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Moulds are a type of fungus commonly found all over the world. Moulds are spread through the air by spores that are too small to be seen by the naked eye. View the fact sheet below or this web page for more information on mould.


  • A picture of the first page of the mould fact sheetMould Fact Sheet (PDF 312KB)
    View, download or print this fact sheet about mould.

How do I know if I have mould?

Mould can cause discolouration and odours.


Mould can be any colour: black, white, red, orange, yellow, blue, violet, etc.
Not all discolourations are caused by mould.


Musty or earthy smell may be present.

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What does mould need to grow?

Mould growth requires:

  • warm temperatures,
  • nutrients or a growth medium, and
  • water.

As most homes and buildings provide the right temperature and growth medium, a mould growth problem can most often be caused by a moisture problem or high humidity.1

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What is the Canadian guideline for mould?

No exposure limits have been set by Health Canada. Health risks depend greatly on exposure and each person’s sensitivity.1 In the absence of exposure limits, tests for the presence of fungi in air cannot be used to assess risks to the health of building occupants.1

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Why is mould a public health concern?

Mould growth can influence the indoor air quality, as moulds can release small particles, such as spores and mycelial fragments into the air.1 These small particles are able to be inhaled by humans, and can penetrate deep into the lungs.1

Some people are more susceptible to mould than others, such as; the elderly, young infants, people who are immunocompromised, or those with an existing respiratory condition. Exposure to mould can be associated with asthma-related symptoms such as chronic wheezing, irritation symptoms, and non-specific symptoms.1

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How do I prevent mould growth in my home?

Health Canada recommends:1

  • to control humidity (i.e. prevent high humidity),
  • to diligently repair any water damage in your premises, and
  • to clean thoroughly any visible or concealed mould growing in the building.

These recommendations apply regardless of the mould species found to be growing in the building.

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How do I clean mould?

If you discover mould, follow these two steps:2

  • Clean the mouldy surface with water and dish detergent. There is no need to use bleach.
  • Fix the underlying cause, whether it is due to water damage or too much humidity. In a rental unit, notify your landlord in writing if the mould is a recurring problem.

Note: When removing mould, it is recommended you wear proper protective equipment, including rubber gloves, eye protection and a dust mask.2

You might consider hiring a professional if there is a large amount of mould (if the patch is larger than 3 m²) or if the mould keeps coming back after you clean it. A large amount of mould is often the result of a larger problem, such as a leak in the foundation or a major flood, which may require professional help to fix.2

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What should I do if I am concerned about mould in my rental unit?

  • For mould in bathrooms or window sills, simply clean the area with soap and water.
  • If the mould is caused by a water leakage either from outside or a broken pipe, or a flooding incident, notify your landlord or property manager in writing, and allow a reasonable amount of time for actions to be taken.
  • If no actions are taken, or your property manager refuses to address the issue, you may contact the Middlesex-London Health Unit’s Environmental Health line, 519-663-5317 ext. 2300, and discuss it with a public health inspector.
  • In the meantime, increase ventilation by opening windows and using fans to circulate air.

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When will the Middlesex-London Health Unit do an inspection?

Middlesex-London Health Unit inspectors will carry out an inspection if all of the following conditions are met:

  • mould is visible,
  • the property manager has been notified of the concern in writing, and
  • the concern has not been addressed in a reasonable amount of time.

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Additional Information

For more information visit the Government of Canada's Reduce humidity, moisture and mould web page or the Canadian Mortgage and Housing Corporation’s Mould in Housing web page.

View Health Canada's Indoor Air Quality Checklist (PDF 307KB) for steps you can take to protect yourself and your family from indoor air pollutants, including mould.

To speak to a Public Health Inspector on the Environmental Health Team about mould, please call:

  • 519-663-5317 ext. 2300


Please note: Where indicated, the source of the information on this web page is the Government of Canada’s Residential Indoor Air Quality Guideline: Moulds or Reduce humidity, moisture and mould websites. The information is a copy of the version available at the URL in the references section below.

Date of creation: September 11, 2013
Last modified on: July 17, 2019




1Government of Canada. (2016, January 12). Residential indoor air quality guideline: moulds. Retrieved from