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The purpose of this guidance is to summarize ways to identify, remediate, and prevent moisture and mould issues indoors. This guidance provides practical recommendations to address this potential health hazard, including guidance for assessing the magnitude of the problem, a prevention checklist, and considerations when hiring a professional to remediate mould and moisture issues indoors. This guidance aligns with what is available internationally and is intended for the general public, including property owners, landlords and tenants, as well as public health and building professionals.
The word "mould" is the common name referring to fungi that grows on food or materials in homes or other buildings. Health Canada has concluded that indoor mould growth may pose a health hazard. Health Canada and other internationally recognized organizations do not propose a health-based exposure limit for mould exposure indoors, as current scientific information is not available to support its derivation.
People living in homes with mould and damp conditions are more likely than others to haveFootnote 1:
More recently, there is increased recognition that exposure to indoor mould and dampness may contribute to the development of asthma, bronchitis and other respiratory infections, as well as eczemaFootnote 2.
The level of risk depends on the extent of mould growth, regardless of the species, how long it has been present, and the susceptibility and overall health of the individuals exposed. Some people are considered to be at greater risk of experiencing adverse health effects from mould exposure, such as infants, children, seniors, pregnant people and those with respiratory conditions such as asthma.
Any health concerns suspected to be caused by poor indoor air quality, including mould exposure, should be discussed with a healthcare professional. These professionals are best suited to determine whether symptoms may be related to environmental factors or other underlying causes. In addition, tools exist to support healthcare workers in identifying health conditions associated with or worsened by exposure to mouldFootnote 3.
In some regions of Canada, a changing climate is predicted to result in increased moisture levels and mould exposure in the indoor environment. Changing or intensifying weather patterns such as rainfall, storms and flooding may produce higher levels of water infiltration and dampness in buildings; these are scenarios favourable for increased mould growth. Energy saving measures such as increased building tightness can also result in higher relative humidity and moisture indoors if there is not sufficient ventilationFootnote 4.
Health Canada recommends controlling dampness indoors and cleaning up any visible mould regardless of the type of mould presentFootnote 1.
Mould will typically grow indoors if moisture is present. Moisture problems may result from:
Moisture accumulates indoors when it cannot be vented outside and becomes a problem when building or other materials (e.g., drywall, wood, paper, textiles) become damp or wet.
Unless the cause of the moisture problem has been identified and solved, mould will reappear. To prevent future problems, measures should be put in place to control sources of moisture indoors, includingFootnote 6:
Run a dehumidifier in damp areas such as basements, and if moisture is condensing on cold surfaces such as window panes or relative humidity is > 50%.
See Appendix A for a moisture and mould prevention checklist designed for use in homes.
Personal protective equipment (PPE) should be worn while identifying and evaluating the extent of mould growth, as well as preparing for and undertaking the cleanup. Using PPE can help prevent health effects from exposure to mould. Below is a list of the minimum protective equipment recommended:
N95 respirators can be purchased from most home improvement stores. Make sure that the label says "N95" Footnote 2;Footnote 7. If N95 respirators are unavailable, consider using one with greater protection such as a P100 or N100 respirator. Other masks are designed to provide limited protection against dust and are not suitable for protecting against mould exposure. Make sure masks are properly fitted and you can breathe through them.
Mould growth can be hidden. It can grow behind walls or above ceiling tiles, so it is important to check for the presence of mould anywhere that is damp, and especially where water damage is known to have occurred.
In cases where a moisture or mould problem is suspected, a walk-through should be done of all rooms, as well as attics, basements, crawl spaces and storage spaces. Start with a visual inspectionFootnote 6; Footnote 8. Look for signs of mould as well as for indications of excessive moisture, such as stains or discolouration on floors, walls, window panes, ceiling tiles, fabrics and carpets. Additional signs of excessive moisture may include peeling paint, wrinkled wallpaper, cracks in plaster, warped wood, or efflorescence (white powdery salt crystals on the surface of walls or masonry). Check for obvious signs of leaks, condensation, or flooding, and for musty/earthy odours.
Immediate action is important. Mould will begin to grow in an area with excessive moisture within 48 hours.
If strong, musty odours are detected but no clear signs of water damage or mould are seen, consider consulting with a professional about hidden mould within walls and ceiling/floor cavitiesFootnote 5 (see Hiring a professional).
Inspect surfaces around plumbing, under and adjacent to windows and doors, and places where items such as wires pass through openings in the building's exterior. Condensation observed on windows and pipes should be noted. An association of odours with the start-up of fan-driven heating or cooling systems should lead to inspection of any accessible filters and humidifiers. If problems persist after these parts are changed or cleaned, the systems may need to be evaluated by qualified individuals.
On the building exterior, look for obvious signs of issues with building integrity (such as damage that may allow water intrusion), or deteriorated materials or surfaces (including missing downspouts or staining)Footnote 6; Footnote 8.
It is important to determine the extent of the mould problem to help decide on the path forward for cleanup. The following table can be used as a decision chart to facilitate categorization of mould area by size and the appropriate path forward for remediation, as well as whether professional assistance is required.
Three or fewer patches and the total area is less than 1 m2
Small mould patch on painted ceiling.
If there are more than 3 patches or if the patches are greater than 1 m² but less than 3 m²
Medium mould patch on painted ceiling.
