Retired air tightness technician Trevor Clark talks about mould and steps you can take to reduce the likeliness of it in your home in his monthly column.

Mould is always associated with moisture, (from condensation and damp) and its growth can be inhibited by keeping humidity levels below 50%.

There are two common classes, which are: moisture induced growth of mould colonies and natural substances released into the air such as animal dander and plant pollen.

Moisture build-up inside buildings may arise from water penetrating compromised areas of the building envelope or skin, from plumbing leaks, condensation due to improper ventilation, or from ground moisture penetrating a building part.

Even something as simple as drying clothes indoors on radiators can increase the risk of exposure to (amongst other things) Aspergillus, a highly dangerous mould that can be fatal for asthma sufferers and the elderly.

In areas where cellulosic materials such as paper, wood and plasterboard drywall, become moist and fail to dry within 48 hours, mould mildew can propagate and release allergenic spores into the air.

In many cases, mould growth is suspected within wall cavities even if it is not immediately visible.

In a situation where there is visible mould the indoor air quality has been compromised, mould remediation may be needed.

Mould testing and inspections should be carried out by an independent investigator to avoid any conflict of interest and to insure accurate results.

The primary hazard of mould growth, as it relates to indoor air quality, comes from the allergenic properties of the spore cell wall.

More serious than most allergenic properties is the ability of mould to trigger episodes in persons that already have asthma or problems with breathing.

Below are types of mould and other allergens that can arise in households:

Carbon Monoxide (CO) acutely toxic indoor air contaminants is a colourless and odourless gas that is a by-product of incomplete combustion. Sources of carbon monoxide are tobacco smoke, space heaters using fossil fuels, defective central heating boilers and automobile exhaust. By depriving the brain of oxygen, high levels of carbon monoxide can lead to nausea, unconsciousness and death.

Volatile Organic Compounds (VOC) are emitted as gases from solids or liquids.

VOCs include a variety of chemicals, some of which may have short and long term adverse health effects.

Concentrations of many VOCs are consistently higher indoors (up to ten times higher) than outdoors. Legionella - disease is caused by a waterborne bacterium legionella that grows in slow-moving or still, warm water.

In temperatures below 60°C, exposure is through the creation of an aerosol effect, most commonly from evaporative cooling towers or showerheads.

Other bacteria - There are many bacteria of health significance found in indoor air and on indoor surfaces.

A large fraction of the bacteria found in indoor air and dust are shed from humans.

Among the most important bacteria known to occur in indoor air are Mycobacterium Tuberculosis, Staphylococcus Aureus, and Stretococcus Pneumoniae.

Asbestos Fibres Common building materials used before 1975 contain asbestos, such as floor and ceiling tiles, shingles, fireproofing, heating systems, pipe wrap, mastics, and other insulation materials. Normally, significant releases of asbestos fibre do not occur unless the building materials are disturbed, such as by cutting, sanding, drilling, or building remodelling.

Carbon Dioxide (CO₂) is a relatively easy to measure for indoor pollutants emitted by humans, and correlates with human metabolic activity. Carbon dioxide at levels that are unusually high indoors may cause occupants to grow drowsy, to get headaches, or to function at lower activity levels.

Ozone - is produced by ultraviolet light from the Sun hitting the Earth’s atmosphere (especially in the ozone layer), lightning, certain high voltage electric devices (such as air ionizers), and as a by-product of other types of pollution. Ozone exists in greater concentrations at altitudes commonly flown by passenger jets.

Particulates can be found indoors and can affect the health of occupants. Authorities have established standards for the maximum concentration of particulates to ensure indoor air quality.

Houseplants together with the medium in which they are grown can reduce components of indoor air pollution, particularly volatile organic compounds. Plants remove CO 2 and release oxygen and water, although the impact for house plants is small.

Plants also appear to reduce airborne microbes and mould. However, the increased humidity can itself lead to increased levels of mould and even VOCs.