How to work it out
Use both the number of patches and the largest patch area to choose the Health Canada category.
- Convert square feet to square metres before comparison.
- Record the largest individual patch as well as the total.
- Recurring growth or an unresolved moisture source still requires escalation.
Health Canada visible mould categories
| Category | Patch guidance | Response |
|---|---|---|
| Small | total ≤1 m² and no more than 3 patches | follow small-area guidance |
| Medium | >3 patches or a patch >1 m² but <3 m² | use the medium-area precautions |
| Large | one patch >3 m² | professional assessment and cleanup |
Plan the job
Count and measure separately
Record each accessible patch and calculate both total visible area and largest individual patch.
Correct moisture
Find and address leakage, condensation, drainage, or ventilation conditions that enabled growth.
Escalate the full condition
Use professional help for a large patch and for recurring, concealed, contaminated, or otherwise unsafe conditions.
Before you start
- Do not disturb suspect asbestos- or lead-containing material while investigating mould.
- Follow Health Canada protective and cleanup directions, not bleach folklore.
Sources and notes
Source links reviewed July 16, 2026. A review date is not the document's publication date.
- Health Canada: Addressing moisture and mould in your homeCanada · government guide
The moisture source must be corrected; large or recurring growth calls for professional assessment.
- National Institute of Standards and Technology: NIST Guide to the SI, Appendix B — Conversion FactorsUnited States · government standard
Code retains exact defining constants where NIST identifies an exact relationship.