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Environmental and Workplace Health

Mould in Indoor Air

Mould is a common term referring to various species of fungi that are a natural part of the environment. If mould finds a damp place to grow inside a home, however; it can lead to respiratory problems and allergic reactions among some people. Mould is one of the most commonly found indoor air pollutants.

What are the health risks of mould?

Health Canada considers indoor mould growth to be a potential health hazard. Scientific evidence links mould and damp conditions in homes to increased risk of respiratory symptoms, including

  • Eye, nose and throat irritation
  • Coughing and phlegm build-up
  • Wheezing and shortness of breath
  • Other allergic reactions

People have different sensitivities to mould, so not all people will react when exposed. Although it seems clear that exposure to mould can worsen the symptoms of asthma, it is still unclear whether or not it actually causes asthma in otherwise healthy people. Some airborne moulds can cause severe infections in people with severely weakened immune systems, such as transplant recipients or people with leukemia or AIDS. People who are concerned their health is being affected by mould should talk to a physician.

What do I do about mould in my home?

Mould growth indoors is the result of too much moisture from, for example, leaks in plumbing or the building's exterior, flooding or high humidity. Mould can grow on a variety of surfaces and materials (for example, cloth, wood, paper, insulation, drywall) if there is enough moisture. The key to preventing mould is preventing excessive moisture through proper home maintenance and by following these simple steps:

  • Promptly repair any leaks
  • Use kitchen and bathroom exhaust fans when cooking and showering
  • Make sure clothes dryers are properly vented to the outside
  • Make sure tubs and sinks are sealed tightly so water doesn't leak into the walls
  • Humidity in the house should be kept at or below 50% in the summer, and 30-35% in the winter, using a dehumidifier if necessary
  • Mould can grow on fabrics, food, paper, wood and practically anything that holds moisture, so these items shouldn't be stored in damp areas such as basements
  • After a flood be sure to completely dry the affected area within 48 hours

If you find a small or moderate amount of mould, you can generally remove the mould yourself. For hard surfaces, simply wash the mould off using soap and water. There is generally no need to use bleach, as it can also be a respiratory irritant. Be sure to fix the moisture problem that allowed mould growth in the first place or it will come back again. You might consider hiring a professional if there is a large amount of mould or if the mould keeps coming back after it has been removed.

What are Health Canada's recommended levels for mould?

Health Canada does not have any numerical exposure limits for mould. Since people have different sensitivities, it is not possible to establish a "safe" limit for mould. Health Canada recommends removing any mould found growing indoors and fixing the underlying moisture problem.

A publication entitled Indoor Air Quality in Office Buildings: A Technical Guide, published by Health Canada in 1995, did provide some advice on how to interpret mould air testing done in large office buildings. It included some numerical limits for spore counts (in CFU / m³) that are often misinterpreted as Health Canada guidelines. The numbers provided are typical values found in sampling done in federal government buildings by the author, using a specific sampling method. They are meant to help guide investigations in office buildings, where mould is suspected but was not located by a visual inspection. They do not represent a "safe level" of mould, are not applicable to homes, and in no way represent an official Health Canada guideline or recommendation.

The document has been removed from the Health Canada Indoor Air website. Health Canada considers that this publication requires updating but that this task does not fall within the current mandate of the Indoor Air program. The guide was originally published when Health Canada was responsible for responding to requests for air quality investigations in federal buildings. This activity is now carried out by Public Works and Government Services Canada.

What about toxic moulds?

Other non-respiratory symptoms have been attributed to exposure to mould in buildings, especially to so-called "toxic" mould species often referring to "black mould" or the genus Stachybotrys. While it is true that some moulds do naturally produce toxic substances (known as mycotoxins), there is insufficient evidence that these moulds have different or greater health effects than the other moulds. All moulds pose a risk to respiratory health and should be removed.

For more information, please see the Next link will take you to another Web site NCCEH evidence review on the Health Effects of Mould Exposure in Indoor Environments.

How do I find mould in my home?

If you are worried you may have mould, start with a visual inspection of your home. Look for signs of mould or excessive moisture, such as stains or discolouration on floors, walls, window panes, ceiling tiles, fabrics and carpets, obvious signs of leaks or flooding, or a musty "earthy" odour. Not all mould is obvious. It can grow inside 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 has occurred. If necessary, you may contact a home inspector or contractor with experience dealing with moisture issues and mould.

Should I test for mould?

Health Canada generally does not recommend testing the air in homes for mould. Mould is a natural part of the environment and there are always mould spores in air. It only becomes a problem if it finds a damp area to grow indoors. So simply finding mould spores in air tests doesn't necessarily mean there is a problem.

