. Surface samples show low spore counts of common genera without active growth. Air samples are consistent with or below outdoor levels. No remediation is required. This is the target condition for any building and the benchmark a post-remediation verification aims to confirm.
Condition 2 — Settled Spores
Elevated spore levels are present on surfaces, but microscopy shows settled spores rather than active growth — the contamination originated from a source elsewhere (another room, the HVAC system, or outdoor intrusion). Surfaces may need professional cleaning, and the actual source of contamination should be investigated and addressed. Condition 2 areas do not require full containment remediation but should not be ignored, as they indicate a contamination pathway exists.
Condition 3 — Active Growth
Confirmed mould colonisation with active growth on building materials — hyphae, spore-bearing structures, and often visible growth. This is the most serious rating and indicates that moisture conditions have been sufficient for mould to establish and reproduce on the substrate. Professional remediation by a qualified contractor is recommended, with containment to prevent cross-contamination. The scope of remediation depends on the species identified, the extent of growth, and the materials affected. Mould damage to porous materials is permanent — fungal enzymes break down the substrate itself, so cleaning cannot reverse structural degradation. Porous materials (plasterboard, carpet, soft furnishings) with Condition 3 growth typically require removal rather than cleaning.
What to Do With Your Results
Your laboratory report is a diagnostic tool — it tells you what is present, how severe it is, and what conditions are driving it. Here is how to act on common findings:
- All areas Condition 1: Your property has normal mould conditions. No action required beyond maintaining good ventilation and managing humidity
- Condition 2 areas present: Investigate the source of spore contamination. Improve ventilation, address any moisture issues, and consider professional cleaning of affected surfaces
- Condition 3 areas present: Engage a qualified remediation contractor with IICRC certification (WRT or AMRT). Remediation should include containment, HEPA filtration, removal of contaminated porous materials, and a Post-Remediation Verification (PRV) to confirm the job has returned conditions to Condition 1
- Hydrophilic genera identified (Stachybotrys, Chaetomium): Prioritise identifying and repairing the moisture source. These moulds do not grow without sustained, heavy moisture — the water problem must be resolved, or mould will return after remediation
- Elevated Aspergillus/Penicillium indoors: Review ventilation and humidity management. These genera indicate elevated moisture but may not require major remediation if addressed early through environmental controls
If you have questions about your results, or need help understanding what action to take, get in touch — our team can walk you through your report and recommend appropriate next steps. For properties that haven't yet been tested, learn more about our professional mould assessment service.