Moulds are from the fungus group and they need water to grow. While their main function in the outdoor environment is degradation of wood, bark, shrubs and grasses, indoors they range from being unobtrusive microbial guests to obtrusive microbial pests.

Moulds can cause respiratory issues, and they may affect well-being in many ways. They are definitely a challenge to the immune system. When mould spores are encountered in vanishingly small amounts, the average uncompromised immune system may cope and even be strengthened by the odd encounter, but for the very same person encountering spores in larger amounts, it can lead to serious issues.

Indoors, a trained eye is a useful attribute when distinguishing what’s mould from what isn’t. If you are untrained in spotting mould, hire a trained tradesperson or microbiologist to come on site and do an inspection. The inspector always wears personal protective equipment (PPE), including a full-face breathing mask, tightly sealed to the face by rubber contours, one that also covers the eyes. That way, spores are not breathed in by the inspector nor wafted in the eyes when the inspector gets anywhere near to the mouldy area.

One key method the inspector uses, is to look for texture: a hairy appearance, a soft or feathery appearance, silky, like fine hairs, and blotchy. A trained operator will never touch the mould, as touching would spread the spores.

Another useful tool the inspector uses is judging the smell of a room. The human nose is naturally a sensitive detector of ”musty” odours, and heavy dusty odours. However, it is dangerous to assume that moulds will produce an odour, as they don’t always do so.

Finally, any telltale signs of moisture, showing that active mould growth may be occurring on or behind surfaces, could indicate to the inspector that there may potentially be a high spore concentration in the air.

In examining how moulds behave, an observer often poses questions about the invisible spores. Interestingly when explaining the behaviour of spores, it is essentially the same as the behaviour of other dry dust particles.

Many people have an intuition that spores are like gliders. This is actually correct. Spores have no power of their own. Wind movement and even air disturbance through a property, such as by human body movement, gets spores airborne, and then those spores eventually settle again after a few days or even weeks. Spores and other dust particles settle in undisturbed places, nooks and crannies. The indoors has many undisturbed areas, due to the lack of strong wind.

Consider a homeowner asking a tradesperson “what differences are there between indoor and outdoor dust?” If the home is clean, a part of the answer would be “human skin flakes.” These flakes result from the natural shedding of our outer layers, no matter how often we shower. These flakes build up indoors in those same still places where spores and other dust build up. Flakes can get into the pile of a carpet for instance, among other places. Shedding is relevant to mould for one reason: moulds can consume human skin flakes, as a nutrient base.

It’s important when vacuuming any space to use a HEPA vacuum, so that the finest most allergenic particles are not merely resuspended in the atmosphere when they exit out of the hot end of the vacuum cleaner. Skin flakes are clearly not allergenic, but you want them to be gone to avoid “succession” by moulds and then dustmites (miniature insects) in turn. Dustmites are not fussy and will eat skin flakes or mould, but correlations show that they either prefer the skin half broken down by mould, or that they actually prefer mould as food. Given that dustmites can be a cause of allergic reactions for some people, vacuuming to remove the food sources of dustmites starts to look mighty appealing for the people occupying a property.

While skin flakes are an appetising treat for moulds, the more important observation is that moulds can easily use not just wooden products but also paper products as nutrients. Specifically, they consume the component called cellulose, – the fibres present in the wood and paper. Moulds break these fibres down microscopically, right on the surface that contacts air. Notably, this ability includes infesting the paper finish present on gyprock/sheetrock and using it as a nutrient. Moulds need oxygen, like people do, which is why they like the surface of the gyprock paper rather than penetrating deeper into that paper layer by habit.

There is almost nothing that moulds cannot eat, including paint, varnish, some grouts and even concrete at a pinch. They need a carbon source, and they thrive best when there is a rich source present. Wood, paper and exfoliated skin contain carbon. Oils and proteins from our cooking are carbonaceous too. In short, moulds can metabolise any material that is derived from living things. They may also colonise mineral-based materials, because those materials can gather dust and oils or they may naturally contain traces of organic matter.

Moulds are sometimes hidden. The growth phase may have occurred far from the site where spores land, or the growth site may be hidden below or behind a building material, such as walls, fabric, furniture, or flooring. In such cases people may be largely unaware of mould particles.

Once we are aware of mould particles in a property, it is important to know how we can control them.

In a property that has no water intrusion and no condensation issues, keeping spores at a healthy level is related to dust control. Control can be as simple as regular vacuuming, plus occasional spring cleaning in any season, and wiping all horizontal surfaces regularly with a damp soapy cloth while rinsing well between wipes. Keeping the house dry is the conscious aim, and dust removal is another objective that’s just as necessary. Rather than spreading dust from place to place, removal means completely removing it from the premises. This can be as simple as emptying the vacuum’s dustbag into the bin outside.