An air test for mould will not help identify the source of the moisture or how to fix it. You also do not need to know the type of mould. All mould species can cause health problems and there is no "safe" level of indoor mould, as everybody is different in their sensitivity to mould. Instead, if you are worried you may have mould, start with a visual inspection of your home.

Air testing for mould may be helpful in large office or residential buildings where it is difficult to do a complete visual inspection to find mould or to confirm that all mould has been removed following remediation. Such mould testing must be designed and conducted properly and should only be done by an experienced professional.

For more information see the Next link will take you to another Web site NCCEH evidence review on Mould Assessment.

What should I do after a flood?

In the event of a flood, take action immediately to remove the water and dry any items that are wet. Items made of material that may hold moisture such as fabric, wood or paper may have to be thrown away. Drying your house and furnishings within two days will reduce or prevent mould growth and greatly reduce repair costs. For more detailed information on dealing with floods, please see the CMHC publication Next link will take you to another Web site After the Flood.

How do I remove mould growing in my home?

The amount of mould is generally defined by the size of the area covered. The CMHC, for example, classifies the amount of mould as

  • Small, if there are 1 - 3 patches, each less than 1 m² (10 square feet) in size
  • Moderate, if there more than 3 patches or if the patches are greater than 1 m² but less than 3 m² (32 square feet)
  • Extensive if the patch is larger than 3 m²

People can generally clean small and moderate areas themselves with soap and water, but you should consider getting professional help with extensive mould growth. The person doing the work should wear proper protective equipment, including rubber gloves, eye protection and a dust mask. You may also want to isolate the area by taping plastic sheeting to walls and ceiling to prevent the spread of dust and mould particles. Sensitive individuals should not be in the same or adjacent rooms during the work and may choose to leave the house all together until the mould is removed.

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

Workplace health and safety organizations often employ a similar, sized-based classification system that classifies mould as Level I (< 1 m²), Level II (1-3 m²), Level III (3 - 10 m²) or Level IV (> 10 m²). For each level, there are recommended procedures to protect the health of clean-up personnel and people working nearby during the remediation. Some mould inspection and remediation companies may employ these classification systems when dealing with mould problems in homes. For more information on how to interpret these types of classification systems, please refer to a respected workplace safety agency or organization such as the Next link will take you to another Web site Canadian Centre for Occupational Health and Safety or the Next link will take you to another Web site United States' Occupational Safety and Health Administration.

For more detailed information on removing mould see the Next link will take you to another Web site CMHC publications or the Next link will take you to another Web site NCCEH evidence review on Mould Remediation.

Where do I go for more information?

For a review of the science detailing the health effects of mould, please see Health Canada's Residential Indoor Air Quality Guideline for Mould.

The Next link will take you to another Web site National Collaborating Centre for Environmental Health (NCCEH) has also published evidence reviews on the Health Effects of Mould Exposure in Indoor Environments; Mould Assessment and Mould Remediation.

The Canada Mortgage and Housing Corporation (CMHC) [Link to] has information on how to fix moisture problems in your home and how to remove mould, including:

  • Moisture and Air: Householder's Guide -- Problems and Remedies
  • Choosing a dehumidifier
  • The Condominiums Owners' Guide to Mould

If you rent your home, there may be a limit to what you can do to correct indoor air problems. Ideally, you and your landlord will be able to come to an arrangement to deal with any problems. CMHC's Next link will take you to another Web site The Tenant's Guide to Mold is written specifically for people who rent.

The provinces and territories regulate landlord and tenant relationships. You can contact your provincial or territorial government for more information. You may also want to contact your local municipal government, as they may have by-laws on acceptable standards for rental properties.

If you suspect an indoor air quality problem in your workplace, you should first contact the Occupational Health and Safety Officer or Committee of your workplace. The provinces and territories regulate workplace health and safety, except for federally-regulated sectors (like federal buildings, banks and transportation sectors) which fall under the Canada Labour Code or workers in federal buildings.

What are the Residential Indoor Air Quality Guidelines?

The Residential Indoor Air Quality Guidelines are Health Canada's official position on the health risks posed by a specific indoor contaminant, based on a review of the best scientific information available. They summarize the known health effects, detail the indoor sources, and provide a recommended exposure level below which health effects are unlikely to occur. The Guidelines are recommendations only and are not an enforceable standard under any regulation. They are meant to serve as a scientific basis for activities to reduce the risk from indoor contaminants. This could include the development of regulations or standards or the production of communication materials aimed at the general public.