Indoor Moulds – The Definitive Guide


Mould is recognized by its fetid smell, nauseating tones of colour, and its fuzzy texture. But what is mould? Are moulds just a biological nuisance that happens to appear, or are they something more? 

While there is a diverse set of moulds, the most commonly seen and interacted with are considered to be fungi. Specifically, moulds are multicellular organisms that are made up of expanding and interconnecting branches known as ‘hyphae.’ These branches are collectively referred to as ‘Mycelium’, and despite being made of numerous segments, are all considered one organism. Mould hyphae spread themselves out to find and consume food sources to aid in asexual reproduction. This expansion is what causes the ‘fluffy’ texture that the fungus is infamous for. 

Classifications of Moulds

There are different kinds of moulds, with some being considered fungi and others as separate ‘eukaryotic’ organisms.

Fungal Moulds

The most commonly encountered moulds are considered to be fungal moulds, which encompass hundreds of species. These species of microorganisms that comprise fungal mould form colonies in the form of interconnecting hyphae. They use this network of hyphae to consume biological matter. The ideal food for moulds range from starches, cellulose, and other biopolymers, which is why moulds are typically found growing on food. 

Slime Moulds

Unlike fungal moulds, slime moulds are a collection of single-celled organisms that can communicate and form together. They are characterized by the distinctly bulbous, slimy, gelatinous mass that they develop into when food sources are scarce. Otherwise, these organisms will spread out to find food. This forms vein-like structures to create an interconnected network that represents the entirety of the slime mould. 

While being brainless and technically composed of smaller individuals, these moulds are excellent at creating efficient paths to get to food sources. When the separate microorganisms find enough food, they form a structure called a ‘plasmodium,’ which is the slimy structure the mould is recognized as. 

Slime moulds feed off of decaying plant matter, and other microorganisms. For this reason, slime moulds are commonly found on rotting logs, mulch, lawns, rain gutters, and anywhere that has wet, decaying plant-life. 

Water Moulds

Also known as ‘Oomycete’, water moulds are a pathogenic mould species that afflict plants and some animals. Water moulds attained their namesake for the conditions in which they grow, including humid environments with running surface water. 


Common Indoor Moulds


Alternaria is commonly known as a plant pathogen, being a significant source of agricultural food spoilage. When found indoors, it poses as an allergen to humans that can sometimes lead to asthma and other irritations. Colours associated with this mould are green, black, and grey.



Aspergillus mould spores can be found everywhere, though they cannot be seen. Like most moulds, aspergillus will consume plant material. Additionally, this species is known for invading buildings, commonly favouring humid spaces like bathrooms. For individuals with a low immune system, the mould can become a threat as it evolves into a condition called aspergillosis. Aspergillosis occurs when the mould can enter into a human airway, causing infection. 



This fungal mould is described as a ‘black yeast’, referring to its appearance as a black, flat, shiny, moist mass. Aureobasidium is a species known for infesting produce such as cabbages and potatoes without being apparent. 



Chaetomium moulds are described as being white and fluffy, and are commonly found in soil, dung, and plant matter. This species is known for its dusty smell, and is known to infest buildings with water damage. The mycotoxins that this mould produces can be toxic to humans. 



Cladosporium is commonly found both indoors and outdoors and is known for its olive-colored colonies. Like other moulds, it feeds off organic materials like leaves and grows on humid or wet surfaces. 


Fusarium is considered one of the most significant plant pathogens, with certain Fusarium species causing Panama disease. Panama disease is known to wipe out large quantities of bananas. Additionally, some species are known for attacking barley plants, which has a significant impact on agricultural industries. 


Penicillium is one of the most common moulds encountered day-to-day, and is also one of the most useful. Some species are used to make cheese, while others are used for penicillin production for antibiotics. The appearance of penicillium appears as white fluff, which gradually turns green. 

Stachybotrys Chartarum

Known as ‘black mould’, Stachybotrys Chartarum is found in damp building materials like wallpaper, drywall, and timber. S. Chartarum has a difficult time growing, specifically favoring damp areas with no light. Additionally, S.Chartarum requires no other moulds to interfere with its growth. There is evidence that this mould may produce mycotoxins, which may cause toxicity in humans and animals over time if exposed. 

Serpula Lacrymans

Serpula Lacrymans is a mould known for causing dry rot in wood and other organic materials. Dry rot is recognizable in the way it decomposes wood structures to an unrecognizable extent. 


This mould is found in soil, and is known to have mutualistic relationships with some plants. Later growth reveals a characteristic green colour to this mould. 


