Mould in damp buildings

Last updated: 8 March 2019

If you have mould growing in your property, you need be aware of possible health implications. Find out how to identify mould, clean it up safely, and prevent it from growing.

This page summarises information released by the Ministry of Health, BRANZ Ltd and WorkSafe New Zealand. It is a guide only.

Most moulds are harmless to healthy individuals and occur naturally in our environment.

However, some moulds can release substances that are potentially toxic and should be treated with caution.

If you are in any doubt about whether you are at risk, contact your local council to have moulds checked out or your GP if you have any health concerns.

Health effects

When moulds and fungi reproduce, they release countless tiny spores that can become airborne. Health problems can arise when large numbers of these spores are inhaled, ingested or come into contact with the skin.

Some moulds can produce adverse health effects such as:

  • allergies
  • aggravation of respiratory problems
  • eye and skin irritation
  • headaches and nausea
  • flu-like symptoms.

Those with pre-existing asthma and those with weakened immune systems, as well as infants and the elderly are at the greatest risk.

Most people who experience adverse effects associated with mouldy buildings fully recover following removal and clean-up of the mould contamination.


If mould is present in a form that is likely to cause health effects, it will be very visible. Stachybotrys and other toxic moulds often grow only inside wall cavities.

When mould is contained in a sealed environment, it is not a threat to the home’s occupants. However, if mould spreads to walls and floors and becomes visible, or if wall cavities are opened, the spores can be released.

Toxic mould

Formal identification of harmful moulds, such as Stachybotrys chartarum (S. chartarum), can only be carried out at laboratories.

S. chartarum is a greenish-black mould that grows on material with high cellulose content, such as fibreboard, that has become extremely wet and has remained wet for some time.

Excessive indoor humidity resulting in condensation on the walls will further promote its growth.

While S. chartarum is growing and is still wet, a wet slime covers its spores and prevents the spores from becoming airborne. It is only when the mould dries out and the spores become airborne that they can become a problem.

Clean-up procedures

Spores are more easily released when mouldy materials dry out, so it is important to clean up any mouldy areas while they are still wet.

You should wear protective clothing that is easily cleaned or discarded, rubber gloves and a face mask (ask for a respirator with P1 particulate filters at a hardware store).

  • The Ministry of Health and BRANZ Ltd recommend cleaning away any mould with commercial mould cleaner or hypochlorite bleach as soon as it appears on internal surfaces. Apply 1½ cups of household bleach to 4 litres of water and leave for 10 minutes before rinsing and drying.
  • Any affected materials that are removed should be wrapped in plastic before being disposed of. In extreme cases, porous materials such as carpets and curtains may be difficult to thoroughly clean and may have to be thrown out. Your local council will be able to advise you on this (see more information below).
  • If there are extensive areas of mould and dampness in your home or if you have allergic reactions, it may be best to seek experienced advice on its removal, including whether to employ a professional cleaner.


If you have mould growing in your home, it is important you clean it up before you dry out the house.

Better ventilation, more heating and higher levels of insulation can prevent the growth of moulds.

Simple measures such as using an extractor fan in the bathroom and kitchen are effective forms of ventilation.

Heating and insulation increase the capacity of the air to hold moisture and prevent high humidity. Mould cannot grow without high humidity or condensation.

More information

If the occupants of your house suffer health problems after exposure to moulds, we recommend that you contact your GP for a medical assessment.

If your doctor thinks your symptoms are related to living in a leaky home, ask them to refer your case to a health protection officer at the Public Health Service.

Contact your local council if you want to find out if the mould in your house is a health risk to the occupants.

Dealing with mould is a brochure that can be purchased from the BRANZ website.

Mould resources on the US Environmental Protection Agency website.

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: