What are moulds?

Moulds are members of the fungus family, along with mushrooms and yeast. There are thousands of different types of moulds, and their spores are present in varying quantities in air, indoors and outdoors, at all times of the year. Moulds play an important role in helping to compost decaying materials like plants, in the fermentation of food, and in some antibiotics, but they can pose health risks as well.

How do moulds grow indoors?

Mould spores can get inside buildings through doors, windows, ventilation, heating and cooling 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.

Indoors, mould spores need nutrients (food) and moisture to grow. Nutrients available indoors include dust, pet dander, some building materials and furnishings. Sources of moisture include shower and bath areas, humidifiers, cooking, plants, water leaks or penetration, and even people themselves. 

Where do moulds grow?

Moulds usually grow in areas, which are damp or humid, such as bathrooms. They can also grow in humidifiers, porous insulation, fan-coil units, and in condensation or drip pans. If building materials get wet, these surfaces can also support mould growth. These surfaces include drywall, ceiling tiles, carpet, wallpaper, window casings and even building foundations. 

How do I know if I have mould in my home?

Common household moulds come in many colours and often looks like a stain or smudge or have a hairy appearance and it may smell musty. It can take as little as 48 hours for mould to appear on damp materials.

How can moulds affect your health?

Moulds can release spores and various chemicals into the air when they grow. When left to grow indoors, some people may experience allergic reactions or and asthma attacks. There are also hazardous mould species that produce toxins, which can impact lung function and impact other organs. Health effects are characterized as being:

  • An irritant – which can cause symptoms like eye, throat, and skin irritation
  • An allergen – which can cause symptoms similar to hay fever, asthma attacks (between 10% and 30% of persons with asthma are allergic to moulds), and dermatitis
  • A toxin – which can cause headaches and flu-like symptoms like fever and cough, diarrhea and fatigue
  • An infection – including lung infections, which are normally only a risk for people with severely weakened immune systems, such as those on chemotherapy, people living with HIV/AIDS, and for victims of severe burns whose skin has been damaged.

If you are concerned that you may be experiencing symptoms linked to mould exposure, please speak to your physician.

Who is at risk?

Anyone can be affected by moulds, but some people are more susceptible than others including:

  • People with asthma or allergies to moulds
  • Infants and young children, whose lungs are still developing
  • People with weakened immune systems

Factors that increase the risk to health include:

  • Exposure to large volumes of moulds
  • Exposure to moulds for a long period of time, or repeated exposures to elevated levels for short periods
  • Exposure, in sufficient amount, to toxins from toxin-producing mould species

Experts recommend that people should not live or work in mouldy buildings. If you or a family member are exposed to indoor mould and are experiencing health effects, consult your doctor to determine if moulds are a possible cause. At the same time, ensure that the conditions that are allowing indoor mould growth are corrected.

What can you do to prevent growth of indoor mould?

Homeowners and Tenants

The best way to prevent the growth of indoor mould is to make sure mould spores do not have the food and moisture that will permit their growth:

  • Clean your bathrooms regularly and keep them dry by using your bathroom fan when showering or bathing.
  • Use the kitchen fan to limit the moisture created in your home through cooking.
  • Fix water leaks and condensation problems as soon as possible. Windows that have condensation problems in the winter should be cleaned regularly. 
  • Dry any water damaged building materials or furnishings within 48 hours. If something cannot be dried quickly it should be discarded. 
  • Get professional advice for serious contamination problems, such as after a flood. For more information on dealing with mould after a flood or related issues on flood clean up, take a look at what to do if your sewer backs up or your basement floods.
  • If you are a tenant, all the precautions above apply to you also. If there is a problem you cannot resolve on your own, advise your landlord as soon as possible, and if the landlord has not taken action to resolve a mould problem in a reasonable amount of time contact the City of Ottawa by calling 3-1-1 and report the problem to Bylaw and Regulatory Services (Property Standards). 
Cleaning up mould
Health Canada suggests seeking professional cleaning assistance if you have:
  • an area of mould greater than a square meter, or
  • more than three patches of mould each less than a square metre in area.

When cleaning up mould impacted areas on your own, consider the following:

  • Think of your personal safety when cleaning up mould: Wear protective clothing such as rubber gloves, eye protection and a mask when cleaning up mould. A disposable respirator, such as an N95 mask, should fit well to your face covering your nose and mouth to protect against mould and dust in the air while cleaning and follow the manufacturer’s instructions for proper use.
  • Use soap and water to clean mould on hard surfaces: Clean with soap and water and rinse with clean water. Bleach is not necessary to clean up mould that is not related to a flooding event. Learn more about flood cleanup.
  • Discard absorbent or porous materials that cannot be washed and dried. Materials such as ceiling tiles, upholstery and carpet may need to be thrown away if they become mouldy, as they can be very difficult to clean.

Prevent Mould Growth

Prevent mould from growing by keeping rooms dry. Always fix water leaks right away, whether from plumbing or other indoor systems or from water penetrating from outside. Use fans, dehumidifiers and open your windows to reduce moisture in your home, especially in the summer. In the winter it is important to adequately heat rooms since relative humidity will be higher in cold rooms.

Where can you get more information?

The following agencies can provide information on indoor moulds, their health effects, proper mould clean-up procedures, and advice on health problems related to indoor air quality.

  • Ottawa Public Health
    • Telephone: 613-580-6744, Selection option 1 for English, or 2 for French. Then select option 5 to speak with a Public Health Inspector.
    • E-mail:

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