Mould is everywhere outdoors, and a little mould indoors is normal. Health problems become more likely when mould grows inside a building on damp materials, because the people living or working there breathe in more spores and fragments than usual. Most people have mild symptoms or none at all, but some people are more sensitive than others. This guide explains the common symptoms, who is most at risk, what the different types of mould mean for health, and what to do if you're worried.
Common symptoms of mould exposure
Most symptoms are like hay fever, or a cold that doesn't go away:
- sneezing and a runny or blocked nose
- itchy, red or watery eyes
- a sore throat or a lingering cough
- wheezing, shortness of breath or asthma flare-ups
- skin irritation or rashes
Some people also report headaches or tiredness, although these have many other possible causes. A useful clue is timing. If symptoms ease when you're away from the building and come back when you return, the building may be the cause, and that pattern is worth mentioning to your GP.
How mould affects health
Mould mainly affects people when they breathe in its spores and tiny fragments. It can cause problems in several ways:
- Allergy and irritation. This is by far the most common effect, and it causes the hay-fever-like symptoms above.
- Asthma. Mould can trigger attacks in people who already have asthma. Damp, mouldy homes are also linked with children developing asthma.
- Infection. This is rare. It mainly affects people whose immune systems are weakened by illness or medical treatment. Aspergillosis, a lung infection caused by Aspergillus, is the best-known example.
- Hypersensitivity pneumonitis. In a small number of people, repeated exposure causes inflammation in the lungs. It is more common after heavy exposure, such as on farms, but it can happen in homes too.
Some moulds also produce toxic substances called mycotoxins. Researchers are still working out how much they affect health at the levels found in homes. They are a good reason to deal with mould promptly, but not a reason to panic.
Who is most at risk
Anyone can react to mould, but some people are more sensitive:
- babies and young children
- older people
- people with asthma, allergies or other lung conditions such as COPD
- people with weakened immune systems, for example during chemotherapy, after an organ transplant or on kidney dialysis
- pregnant people
If someone in one of these groups lives or works in a mouldy building, it's worth acting sooner rather than later.
What different moulds mean for health
Our laboratory identifies mould to genus level, for example Aspergillus or Penicillium. The genus helps explain both the likely health effects and the moisture problem behind the growth. These are the moulds we see most often in New Zealand buildings. For what each one says about moisture, see our guide to the types of mould.
Aspergillus
One of the most common moulds indoors. Its spores are very small (typically 2 to 5 µm), so they can be breathed deep into the lungs. For most healthy people the main effect is allergy. In people with weakened immune systems, some species can cause aspergillosis, a serious lung infection. Repeated exposure can also lead to hypersensitivity pneumonitis in people who have become sensitised.
Stachybotrys
Stachybotrys chartarum is the mould most people mean by "toxic black mould". It only grows on materials that stay wet for a long time, such as plasterboard, paper and timber, so finding it usually means a long-running leak or water damage. It can produce mycotoxins called trichothecenes. Its spores are sticky and heavy and don't float in the air easily, so exposure mostly happens when the growth is disturbed or dries out. Not every dark mould is Stachybotrys, and only a lab test can tell them apart. See black mould in New Zealand for more.
Chaetomium
Like Stachybotrys, Chaetomium needs long-term water damage and is often found alongside it on wet plasterboard, paper and timber. Infections are rare. It produces mycotoxins, and research has linked indoor Chaetomium growth with asthma, particularly in children.
Penicillium
Just as common indoors as Aspergillus, and often found with it. Its spores are also small enough to reach deep into the lungs. The main health effect is allergy, and repeated exposure can contribute to hypersensitivity pneumonitis. Infection is rare. Some species produce mycotoxins, including mycophenolic acid, which suppresses the immune system. Penicillium is one of the moulds we find most often in New Zealand homes.
Cladosporium
The most common mould in New Zealand's outdoor air, so some is almost always present indoors too. It rarely causes infection and is not a major producer of mycotoxins, but its spores are a common allergy trigger. Heavy growth indoors usually points to condensation, for example on window frames and bathroom ceilings.
Alternaria / Ulocladium
Alternaria, which now includes Ulocladium, is one of the best-studied allergy moulds in the world. It is strongly linked with hay fever and with asthma in children. It is common outdoors, but significant growth indoors points to water-damaged materials.
Aureobasidium
Often found on damp window frames, painted wood and bathroom surfaces. It carries little risk of infection for healthy people, although, like any mould, its spores can trigger allergies.
Wallemia
An unusual mould that grows in very dry conditions, on dust, carpets and sugary or salty materials, so it can turn up in homes with no obvious damp. It has been linked with hypersensitivity pneumonitis and with allergies.
Mould and the law in New Zealand
New Zealand has no legal limit for mould indoors, and mould is not listed in WorkSafe's Workplace Exposure Standards (16th edition, July 2026).
Employers still have a duty under the Health and Safety at Work Act 2015 to eliminate or minimise risks to workers' health, and that can include exposure to mould.
For rental homes, the Healthy Homes Standards set minimum requirements for heating, insulation, ventilation, moisture and drainage, and draught-stopping. Meeting them makes mould much less likely.
What to do if you're worried
- Talk to your GP about any symptoms, especially if they ease when you're away from home, or if someone in the house is in a higher-risk group.
- Find and fix the moisture. Mould needs water to grow. Unless leaks, condensation or poor ventilation are dealt with, it will come back however well it's cleaned. Our article on why bleach doesn't kill mould explains why.
- Find out what's growing. A lab test identifies the mould and shows whether it is actively growing. For a single patch, you can post us a tape lift. For a musty smell with nothing visible, or mould in more than one room, a mould assessment is the better option.
- Don't scrub large areas yourself. Disturbing mould releases spores into the air. For anything bigger than about 1 m², use a professional remediator.
References
- Burge, H. & Otten, J. (1999). Fungi. In Bioaerosols: Assessment and Control (pp. 19.1–19.7). American Conference of Governmental Industrial Hygienists.
- Kidd, S., Halliday, C., Alexiou, H., & Ellis, D. (2016). Descriptions of medical fungi (3rd ed.). Newstyle Printing. ISBN: 9780646951294.
- World Health Organization (2009). WHO guidelines for indoor air quality: dampness and mould. WHO Regional Office for Europe.
- WorkSafe New Zealand (2026). Workplace exposure standards and biological exposure indices (16th ed.).