Mould is ubiquitous in the external environment — a core part of the ecological biome, colonising soil and plant matter. Indoors, where water activity and organic substrates are sufficient, mould colonises building materials, producing enzymes that break down lignin, cellulose, and other polymers. Secondary metabolites of this process — mycotoxins and microbial volatile organic compounds (MVOCs) — alongside airborne spores and hyphal fragments, are the primary health vectors. The genera found on-site and the substrates colonised determine the nature and severity of potential exposure.
Health Effects of Mould Exposure
The primary health risks from mould in buildings arise from the inhalation of airborne spores, spore fragments containing toxins (mycotoxins, MVOCs), and fragments of hyphae. Exposure may result in a range of illnesses, from allergy-like symptoms — rhinitis, sinusitis, otitis, keratitis — through to genera-specific disease such as Aspergillosis (Aspergillus), Mucormycosis (Mucor), and Histoplasmosis (Histoplasma).
Prolonged or heavy exposure is associated with more serious acute and chronic health effects. Where sensitisation of the immune system persists due to elevated mould exposure, chronic disease may arise — most notably Chronic Inflammatory Response Syndrome (CIRS), now increasingly recognised clinically as Hypersensitivity Pneumonitis (HP). Symptoms of CIRS/HP are variable between individuals and difficult to distinguish from other medical conditions without specific blood and urine analysis for mould metabolites and toxins. Where the source of exposure is confirmed and removed, health outcomes typically improve.
Immunocompromised individuals are at substantially greater risk of mould-specific disease — including the elderly, those in pregnancy, the chronically ill, and young children and infants.
Vulnerable Populations
Certain groups are particularly susceptible to mould-related health effects:
- Children and infants (developing immune and respiratory systems)
- Elderly individuals
- Immunocompromised persons (e.g., HIV/AIDS, transplant recipients, renal dialysis patients)
- Those with existing respiratory conditions such as asthma or COPD
- Patients in healthcare settings
- Pregnant individuals
For occupants in rental properties, it is worth noting that the Residential Tenancies Act 1986 and the Healthy Homes Standards Regulations 2019 set minimum requirements for heating, insulation, ventilation, and moisture ingress control — non-compliance with these standards increases the likelihood of mould growth and subsequent occupant exposure.
Mould Genera: Health Profiles
All mould is potentially hazardous — regardless of genera, sensitivity to mould mycotoxins and particulate is highly variable between individuals. Below is a brief profile of the genera most commonly encountered in New Zealand indoor environments, and their known health implications.
Aspergillus
Aspergillus spores are among the most clinically significant of indoor mould genera, primarily because they are respirable — at under 3 µm, they penetrate to the alveoli of the lung. In immunocompromised individuals this can cause Aspergillosis, a pulmonary infection. In otherwise healthy individuals, chronic exposure leads to repeated immune responses driven by the (1→3)-β-D-glucan polymer present in the spore wall, causing cumulative allergenic sensitisation. This pathway is increasingly recognised clinically as Hypersensitivity Pneumonitis (HP) or Chronic Inflammatory Response Syndrome (CIRS). Aspergillus spores are more commonly reported as causative than many other genera because of their small size, ready airborne dispersal, and high environmental prevalence.
Stachybotrys
Stachybotrys — colloquially known as "toxic black mould" — presents a health risk that is distinct from most other genera: its spores are not readily airborne. The primary hazard stems instead from the production of Trichothecene mycotoxins, some of which are volatile and become airborne independently of spore dispersal. Trichothecenes inhibit ribosomal protein synthesis and interfere with DNA and RNA replication in exposed persons. The severity of health effects is dependent on route and duration of exposure. Stachybotrys colonises cellulosic substrates (paper, cardboard, gypsum board facing) that have been persistently wet, and is therefore a strong indicator of chronic or concealed water ingress.
Chaetomium
While direct infection of the body by Chaetomium genera is clinically rare, species within this genus produce a range of mycotoxins on colonised surfaces — including chaetoglobusins (A, C, F), chetomin, and chaetomugilin-C. These mycotoxins have been found to be highly allergenic even at low doses, are capable of triggering asthma, and impact cell motility and cilia function in the lung. Emerging epidemiological evidence also suggests a causative link between indoor Chaetomium growth and the development of asthma in young children and adults. Chaetomium is commonly associated with paper, damp drywall, and water-damaged structural timbers.
