Mould and ventilation management

Risks associated with mould

The science

Moulds do not need light and can therefore flourish in dark places. Moulds, which are fungi, commonly live on dead organic matter. They are found growing indoors and outdoors.


Mould fungi are mainly aerobic – they obtain oxygen for growth from the air. When germination occurs, thin tubes called hyphae grow from a spore. Collections of hyphae are called mycelium. The hyphae spread over and through the substrate.


Mould spores may land on damp material and then begin growing, digesting what they need to survive and spread. Moulds absorb nutrients outside their bodies.


Organic matter can be found in building materials or finishes, or in build up of waste material, dust and grime. For example, mould will not grow on clean glass, but it may on dirty glass. Also, it could not obtain energy from Rockwool insulation, which is inorganic, but such inorganic insulation could be damp and covered by organic dust, containing the nutrient fungi need to grow.


Plasters containing salts might not support growth of moulds, but when wallpapered, moulds can thrive, enjoying nutrition from the paper and paste. Other organic material that provide nutrients for mould include:

  • wood;
  • chipboard;
  • paper;
  • cotton;
  • wool; and
  • some paints.

Carpets often support mould, especially in damp environments. Many moulds can feed off materials containing cellulose. Avoid using plasterboards in wet rooms, as such materials can support mould and can physically break down from repeated wetting.


Mould has the same kind of life cycle as other fungi such as dry or wet rot. Spores can loosely be described as the seeds of fungus, and there are always mould spores in the air, both inside and outside buildings. However, inside a building suffering from mould contamination you would be likely to find higher concentrations of spores than outside. Mould spores are present in dust that can accumulate on both vertical and horizontal surfaces.


Moulds can often cope with changing moisture availability, and the spores can remain dormant in dry conditions. They can then germinate, either where free water is present (although some moulds hate free water), on damp surfaces, or even in moist air – but for all moulds water is a key requirement, in whatever form.


Moulds proliferate in quite a range of conditions. Jagjit Singh reminds us in Building Mycology that, although mould fungi may have wide temperature and humidity tolerances, relative humidities exceeding 70% and temperatures in the range of 15–20°C are generally required.


Some fungi can grow in much higher temperatures – up to 30°C and even below freezing.

Conditions needed for mould

Experts consider that, for spores to germinate, certain critical conditions are needed:

  • moisture;
  • food source;
  • oxygen;
  • temperature 5–40°C;
  • humidity 55%+; or 
  • most commonly a threshold of 65%.

How long does mould take to form?

  • In the drying industry the first 24 hours after a flood is considered enough for mould to take hold.
  • Spores can germinate within 12 hours of wetting.
  • Enzymes are released into the substrate to absorb nutrients.
  • Mould spores can form within 5–16 days.

A key factor in assessing condensation risk in homes is how often relative humidity is above a critical percentage – often considered 70–75%. So in order to combat mould colonisation target RH directly – either reducing moisture in the air or raising air temperature will reduce RH.

Measures that raise surface temperatures may also be used; for example, insulating sheet polystyrene glued to walls.

A high RH may not necessarily mean there actually is any surface condensation, just that surface condensation will be more likely. This is where monitoring can help you understand exactly what is happening in a property over a given time period – when RH and air temperature are logged, with surface temperature logged too at key room positions.

Health issues

People vary in how aware they are of stale air, and may not relate any health problems to indoor air quality. We can detect quite a few pollutants using sophisticated equipment, and can quite easily measure the relative dampness of air using humidity and temperature sensors.

There is a huge range of indoor contaminants, and when bad building air impairs health, the term 'sick building syndrome' is often used. Causes of sick building syndrome can be bad ventilation, ventilation ducting contaminated by bacteria and fungi (including moulds), and ineffective air filters enabling the passage of dust and other pollutants.

Measures to remedy condensation not only reduce mould colonisation, but can also diminish other pollutants. For example, filters used in conjunction with ventilation equipment may filter out a range of pollutants such as tobacco smoke, pollen, etc.

Scientists are beginning to more fully understand moulds. Some estimate there are more than 100,000 species, with probably only 200 studied with respect to health implications. So no wonder there is caution concerning mould risk. It is not known how many moulds are toxic, although certain species, such as stachybotrys chartarum or various aspergillus moulds, particularly aspergillus fumigatus, are considered potentially harmful to humans. The US Environmental Protection Agency warns that any mould 'in profusion' can cause health problems.

Health issues relating to mould can be summarised as follows:

  • Allergic reactions from inhaling or touching mould or spores can be immediate or delayed, and can include hay fever-type symptoms, sneezing, runny nose, red eyes and skin rash. Repeated exposure increases sensitivity to mould. Once an allergy develops, it can be a permanent lifetime condition.
  • Asthma attacks can be triggered in people allergic to mould.
  • Hypersensitivity pneumonitis can follow short-or long-term exposure to mould. The disease resembles bacterial pneumonia but is uncommon.
  • Irritant effects can include irritation of eyes, skin, nose, throat and lungs – sometimes experienced as a burning sensation in these areas.
  • Opportunistic infections may affect those with weakened immune systems – notably lung infections by Aspergillus fumigatus. Trichoderma has affected immune-compromised children. Healthy individuals are not likely to be affected.
  • Mould toxins – moulds can produce toxic substances called mycotoxins. These can be from within a spore or on the outside of a spore. Mycotoxins can affect individuals who inhale, ingest or touch them.
  • Aspergillus versicor and stachybotrys atra (also known as stachybotrys chartarum) produce potent toxins – there is a possible link between stachybotrys chartarum and pulmonary haemorrhage in infants. Children, the elderly and those in poor health seem most vulnerable to mould toxins.
  • Microbial volatile organic compounds (mVOCs) are compounds produced by moulds that are volatile and released directly into the air. Often strong odours are produced. Exposure to mVOCs has been linked to symptoms such as headaches, nasal irritation, dizziness, fatigue, and nausea.
  • Spores may measure 2–10 microns, i.e. 0.002–0.01mm. The size of spore dictates how far into the human body the spore will infiltrate. The smaller the spore, the further it can travel into the respiratory tract. Apparently spores greater than 10 microns are trapped in the nasal passages, whereas spores smaller than 4 microns can travel into the lungs. Breathing through the mouth can allow larger spores to travel as far as the lungs. As well as breathing them in, we can be affected by spores through skin contact or ingestion, i.e. by eating mould-affected foodstuffs.