Hazard summary

Workers and the public may be exposed to mould on water-damaged building materials inside buildings, and during building maintenance and repair operations. The most common types of mould are generally not hazardous to healthy individuals–but some moulds may be hazardous to certain individuals.

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.

This resource does not replace the Occupational Health and Safety Act (OHSA) and its regulations, and should not be used as or considered legal advice. Health and safety inspectors apply the law based on the facts in the workplace.

Hazard locations

Moulds (fungi) are present everywhere–indoors and outdoors.

Any building may have mould. However, 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.


The sustained and/or extensive growth of any visible mould on the interior surfaces of a building is unacceptable. Mould growth on the interior surfaces of buildings is a risk factor for health problems.

Moisture problems (flooding, leaks, water intrusion, condensation, etc.) in buildings are the primary reason for mould growth. These moisture problems should be the focus of assessment and control efforts, followed by clean-up, remediation of contaminated materials, periodic inspections, and preventive and remedial maintenance. The Internet resources below contain detailed information. If mould contamination is extensive, the employer should seek professional assistance from private sector consultants who specialize in mould and mould remediation.

Occupants of buildings contaminated with mould should be advised of the presence of the mould and given information on the health effects of mould.

Legal requirements

Employers are required by section 25(2)(h) of the Occupational Health and Safety Act 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. Various sections of the Industrial, Construction, Mining or Health Care regulations may also apply to maintenance and remediation activities.

Resources on corrective measures, mould remediation and worker training are listed below.


Health Canada

Environmental Abatement Council of Ontario (EACO)

U.S. Environmental Protection Agency (EPA)

U.S. Centers for Disease Control (CDC)