If a single patch is larger than 3 m2
Large mould patch on painted ceiling.
An area of mould is considered small if it covers one square metre or less. There should be no more than three patches of mould, with the total combined area staying within one square metre. Many small patches of mould in one area or throughout the home or building can be a sign of a larger moisture issue that needs to be investigated and addressed immediately.
If ignored, small areas of mould may become larger over time, so it is important to address them immediately. A small amount of mould may be cleaned up using proper procedures and protective equipment.
The area of mould is considered medium if there are more than three patches of mould (each patch smaller than one square metre), but the total combined area is less than three square metres. In this case, while assessment by a qualified professional is recommended, in most cases a medium amount of mould may be cleaned up using proper procedures and protective equipment without involving a professional.
An area of mould is considered large if a single patch of mould is larger than three square metres. Immediate action and an assessment by a professional is required to determine the cause of the extensive mould growth and develop/implement a remediation plan.
Health Canada recommends that a qualified professional assess and clean up large areas of mould.
The presence of odour may also help determine the extent of mould growth. Subjectively rate mouldy or musty odours in each room as "mild," "moderate" or "strong," and take immediate action, in consultation with professionals, if there are any ratings categorized as "strong." Consider further investigation in cases where levels of mustiness are rated "mild" (but detectable) or "moderate" if there are any additional signs of mould, water damage, water stains, wetness or dampnessFootnote 2, Footnote 6, Footnote 8.
Any indoor mould growth should be removed. As discussed above, the underlying water or moisture problem that led to mould growth should be fixed first to prevent mould from returning. For example, the building envelope (i.e., roof, walls, windows, doors, foundation) must be repaired if moisture or water is entering your home from the outside.
As mentioned previously, anyone involved in mould cleanup should ensure that they are using appropriate PPE. Susceptible individuals, such as infants, children, seniors, pregnant people and those with respiratory conditions such as asthma, should not be in or near the area where the mould is being cleaned up. It is recommended to keep pets away from the area as well.
In a flood situation, put your own safety first. Avoid electrical shock. Shut the power off to the flooded area at the breaker box and ask the local electrical utility for help if needed. Wear rubber boots at all times while standing in water, and keep extension cords out of the water. See Flood clean up and indoor air quality for more informationFootnote 9 on how to properly clean up after a flood.
The following six-step process is recommended to prepare for mould cleanupFootnote 2;Footnote 10:
Step 1: Discard mouldy or damaged materials
Items that may need to be discarded include:
Step 6: Seek professional assistance, if necessary
As discussed above, a professional is generally not required to clean up small areas of mould if the proper procedures are followed and the right protective equipment is used.
In most cases, medium areas of mould can also be cleaned up safely and effectively by following the proper procedures and using the right protective equipment. Materials damaged by mould must be physically removed and disposed of under safe conditions. However, it is important to seek professional assistance if there is a large mould problem or if mould comes back after cleaning.
Materials with mould contamination that can be properly cleaned must be non-porous, such as glass, metals, leather, plastics and vinyl. Examples of washable surfaces include window sills, wood, hard surfaces and tiles.
Do not use bleach to clean up mould.
Mould that comes back after cleaning is usually an indication that the source of moisture has not been removed. Seek professional assistance.
In some cases, seeking advice on how to address moisture and mould problems indoors is necessary. A qualified professional with experience dealing with moisture issues and mould remediation can examine the condition of the home or building and document concerns. Qualified professionals can identify the moisture issues and sources and suggest a prioritized action plan consisting of various options on mitigation and remediation of moisture and mould problems.
Situations that require the involvement of a qualified professional include:
When considering working with a professional, be sure to solicit proposals and interview candidates the same way you would for home renovations. A preliminary site visit may be needed for the professional to understand the problem and generate an appropriate response. Ask the professional for his or her general approach to resolving the problem. A systematic approach based on a thorough visual inspection is usually more effective than relying on extensive air, bulk, or swab testing. Proposals should indicate the estimated fees and expenses for each planned phase of the project. Find out what criteria will be used to decide on testing or remediation strategies, and what potential follow-up investigation activities may cost. For major projects involving extensive mould growth, it is a good idea to make a written outline in advance laying out:
Here are some ways to recognize qualified professionals:
Health Canada does not recommend testing the air for mould.
In most situations, Health Canada does not recommend testing the air for mould. An air test does not provide information that can predict health effects and often offers little information on the cause of mould damage in the house. In some cases, however, where concerns remain unexplained after inspection for dampness and mould, especially if there is a history of flooding, roof leaks or pipe leaks possibly affecting wall cavities, a qualified professional may recommend a 'non-viable' air test for mould spores. This test involves microscopy done on machine-collected samples without culturing, and can be used to detect specific indicator mould types. High counts for these mould types may help direct qualified professionals to hidden problems within wall and ceiling cavitiesFootnote 2,Footnote 11.
The key to mould prevention is to remove excessive moisture build-up and control relative humidity through proper home maintenance and by following these steps:
Note: If renting a house or an apartment unit speak to the property owner about any moisture or mould problems. Information on landlord/tenant issues, rights and responsibilities is available from your provincial/territorial government.
For more information, please visit Health Canada's indoor air quality webpage or contact us at air@hc-sc.gc.ca.
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