Penicillium
Human infection by Penicillium is rare but may manifest as mycotic keratitis, otomycosis, or endocarditis. More broadly, some species produce mycophenolic acid as a mycotoxin — a potent immunosuppressant that, at elevated environmental concentrations, may impair immune function in exposed individuals. The spores of Penicillium are respirable and share similar exposure pathways to Aspergillus, including chronic sensitisation via (1→3)-β-D-glucan and potential induction of HP. Penicillium is one of the most commonly encountered indoor mould genera in New Zealand, frequently colonising cool, damp surfaces and stored organic material.
Cladosporium
Cladosporium is not generally associated with significant mycotoxin production, and direct infection of the body in immunocompetent individuals is rare. Where infection does occur, reported cases have involved the lungs (55% of cases), superficial sites (28%), and deep tissues or fluids (15%). There is some emerging evidence that the (1→3)-β-D-glucan polymer structure in Cladosporium species may elicit a weaker immune response than that of other genera — though inhalation of spores and particulate will still provoke allergenic or irritant responses in sensitive individuals. Cladosporium is among the most prevalent outdoor and indoor airborne genera in New Zealand and is a common background constituent of indoor air samples.
Alternaria / Ulocladium
Alternaria (and its morphological synonym Ulocladium) is not commonly associated with invasive infection in immunocompetent individuals. The primary health concern is allergenic exposure — Alternaria is one of the most well-documented allergenic mould genera worldwide, and a significant trigger of IgE-mediated hypersensitivity. There are strong epidemiological associations between Alternaria exposure and allergic rhinitis, and notably, the exacerbation and development of asthma in children. Indoor detection at elevated concentrations warrants investigation of moisture sources, as it is a reliable indicator of water-damaged materials or soil ingress.
Aureobasidium
Aureobasidium presents a low pathogenic risk to healthy occupants in the indoor environment. Infections are uncommon in immunocompetent individuals and typically occur via traumatic inoculation rather than inhalation. Reported clinical cases include keratitis, cutaneous infection, pulmonary infection, peritonitis, and systemic infection — the latter primarily in immunocompromised individuals (e.g., those with AIDS or on renal dialysis). It is also a recognised, though rare, causative agent of phaeohyphomycosis — internal body or flesh infections involving pigmented fungal elements. In the context of environmental mould exposure in healthy occupants, Aureobasidium poses minimal infectious risk; however, as with all mould genera, inhalation of spores and particulate may still provoke allergenic or irritant responses in sensitive individuals due to the presence of (1→3)-β-D-glucan.
Wallemia
Wallemia is not commonly associated with invasive infection in immunocompetent individuals, though rare subcutaneous infections have been reported. More significantly, Wallemia species have been implicated in Hypersensitivity Pneumonitis (HP) and allergic respiratory disease. What makes this genus particularly notable is its xerophilic nature — it thrives at water activity levels below 0.75 (aw < 0.75), where most other mould cannot proliferate. This means it can colonise heavily sugar- or salt-laden substrates, and is commonly found colonising carpeted surfaces in the indoor environment. Its ability to grow under dry, low-moisture conditions makes it an unexpected finding in buildings that show no obvious signs of water damage or elevated humidity.
New Zealand Regulatory Context
Currently, there is no New Zealand legislation that establishes thresholds to delineate between normal background mould and elevated indoor mould growth, nor specific regulation for personal mould particulate exposure. Mould is not listed as a biological contaminant in the WorkSafe NZ Workplace Exposure Standards and Biological Exposure Indices (15th edition, February 2025).
However, the Health and Safety at Work Act 2015 (HSWA) requires that PCBUs (persons conducting a business or undertaking) must eliminate or minimise health risks where practicable. This obligation can be reasonably interpreted to encompass biological hazards, including the exposure of workers to mould particulate at concentrations with the potential to affect health.
For residential premises, conformance to the Residential Tenancies (Healthy Homes Standards) Regulations 2019 — under the Residential Tenancies Act 1986 — is required where a premises is occupied under a rental agreement. These regulations set minimum standards for heating, insulation, ventilation, and moisture ingress and drainage. Non-compliance increases the risk of conditions conducive to mould growth and subsequent occupant exposure events.
References
- Kidd, S., Halliday, C., Alexiou, H., & Ellis, D. (2016). Descriptions of medical fungi (3rd ed.). Newstyle Printing. ISBN: 9780646951294.
- Burge, H. & Otten, J. (1999). Bioaerosols assessment and control. American Conference of Governmental Hygienists, 19.1–19.7.
Concerned About Mould?
Contact our laboratory to discuss testing options for your property or workplace.
Get in Touch