Mould in Buildings

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 aggravated 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 OHSA 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 for mould growth. Water-damaged drywall, wood materials, jute, wallpaper, and cardboard are all 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). Using dehumidifiers to reduce the relative humidity in the flooded space is highly recommended. See below for more information on cleaning procedures.
  • Vent moisture-generating sources directly to the outside.
  • Control humidity with air conditioners and/or dehumidifiers.
  • Use exhaust fans.
  • Insulate cold surfaces to prevent condensation on piping, windows, exterior walls, roofs, and floors where possible.
  • Keep building and heating ventilation and air conditioning (HVAC) systems in good repair.
  • Reduce the amount of water used when cleaning carpets (as much as possible) and ensure adequate ventilation and humidity control.
  • Avoid installing 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. 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. 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.

When looking for mould, inspect:

  • Ceiling tiles
  • Walls (including wallpaper and the condition of drywall) 
  • Floors
  • Window sills
  • Insulation
  • Carpet
  • Furniture (condition of fabric, upholstery, etc.)
  • Ductwork and walls (use a mirror to observe hidden locations)
  • Condition of cardboard or paper

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

Cleaning procedures:

Level 1 - Small isolated areas

Level 1 cleanup refers to 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).


  • Occupants in the area should be advised of the presence of mould and the work area should be unoccupied. It is not necessary to vacate the entire building, but those who work close by, are immunosuppressed, have chronic lung problems (such as asthma or allergies), or are recovering from surgery, should be relocated during clean-up.
  • Workers must use a disposable respirator (N95 or better) as well as glove and eye protection. Impervious disposable coveralls should also be worn. A high-efficiency particulate air filter (HEPA) respirator will provide a higher level of protection.  
  • Seal openings immediately adjacent to work area. Drop sheeting can be used to isolate work areas.
  • Pre-clean any contents to be salvaged.
  • Dust suppression methods such as misting the surface lightly before cleaning is recommended. Remove mouldy materials 30 cm past the last visible mould line.
  • Clean the area with water and detergent.
  • Mouldy materials should be disposed of in sealed plastic bags.
  • Area should be dry and free of any visible contamination when the work is completed.   
  • Ensure the source of the moisture or leak is fixed to prevent conditions that may support the further mould growth.


  • The use of chemical disinfectants such as chlorine for remedial purposes is not recommended as use can pose health concerns for people in occupied spaces of the building.
  • Vacuuming may increase exposure to mould and spores by making them airborne. Central vacuums that exhaust to the outside, or those equipped with high-efficiency particulate air filters (HEPA) will minimize this exposure. The filter and contents of the HEPA vacuum must be disposed of in a well-sealed plastic bag.
  • Workers performing clean-up should follow good hygiene practices. Dispose of used PPE in sealed bags and wash hands and face.

Level 2 - Larger isolated areas

Level 2 cleanup refers to larger isolated areas (10 – 100 square feet, or less than 10 square feet in HVAC equipment in occupied spaces).

Inform the Safety Office upon discovery as it is necessary to have trained professionals assess the cleaning procedures, inspect, and perform environmental sampling. The OHSA requires that notification be made to the Joint Health and Safety Committee (worker member in attendance for environmental sampling).

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

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.

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.