Growth of mould on building materials is determined by the water activity (a_w) among other factors. The a_w is a measure of the moistness of the material. Some moulds are capable of growing over a wide range of aw but there is always an optimal range. While these moulds grow, they reach a stage when they produce the toxins depending on the growth conditions. Some of the indoor moulds that produce mycotoxins are discussed below.

Stachybotrys chartarum
Stachybotrys chartarum is cosmopolitan and grows naturally on straw and other cellulose containing materials in soil. In the indoor environment, this mould is commonly found together with other water-loving moulds on cellulose containing materials including paper, canvas and jute which are wetted to a water activity > 0.98. Stachybotrys chartarum produces a number of mycotoxins inclduding macrocyclic trichothecenes, satratoxins and roridins when growing on building materials. The optimum temperature for growth of Stachybotrys chartarum is 23 ^oC with a minimum and maximum temperature of 2 and 37 ^oC respectively. The optimal water activity is 0.98 with a minimum at 0.89.

Aspergillus flavus
Aspergillus flavus is widely distributed in soil. It is associated with a wide range of stored products such as maize and nuts. In indoor environment it is commonly found on damp walls, wallpaper, floor and carpet dust, tarred wooden flooring, humidifiers and HVAC fans, bakeries, shoes, leather, and bird droppings. Strains of this mould may produce aflatoxin, a class 1 carcinogen. The minimum and maximum temperatures for growth are 6 and 45 ^oC, with an optimum at 40 ^oC. The minimum water activity is 0.78 and an optimum at 0.98.

Aspergillus fumigatus
Aspergillus fumigatus is common in composting plant material, woodchips and garbage. Also common in dust infiltrating from outdoor air, carpet and mattress dust, wet building and finishing material, HVAC insulations, fans, filters, humidifier water and potted plant soil. A. fumigatus is a producer of various mycotoxins including gliotoxin, verrucologen, fumitremorgin A & B and fumigaclavines among others. A. fumigatus has an optimum growth temperature at 43 ^oC and minimum and maximum at 10 and 57 ^oC respectively. Minimum water activity for A. fumigatus is 0.82 and the optimum is 0.97.

Aspergillus niger
Aspergillus niger is found worldwide in soil and plant litter. In indoor environment A. niger is common in floor, carpet and mattress dust, acrylic paint, UFFI, leather, HVAC filters and fans, and potted plant soil. A. niger produces malformins and a few strains also produce ochratoxin A. A. niger requires a minimum temperature for growth of 6 and maximum of 47 ^oC with an optimum at 37 ^oC.

Aspergillus versicolor
Aspergillus versicolor is very common on gypsum board, floor, carpet, mattress and upholstered-furniture dust, and damp walls. A. versicolor produces high quantities of the carcinogenic mycotoxin, sterigmatocystin at water activities (a_w) above 0.95. A. versicolor is generally xerophilic- meaning that it can grow at low water activity (< 0.8). The minimum and maximum growth temperatures for A. versicolor are 4 and 40 ^oC with an optimum at 30 ^oC. Its optimal water activity is 0.95 with a minimum at 0.75.

Penicillium chrysogenum
Penicillium chrysogenum is one of the most common indoor moulds. It is extremely common on damp building materials, walls and wallpaper; floor, carpet mattress and upholstered-furniture dust. P. chrysogenum produces few detectable metabolites and often none when growing on building materials. Lack of observed effects on persons exposed to high quantities of spores and the production of few metabolites suggest this species may not be an important health hazard. However, some strains may cause allergenic reactions to susceptible individuals. P. chrysogenum can grow at a minimum temperature of -4 ^oC, an optimum of 28 ^oC, and a maximum of 38 ^oC. It has minimum water activity of 0.79 and an optimum at 0.98

Penicillium brevicompactum
Penicillium brevicompactum is common on damp walls and building materials e.g., gypsum board; floor, carpet, mattress and upholstered-furniture dust. P. brevicompactum produces mycophenolic acid. P. brevicompactum can grow at -2 and 30 ^oC with an optimum at 25 ^oC. Its water activity requirements are a minimum of 0.75 and an optimum at 0.96.

Chaetomium globosum
Chaetomium globosum is common on cellulose containing building materials that has been very wet such as gypsum board, cellulose board and wood. C. globosum produces high quantities of chaetoglobosins. C. globosum requires a water activity of >0.90.

