Moulds are members of the fungus family, along with mushrooms and yeast. There are thousands of different types of moulds, and they are usually present in air, indoors and outdoors. Moulds play an important role in helping to compost decaying materials like plants. But they can pose health risks as well.

Moulds can get inside buildings through doors, windows, heating, ventilation, and air conditioning systems, and through small openings and cracks in walls and foundations. They can also be brought in on clothing, shoes and the skin and hair of people and pets.

Indoor, moulds need two things in order to grow: nutrients (food) and moisture. Nutrients available indoors include dust, lint, dander, building materials, and furnishings. Moulds usually grow in areas, which are damp or humid in places like bathrooms, air ducts, humidifiers, porous insulation, fan coil units, and condensation or drip pans. They can also grow on building materials such as drywall, ceiling tiles, carpet, wallpaper, window casings, and foundation walls, especially if these are damp or wet.

When most kinds of moulds are present in indoor air at the same level as outdoors, they usually do not pose a health risk. Moulds that grow indoors are usually different from typical outdoor moulds, and can pose more risks to health.

Moulds can release spores (their offspring) and various chemicals into the air. When mould levels build up in indoor air and dust, this can trigger allergies and asthma attacks in some people. Certain types of moulds can be even more hazardous, especially when they actually grow on indoor surfaces. Some moulds can produce toxins, which can poison indoor air and cause illness. Exposure to elevated levels of indoor moulds can affect health in four major ways:

• Irritation, causing symptoms like eye, throat, and skin irritation.
• Allergies, including symptoms similar to hay fever, asthma attacks (between 10% and 30% of asthmatics are allergic to moulds), and dermatitis. Allergies to indoor moulds may not be detected by standard allergy tests, which measure reactions to outdoor moulds.
• Toxicity, which can cause headaches and flu-like symptoms like fever and cough, diarrhoea and fatigue. Breathing in mould toxins has been linked to serious illnesses and with sick building syndrome (SBS).
• Infection, normally a risk only for people with severely weakened immune systems, such as those on chemotherapy and people living with HIV/AIDS, and for victims of severe burns whose skin has been damaged. Only certain moulds can cause infection.