Workers and the public may be exposed to mould on water-damaged building materials inside buildings, and during building maintenance and repair operations.

Mould hazard summary

The most common types of mould are generally not hazardous to healthy individuals, but some moulds may be hazardous to certain individuals.  The sustained and/or extensive growth of any visible mould on the interior surfaces of a building is unacceptable and remediation is required.

People who have asthma, bronchitis, hay fever, other allergies, or have weakened immune systems are more likely to react to mould. The most common symptoms are runny nose, eye irritation, skin rash, cough, congestion and aggravation of asthma. Symptoms usually disappear after mould exposure stops. Most often, there are no known long-term consequences to workplace exposures.

Occupational Health Safety Act requirements

Employers are required by section 25(2)(h) of the Occupational Health and Safety Act (OHSA) to take every precaution reasonable in the circumstances for the protection of workers.

The Occupational Health and Safety Act places a responsibility on constructors (section 23), employers (section 25), and supervisors (section 27) to ensure the health and safety of workers.  This includes protecting workers from mould in workplace buildings. Occupants of buildings contaminated with mould should be advised of the presence of the mould.

Hazard locations

Moulds (fungi) are present everywhere, indoors and outdoors. Buildings with a history of water leaks, floods, fires and problems with indoor air quality (e.g. poor humidity control, lack of fresh air) should be considered at greater risk of mould growth. Water-damaged drywall, wood materials, jute, wallpaper, and cardboard are prone to fungal growth.

All moulds need water to grow. Mould can grow anywhere there is water damage, high humidity or dampness. Most often moulds are confined to areas near the source of water. When mouldy material becomes damaged or disturbed, mould spores can be released into the air. Exposure occurs if people inhale the spores.

In modern buildings, moisture problems may exist as the result of:

  • flooding,
  • leaks in the roof or plumbing,
  • sealed buildings that do not allow excess moisture to escape,
  • excessive humidity.

Preventing mould contamination

The best method of mould prevention is to reduce the amount of moisture by keeping the relative humidity between 30% and 50%. Prevention measures include:

  • Immediate clean-up of any floods or spills (within 24-48 hours). Use of dehumidifiers to reduce relative humidity in the flooded space is highly recommended.  See below for more information on cleaning procedures
  • Venting moisture generating sources directly to the outside
  • Control of humidity with air conditioners and/or dehumidifiers
  • Use of exhaust fans
  • Insulation of cold surfaces to prevent condensation on piping, windows, exterior walls, roofs and floors where possible
  • Keeping building and heating, ventilation and air conditioning (HVAC) systems in good repair
  • Reducing  the amount of water used when cleaning carpets as much as possible, providing adequate ventilation and humidity control
  • Avoiding installation of carpet around fountains, sinks, or directly on top of concrete floors that are prone to leaks or frequent condensation

Note: It is important to keep air conditioners and dehumidifiers in good condition. Empty any water collectors regularly so water does not contribute to moisture problems.  If using humidifiers, ensure they are cleaned regularly.

Inspecting for mould

A visual inspection is the most reliable method of identifying mould problems. Common signs of water damage will be discolouration and staining. Moulds will most often appear as dark spots, stains or patches. Inspect the following:

  • Ceiling tiles
  • Walls including wallpaper, and condition of drywall (Sheetrock (R) [USG], gypsum wall board)
  • Floors
  • Window sills
  • Insulation
  • Carpet
  • Furniture (condition of fabric, upholstery, etc.)
  • Duct work and walls (use a mirror to observe hidden locations)
  • Condition of cardboard or paper present

Look for puddles of water around or under sinks, tubs, drip pans for dehumidifiers, air conditioners, and refrigerators that may be contributing to the moisture in the building and provide conditions for mould growth.

Monitoring devices can be used to measure the moisture level of drywall, wood, etc. These devices will help indicate whether or not moisture levels exist that would promote the growth of mould.

Steps to take if mould is discovered

1) If moisture problems are noticed or mould is discovered, immediately contact Plant Operations Service and Maintenance (ext. 33793) to investigate and determine the extent of damage and cleanup requirements.

2) Mould clean up is broken down into three levels and depends on the extent of damage and whether the space is occupied. 

Contact the Safety Office (ext. 33587) to notify if level 2 or level 3 remediation is required or if there are any questions about steps to take.

3) Communicate with building occupants at all stages of process.

Designate a departmental contact person for questions/comments if the clean-up is more than a level 1 remediation

Level 1 - Small isolated areas

(less than 10 square feet of mould growth, or HVAC equipment with less than 10 square feet of growth in unoccupied areas)

Level 2 - Larger isolated areas

(10 – 100 square feet, or less than 10 square feet in HVAC equipment in occupied areas)

Level 3 – More than 100 square feet of mold-affected material

(or more than 10 square feet in HVAC equipment in occupied areas)

It is recommended that porous materials such as dry wall, ceiling tiles, fabric or carpet be thrown out and replaced rather than cleaned whenever possible. Non-porous materials such as metal, glass, hard plastic and semi-porous materials such as wood and concrete can be cleaned and reused (if structurally sound). In all situations, the underlying moisture problem must be corrected to prevent recurring mould growth.


Ontario Ministry of Labour

Canadian Centre for Occupation Health and Safety

Guidelines on Assessment and Remediation of Fungi in Indoor Environments. New York City Department of Health

Health Canada: Fungal Contamination in Public Buildings