Taking five days to mature, Ulocladium moulds are cottony and brown in colour. This mould thrives in extremely wet or damp environments, with its appearance being an indicator of hidden water damage. This is because Ulocldium requires more water than other moulds to grow. Ulocladium is found in soil, drywall, carpets, and other surfaces. 

Growth Conditions for Mould

Fungal moulds and other types require a mixture of moisture, heat, and a food source to grow. These three conditions are abundant in modern homes, which is why mould infestations are so common. Drywall, wallpaper, and timber can be used as a food source for mould colonies. All that is required is some central heating and a moisture level above 60%, and the conditions are ripe. 


Moulds typically reproduce asexually through spores, which are cultivated in fruiting bodies called ‘sporangium’. These structures release microscopic spores, which form the ‘powdery’ appearance on some moulds. When spores land in a suitable environment, they will germinate and begin to consume any suitable material nearby. The cycle starts again after the mould colony is established, and it begins to produce spores of its own. 


Dangers of Mould Exposure

Exposure to mould over long periods can result in increased irritation of airways. The resulting irritation develops into other conditions such as mycotoxin poisoning, and even infection in some individuals. 

For individuals who inhabit water-damaged homes, mould exposure can irritate the respiratory system, causing wheezing, coughing, and headache. Additionally, exposure to mould spores can bring out conditions like asthma in those who can develop it. 

Mycotoxin poisoning is also a risk. Mycotoxins are produced by mould spores, which, when inhaled, build up in the body over time. If the concentration of mycotoxins goes over a certain level, then effects may be more pronounced. Mycotoxins can cause weakness, inhibit bodily functions, and even cause neurological damage. 

How to Prevent Mould

Mould prevention stems from finding ways to stop or inhibit the growth conditions needed for spore production. The following strategies can aid in mould control. 

Reduce Humidity

The main factor that inspires mould growth within a home is moisture. Since heat and organic materials are all readily available, moisture is the only factor you can control. Remedy this by using a dehumidifier in areas such as basements, bathrooms, and kitchens.

Bathrooms in particular, have concentrated exposure to humid environments after showers, which is why they are equipped with exhaust fans. Exhaust fans can pull the humid air away, preventing the ideal growth conditions. 

If there are any leaks or problems with plumbing, those should be fixed as soon as possible. Mould can grow after 24-48 hours if conditions are right, which does not provide much time for error. Additionally, mould growth usually goes by unseen until it has reached a critical stage, which is why it is key to be extra vigilant. 

For additional resources, the government of Canada has an article detailing specific mould-prevention techniques. 

Mould Remediation

Mould remediation is a practice that identifies, removes, and then fixes mould-afflicted areas. The process involves first testing and locating the source of moisture. Once the moisture source is located and removed, the actual mould removal can take place. 

Air Quality Testing

Air sampling is a standard procedure in which air quality is measured by the amount of mould spores present. It can also determine the extent of respiratory damage that could occur after being exposed to an environment. 

Mould Extermination

Mould removal involves a few practices. Some involve manually disturbing the mould with dry brushes or metal brushes to disturb and peel off mould manually. Detergent is also used in combination due to its ability to remove debris from surfaces. Otherwise, there is the more targeted approach known as dry ice blasting

Dry Ice blasting

Dry ice blasting is a cleaning technique that uses pressure-propelled dry ice pellets to clean surfaces. The pellets can remove debris easily with minimal abrasiveness, unlike sandblasting, which can create scratches and dents. An additional benefit is that any leftover residue almost immediately evaporates, meaning that the leftover debris is the only thing that must be cleaned up. 


Fogging is an extra technique used by mould removal specialists to kill any remaining mould spores that may be present in the environment. Like its namesake suggests, an antimicrobial is sprayed as an aerosol throughout the air. Since mould spores float unseen, the mist of antimicrobials can envelop and kill these ‘floater’ spores. The mist can also latch onto surfaces evenly, making sure that all parts of space are cleared from contamination. 

Mould Cleanup 

Mould cleanup occurs after moisture sources have been eliminated, and all mould has been removed and disinfected from surfaces. The resulting debris is then swept up and disposed of in sealed containers or bags. While this process is occurring, the afflicted room is sealed tight with operating HEPA vacuums.

HEPA stands for a standard of vacuum absorption, also known as ‘high-efficiency particulate absorbing’. The HEPA standard means that it is very efficient at catching particles, especially tiny ones like mould spores. 


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