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What is mould?

Moulds are fungi, a group of organisms that also includes mushrooms and yeasts. Moulds are generally found in nature and are carried indoors from the outside, through open windows or doors, on clothing, pets, food or furniture. More than 270 species of mould have been identified in Canadian homes.

Moulds will grow and multiply under the right conditions needing only:

  • sufficient moisture (e.g. very high humidity, condensation, a leaking pipe)
  • organic material (e.g. ceiling tile, drywall, carpets, wallpaper, wood or food)
  • moderate temperature

What are the potential health effects of mould?

The majority of common moulds are not hazardous to healthy individuals. However, the health risk depends on the type of mould you are exposed to and the amount of time an exposed person spends in the home. Those most likely to experience health effects from mould include:

  • the elderly.
  • pregnant women.
  • infants and young children.
  • people with allergies, chronic respiratory illness and/or chemical sensitivities.
  • people with weakened immune systems.

The most common health problems associated with exposure to mould are:

  • eye, nose and throat irritation.
  • runny nose, sinus congestion, frequent cold symptoms.
  • increased asthma attacks.
  • allergic reactions.

Anyone experiencing any of these symptoms should consult a physician.

How to prevent and reduce mould growth

  • Repair all roof, basement, and pipe leaks as soon as you notice them.
  • Turn on kitchen and bathroom exhaust fans, especially when bathing, showering, cooking and doing laundry, and open windows when weather permits.
  • Following a flood or any water damage, thoroughly clean and dry water-damaged carpets and building materials within 48 hours. Discard material that cannot be cleaned and properly dried.
  • Make sure that clothes dryer hoses are properly connected and vented to the outside.
  • Limit the use of humidifiers.
  • Limit the number of indoor plants and fish tanks as these can raise the humidity level in your home.

How can you tell if it is mould?


Mould growth often appears as slightly furry, discoloured, or slimy patches that increase in size as they grow. Mould may be any colour: black, white, red, orange, yellow, blue or violet. Dab a drop of household bleach onto a suspected spot. If the stain loses its colour or disappears, it may be mould. If there is no change, it probably is not mould.


A musty or earthy odour may be the first indication of the presence of mould. Sometimes moulds are hidden and cannot be seen. It may be necessary to look behind and underneath surfaces such as carpets, cabinets, wallpaper and walls. Even if a smell is not noticeable, wet spots, dampness or evidence of a water leak are indications of moisture problems and mould may follow.

What should I do if I have mould?

Small areas

Small areas of mould can be cleaned using a detergent solution. A small area is defined as fewer than three patches, with each patch less than three feet by three feet in size.

  • Scrub the area with detergent (preferably an unscented detergent).
  • Sponge with a clean, wet rag and dry quickly and thoroughly.
  • Bleach is NOT recommended.

Always wear personal protective equipment, including household rubber gloves, a disposable dust mask (3M 8210 or equivalent) and safety glasses or goggles, when using any cleaner or chemical.

Infants and family members with asthma, allergies or other health problems should not be in the work area or adjacent room during the cleaning.

Larger areas

Removing large areas of mould (i.e. more than three patches, with each patch more than 3 feet by 3 feet in size) will require the cleaning services of professionals. Professional help can be found under Environmental Services in the Yellow Pages.

In all cases, the underlying cause of the mould growth (e.g. water accumulation or prolonged high humidity) must be corrected or mould will continue to grow or reoccur. Follow the previously mentioned tips to prevent mould growth and regularly inspect your home for signs of moisture problems or water damage (musty odours, condensation, and discoloration).

Information contained in this Fact Sheet was adapted from the Canada Mortgage and Housing Corporation’s Homeowners’ Guide on Fighting Mold, available on-line at

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