If any nutrients and liquids are present, then ever-present spores may germinate indoors, and mycelia can then grow to produce more spores. This can potentially occur wherever water intrusion occurs. Also, growing moulds may thrive wherever condensation occurs. Condensation relates to the fact that air contains water molecules. Ambient heat gives kinetic energy to water molecules, preventing them from forming into vapour droplets. We all call this humidity. As humid air cools, some of its water molecules drop out as vapour. This is because the kinetic energy separating the water molecules has lessened.

Condensation occurs in a range of places, including in air conditioner units, on and around window panes, and in bathrooms, laundries, and kitchens. Moulds can thrive on any surface that has a temperature significantly lower than the temperature of humid air which contacts them. Tradesmen’s terminology for this is that a localised temperature low creates a “dew point” issue. Condensation can occur on any type of surface and tends to occur where air movement is limited. Condensation is only an issue with humid air.

Precautions can be taken with humid air to prevent condensation. The following invariably need ventilation: a laundry room with a clothes dryer, a shower room, and a kitchen. Look to your local regulatory authorities to determine the amount of natural or mechanical venting required for each moisture source, including adequate duration of venting.

Moulds thrive at any temperature, because the main determinant of growth in areas containing sufficient nutrients is presence of water. Nevertheless, they will grow fastest in warm tropical and equatorial climates, which happen to also be the ones with higher humidity. This results in damp cellulose to match the damp atmosphere, and potentially more condensation issues. A general rule of thumb for mould or bacterial metabolism, is a doubling of growth rate for every 10 degrees Celsius, but topping out at around 30 degrees Celsius. Since winter is the rainiest season in “temperate” (cool) climates, and brings high humidity, moulds have to take what they can get but they relish dampened areas, whatever the temperature.

Moulds don’t use or need light, and so are not a bright lime-green colour. Green algae are sometimes mistaken for mould. This is because algae can grow in some of the same places that moulds do in the tropical indoors. Moulds are not scared of indirect sunlight, though they shy away from direct light. Their natural place is a forest floor under a canopy. Similarly, bacteria are not moulds. However, some people may be sensitive to aerosolised compounds from bacterial films. Such bacterial biofilms can also be forerunners to the colonisation of a surface by mould.

Blackness stemming from fire damage may not be black mould, even though often fire damage and water damage co-occur. Water damage doesn’t always imply mould, as water can pick up dark material and then stain in dark curvy shapes. These instances of staining differ from mould by lack of surface texture.

Upon close inspection with a magnifying glass, moulds have a 3D aspect. They are slightly raised off the surface, so that after infestation even a smooth decor material is now textured. In short, a trained eye is a useful attribute in several ways, when looking for mould. When professionals get close enough to mould to see it, they don’t breathe in! Don’t try this at home!

  • Nice post Bob!

  • A nice overview Bob. Are you a member of the Indoor Air Quality Association (IAQA)? They are pushing for better recognition of mold and air quality problems here in Australia and would benefit from working with you.

  • A good, informative rundown of the issues associated with mold. I imagine this is a serious issue for many people.

    Thanks Bob.

  • Thanks Sean, Bob and Ben. The aim has been to synthesise information into a wholistic picture that also gives the reader tools to judge a situation when encountered. The initial series of articles will develop this theme. The aim is always empowerment, through education. We are all so naturally curious, and good journalism leverages curiosity, to facilitate learning. Sean, I have many tradesmen contacts, and as you say, this blog fills a gap by walking the fine line between that community and the microbiology discipline. Traditionally each community of specialists has gradually developed a fixed stance, largely because of isolation from the other. I work with industry and academia alike, so yep, peak bodies are a natural draw. Its great if the distinctions between the general public and tradesmen/specialists are also made more porous, so I'll start commenting individual comments in all cases where thats clearly appropriate. I hope I see all comments once they ramp up. The comments are all welcome.

  • Morning Bob,
    I have been actively involved in delivering a solution for building owners to strike the right balance between saving energy and inactivating mould in commercial buildings. The cooling coil component of the air handler is the perfect medium for growing mould and we position high-output ultraviolet lamps in close proximity to the cooling coil to bathe the coil 24/7 in UVC light. This improves the performance of equipment delivering energy savings often returning old equipment back to their as built capacity along with reducing the microbial activity within the ducting system.
    We can validate all that we do and building owners here in the UK are looking to save energy along with improving their indoor air quality.
    Health, Wellbeing & Productivity improvement along with reducing staff absence in commercial buildings are now big drivers for major companies and our UVC Solution is now being introduced to their facilities.

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