Mould is a type of fungus that often looks like fuzzy spots of different colors, including green, gray, or black. More than 100,000 mould species have been identified. Exposure to molds can be linked to negative health effects, depending on the type of mould and the health status of the person.
You may have heard or read about “toxic mould.” Actually, molds themselves are not toxic or poisonous. However, there are mould species that are “toxigenic,” meaning they produce mycotoxins (myco- is a prefix that means fungal). These mycotoxins are chemical byproducts of metabolic processes – any process performed to maintain life, such as converting food into energy or cellular respiration. Some mycotoxins can cause a toxic response in humans and animals, even in small amounts(1).
Molds reproduce by creating spores that can be too small for the naked eye to see. The spores vary in shape and range from 2 to 100 microns in size (for comparison, a strand of a human hair ranges from 17 to 181 microns in diameter). Molds release the reproductive spores, which can spread through the air, water, or on animals. Mould spores can cause health issues by triggering allergies or asthma attacks.
When mould spores land on a damp spot indoors, they may begin growing and digesting whatever they are growing on to survive. Molds can reproduce in any moist place. They can grow on wood, paper, carpet, foods and other surfaces.
General mould categories
Mould species are generally categorized as one of three types:
Allergenic – Unlikely to cause illness (though it may aggravate mild allergies)
Pathogenic – Can cause infection in people who are immunocompromised
Toxigenic – Toxic to all humans and animals who come in contact with it
Everyone breathes in airborne mould spores, but some have an allergic reaction or experience asthma symptoms. Airborne mould spores can get into the nose, causing similar symptoms to other common airborne allergens, such as sneezing, itching, runny nose, congestion and dry skin. Mould spores can also enter the lungs, triggering an asthmatic episode.
Everyone breathes in airborne mould spores.
The most common molds whose spores can act as allergens include:
Alternaria – This genus of molds comprises approximately 50 mould species. It is a common outdoor mould, and its spores are a common allergen. Alternaria species often appear as dark gray spots. The mould genus can grow indoors, especially damp surfaces with a food source, such as wood. The most common Alternaria species are A. tenuissima and A. alternate. A. alternate is often found growing on the following indoor materials if they are damp: iron, tiles, bricks, plaster, wallpaper, paper, and canvas(2).
Aspergillus – This genus of mould grows worldwide and is one of the most commonly occurring molds in hospitals. Its spores can trigger an allergic reaction in some people. Aspergillus is so abundant that it is probably impossible to avoid breathing in at least some of its spores. For most people with healthy immune systems, breathing Aspergillus is not harmful. There are a few species of the genus that can cause serious illness when inhaled by people who are immunocompromised, have a lung disease such as COPD, or have asthma. The range of health issues Aspergillus may cause is known as aspergillosis. Some people will have a severe allergic response to inhaling spores of the species Aspergillus fumigatus. This immune response is called allergic bronchopulmonary aspergillosis (ABPA), which causes airway inflammation(3).
Cladosporium – The Cladosporium genus is one of the most widespread molds. Its spores can act as allergens, triggering reactions in sensitive people. C. herbarum is the most commonly identified allergenic species of Cladosporium.4 Indoors, Cladosporium is common on wet building materials, such as gypsum board, acrylic painted walls, wood, wallpaper, carpet and mattress dust, HVAC fans, and wet insulation in mechanical cooling units(5).
Penicillium – This mould genus includes hundreds of species, some of which are used to produce the antibiotic penicillin. It is a blue-green mould that many people have seen growing on food. Some Penicillium species produce airborne spores that can act as allergens and asthma triggers for sensitive people(6).
Other common molds that produce spores that can act as allergens include the genera Epicoccum, Fusarium, Helminthosporium, Mucor, Rhizopus, and Pullularia.
A pathogenic mould is one that can cause an infection in humans, even if they are in good health. Some genus of molds include species that fall into the allergenic category, as well as species categorized as pathogenic. One such example is the Aspergillus genus. This genus includes several allergenic species, as well as one very pathogenic species, fumigatus.
A pathogenic mould is one that can cause an infection in humans, even if they are in good health.
The most common pathogenic molds include:
Aspergillus species: A. fumigatus and A. flavus – A. fumigatus is the most prevalent species of the genus and is the leading cause of invasive aspergillosis and can cause chronic pulmonary infections. Although airborne A. flavus is more prevalent in some hospitals and locations, it is far less studied than A. fumigatus. A. flavus is also known to produce mycotoxins(7).
Cryptococcus neoformans – This mould lives in environments worldwide. People can develop an infection after inhaling its spores, although it is extremely rare for those with healthy immune systems. If an immunocompromised person inhales spores, they can stay in the body and cause a later infection if the immune system becomes even weaker. Most cases of cryptococcal meningitis (a form of fungal meningitis) are linked to C. neoformans(8).
Histoplasma capsulatum – H. capsulatum causes the infectious disease histoplasmosis. It’s acquired by inhaling airborne H. capsulatum spores. Inhaled spores can also cause a lung infection (pneumonia). Those with a weakened immune system risk the infection spreading throughout the body which can be life-threatening. This mould is usually found in soil; however, it is also found where there are bat and bird droppings. Spores usually become airborne from soil disturbances like digging or building demolition. Outbreaks are most often caused by large-scale construction projects(9).
The toxigenic molds are the misnamed “toxic mould” species that may create the chemical metabolic byproducts called mycotoxins, which can cause a toxic response in humans and animals. Mycotoxins are separate from the spores molds produce for reproduction. However, these chemicals can become airborne and travel with mould spores. Researchers have also demonstrated that mycotoxins also travel through the air on very small particles of dust or tiny pieces of wallpaper, which can easily be inhaled(10).
Mycotoxins travel through the air on very small particles of dust or tiny pieces of wallpaper, which can easily be inhaled.
It should be noted that the presence of a toxigenic mould species does not necessarily mean the presence of mycotoxins. Also, it’s not possible to know if a mould is producing mycotoxins just by looking at it(11).
Some of the most commonly found toxigenic molds found growing indoors include:
Fusarium species – F. solani, F. oxysporum, F. moniliforme(12).
Penicillium species – P. brevicompactum, P. chrysogenum, P. citrinum, P. corylophilum, P. cyclopium, P. expansum. P. fellutanum, P. spinulosum, and P. viridicatum(13).
Aspergillus species – A. versicolor, A. niger and A. flavus(14).
What you can do about mould in your home
The best way to control mould growth is to control moisture. Other actions you can take include:
Fix any sources of water leakage. Homeowners may need a plumber or an HVAC contractor to eliminate water leaks (which is the most likely cause of the moisture on which mould is feeding).Reduce indoor humidity (optimal indoor humidity is around 40%).Use exhaust fans whenever cooking, dishwashing, and cleaning.Use soap and a sponge to remove visible mould.If any moldy areas are dry, lightly spray a water mist on them. This will prevent you from sending mould spores into the air during cleaning.If you use bleach, make sure it won't cause discoloration. Never mix bleach with ammonia. It will create toxic fumes.If there is excessive damage to a building or home after a flood and you are cleaning, be sure to put all materials used in cleaning in a heavy duty garbage bag and seal it. Take the bag outside through the closest exit. You don’t want any spores to become airborne.