Environmental and Occupational Health Pearls: Mould

A bi-monthly feature providing clinical information and resources from Public Health Ontario on matters relating to environmental and occupational health. This edition considers questions about mould for family physicians.

What's important for family physicians to know?

  • What is mould? Mould refers to multicellular fungi. While moulds are part of the natural environment they can also grow indoors. [i] They may contribute to poor indoor air quality as spores and fragments may be inhaled and their metabolites may be toxic. They may appear as a stain that can vary in colour and may have a musty odour.
  • Where can mould be found? Indoor mould growth requires moisture.i As a result, mould can often be found in damp areas created by water leaks, flooding, or daily activities such as showering and cooking.[ii]
  • What are the health effects? Sufficient evidence suggests that presence of mould indoors is associated with upper respiratory symptoms, cough, wheeze, asthma symptoms in sensitized asthmatic individuals, and hypersensitivity pneumonitis.[iii] Limited evidence exists on the association between indoor mould and respiratory illness in otherwise healthy children. Indoor dampness-related factors have also been associated with asthma development, dyspnea, current asthma, and respiratory infections.[iv]
  • What can I do? Patients should be advised to prevent indoor mould growth. Prevention of mould growth requires moisture control, which may include timely repair of water leaks, floods and spills and reducing clutter that may support mould growth.  Indoor humidity should be limited to 50% in summer and 30% in colder weather. ii,[v] Appropriate ventilation during moisture generating activities such as cooking and showering may also help prevent mould growth. Instructions on indoor mould removal are available from Health Canada. ii

[i] Health Canada. Residential indoor air quality guidelines – moulds [internet]. Ottawa: Health Canada; 2007. Available from:

[ii] Government of Canada. Reduce humidity, moisture and mould [internet]. 2015. Available from:

[iii] Institute of Medicine: Committee on Damp Indoor Spaces and Health. Damp Indoor Spaces and Health. Washington  D.C: The National Academy Press;  2004. ISBN 0-309-09193-4.

[iv] WHO Regional Office For Europe. WHO guidelines for indoor air quality: dampness and mould [internet]. Copenhagen: World Health Organization; 2009. Available from:

[v] Canadian Centre for Occupational Health and Safety. OSH answers fact sheets: indoor air quality – moulds and fungi [internet]. 2015. Available from: