CAW National Council 4000
Occupational Health and Safety

Hazardous Substances:  Moulds and Fungus

The Problem

Poor indoor air quality can make workers sick. Moulds (also known as fungi) in buildings can cause a variety of illnesses. Moulds are
tiny microorganisms normally present in indoor air. They thrive in moist conditions. A common mould found in homes is mildew. Some
toxic moulds have caused so much illness that classrooms, courthouses and other public buildings have had to be closed.
Occasionally a particular type of mould can be toxic enough to cause death.

Ironically, moulds have been found in hospitals, the very buildings designed to help people get well. One toxic mould, stachybotrys
chartarum has been particularly troublesome in health care facilities where CAW members work. It is a greenish-black mould that
will grow on materials that have become chronically moist or water damaged due to leaks or flooding. This mould produces toxic
substances including trichothecenes and volatile organic compounds (VOCs). A number of our members have fallen ill as a result of
exposure to this mould.

Moulds multiply easily in certain conditions found inside buildings. Ideal breeding conditions for moulds are:

  • wet surfaces
  • excess humidity
  • poor ventilation
  • improper maintenance of the ventilation (HVAC) system
  • poor maintenance in general
  • windows that do not open
  • dust
  • a lot of people
  • a lot of carpeting

Walls, floors, carpets, ceilings (including the real ceiling behind a drop ceiling), windowsills and frames, stairwells, storage areas,
and basements can all be good growth areas for mould if conditions are right. All parts of ventilation systems such as air intakes,
filter units, cooling/heating fans and coils, spray humidifiers, reservoirs, ducts, insulation, induction and fan coil units, drain and
condensate pans and sumps that are dirty or wet can be sources of moulds. Since mould spores are airborne, they can spread
easily through the ventilation system, so mould in a basement, for example, might circulate throughout the building.

Types of Illness

Some moulds can release toxic substances that can cause allergic reactions and flu-like symptoms. Others can cause cancer or are
immunosuppressants (can weaken the immune system).

It doesn't matter if moulds are classified as pathogenic or non-pathogenic. Both can harm your health.

Specific health effects associated with moulds:

  • runny nose
  • eye irritation
  • cough
  • congestion
  • shortness of breath
  • asthma
  • fatigue
  • headache
  • respiratory infection
  • skin irritation
  • systemic infection
  • neurobehavioral and cognitive effects
  • hypersensitivity pneumonitis (extrinsic allergic alveolitis)
  • long term and cumulative effect - cancer
  • suppression of immune system
  • for those with a suppressed immune system, susceptible to serious infection from moulds

How to Detect Moulds in Buildings

Inspect the building on a regular basis. Look for signs of water damage. Besides water or dirt stains on carpets, walls or ceilings,
other tell-tale signs include musty odours, powdery mineral deposits (remaining after the water has evaporated) on walls or
concrete, and softened wood or drywall. Wet floors and walls and indoor condensation on windows support microbial growth that
may not be visible during an inspection.

Dealing with Mould

If mould is found, do not disturb it. Report it immediately to your employer. Make sure removal is done with proper protective

If you are cleaning mould from hard non-porous surfaces, do it when the building is unoccupied. Use appropriate personal protective
equipment (PPE) such as respirators and gloves. Wash all smooth surfaces that have been contaminated by moulds with diluted 5%
bleach (250mL/4L water).

Cleaning water-damaged porous materials such as carpets, insulation or ceiling tiles is not recommended. These materials should
be completely removed; use appropriate personal protective equipment when disposing of them.

Preventing Growth of Mould


Ensure that there is sufficient ventilation and the HVAC (ventilation) system is working properly. Ensure preventative maintenance,
servicing and cleaning are performed regularly.

Control Moisture

  • Fix leaks and seepage.
  • Put a plastic cover over dirt in crawlspaces, and make sure crawlspaces are well-ventilated.
  • Use exhaust fans in washrooms and kitchens to remove the moisture to the outside.
  • Turn off humidifiers if you notice moisture on windows or other surfaces.
  • Use dehumidifiers and air conditioners to reduce moisture in air but keep them clean.
  • Raise the temperature of cold surfaces where moisture condenses. Use insulation or storm windows. (A storm window
    installed on the inside works better than one installed on the outside.)
  • Open doors between rooms to increase circulation.
  • If using carpet on a concrete floor, it may be necessary to use a vapour barrier (plastic sheeting) over the concrete, covered
    with sub-flooring (insulation covered with plywood) to prevent a moisture problem.

Ensure Ventilation Systems is Well Maintained

Prevent accumulation of stagnant water in and around HVAC system mechanical components such as under cooling coils. Maintain
the relative humidity of indoor spaces at less than 60%.

In HVAC systems, use steam for humidification rather than recirculated water and spray humidifiers where feasible. If spray systems
are used, a rigorous preventive maintenance program must be employed, as these systems can easily become contaminated with
bacteria and moulds. This includes maintenance of slime-free surfaces and the addition of potable water to the reservoir. Humidifiers
should be drained and cleaned with chlorine bleach at intervals of 2-4 months. Rust and scale deposits should be removed from
HVAC system components once or twice a year. HVAC systems should be turned off during cleaning operations, which should be
scheduled during unoccupied periods.

Porous synthetic insulation is often used to line ducts and air handling and induction units. The vapour barrier on fibreglass should
be intact. There should be no standing water or condensation on these surfaces. Dirty, contaminated insulation should be removed,
as the effectiveness of cleaning or encapsulation has not been verified.

It is important to use efficient filters to control the load of spores from moulds entering the air handling system. Use prefilters and
extended surface-type secondary filters with dust-spot efficiency ratings higher than 85% when possible. Replace the filters at
regular intervals. The prefilters are normally changed 4-6 times a year and secondary bag filters once a year, depending on outside
conditions and retrofit safety.

The information in this fact sheet is taken from information provided by the Canadian Centre for Occupational Health and Safety and
Health Canada.

Source:  CAW Health, Safety & Environment Department