Trichoderma species
Trichderma species are frequently found on gypsum board and water saturated wood; floor, carpet and mattress dust; paint; domestic water supply, and HVAC system air. In one study, Trichoderma species were not found to produce detectable quantities of trichothecenes when growing on building materials. Less than 1% of the isolates produced trichodermol or its esters.

References
Kuhn, D. M. and Ghannoum, M. A. (2003). Indoor Mold, Toxigenic Fungi, and Stachybotrys chartarum: Infectious Disease Perspective. Clinical Microbiology Reviews, 16(1):144–172.Nielsen, K.F, (2002).

Mould growth on building materials: Secondary metabolites, mycotoxins and biomarkers, Dissertation, The Mycology Group, Technical University of Denmark. 116p



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Growth of commonly occurring moulds in foods and building materials may result in production of toxic complex secondary metabolic by-products referred to as mycotoxins. Moulds that produce mycotoxins are said to be toxigenic (or incorrectly toxic moulds). According to experts, five kinds of mycotoxins are important in human and animal health: aflatoxins, ochratoxin A, fumonisins, certain trichothecenes and zearalenone. It is believed the effect of mycotoxins as a cause of human mortality is underestimated. Mycotoxins can elicit a wide range of toxic responses including acute and chronic systemic effects in humans and animals that cannot be attributed to mould growth within the host or allergic reactions to foreign proteins. Exposure to mycotoxins can occur through ingestion, contact or inhalation of airborne particulates containing the toxins, including dust and mould components such as spores and mycelial fragments. In agricultural settings, mycotoxicoses (mycotoxin poisoning) in both farm animals and humans can result from oral, dermal, or exposure through inhalation of mycotoxin-contaminated grain or dust. The most widely known mycotoxins are the aflatoxins whose toxic effects were first realized in England in the 1960s when an outbreak of the so called turkey X disease killed over 100,000 fowls following consumption of contaminated peanuts. Aflatoxins are still a problem particularly in developing countries. In mid 2004 more than 100 people died in an East African country after consuming grains contaminated with aflatoxins. Aflatoxins are mainly produced by two species of Aspergillus, Aspergillus flavus and A. parasiticus, in several agricultural commodities, including corn and nuts. Two structural types of aflatoxins are known (B and G types), of which aflatoxin B₁ is a class 1 carcinogen and is considered the most toxic. In experimental animals, exposure through breathing has been shown in some cases being several orders of magnitude more toxic than dermal or even systemic administration. Levels of airborne mycotoxins in indoor environment have not well been established and neither the contribution of airborne mycotoxins to sick-building syndrome.

What Factors Favour Mycotoxin Production?
Mycotoxins are produced by a few strains of moulds at some point during their growth under suboptimal growth conditions or limited nutrients. Production occurs preferentially on materials that both allow toxigenic moulds to grow and provide the conditions for mycotoxin production. From the many studies of the production of mycotoxins by mould isolates derived from agricultural environments, a great deal is known about the mould species that are capable of producing known mycotoxins and about the growth media and conditions that induce production. Conditions that favour production of one type of mycotoxins may not be favourable for production of another type. For example, aflatoxin production by Aspergillus is dependent on concentrations of O₂, CO₂, zinc, and copper, as well as physical location, while production of ochratoxin relates to air exhaustion. Understanding the conditions under which mycotoxins are produced is important since presence of toxigenic moulds in any environment does not prove the presence of a mycotoxin.

What do we know about mycotoxins in indoor environment and their health effects?
Frequently, toxigenic moulds have been isolated from building materials and air samples in buildings where residents have suffered from non-specific symptoms possibly related to mycotoxin production, such as cough; irritation of eyes, skin, and respiratory tract; joint ache; headache; and fatigue. It is only recently that the presence of some mycotoxins has been confirmed in crude building materials. Most mycotoxins have yet to be extracted from either air samples or bulk material derived from indoor environments. Also, very few studies have been conducted to show correlation between mycotoxin exposure and building-related illnesses.


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Aspergillus has close to 200 species and varieties. This mould is widely distributed from the arctic region to the tropics and is frequently found in air, soil, and stored agricultural products.


Harmful effects of Aspergillus to our lives

Species of Aspergillus produce dry, hydrophobic spores that are easily inhaled. Inhaling spores of Aspergillus, particularly those of Aspergillus flavus and Aspergillus fumigatus frequently cause pulmonary aspergillosis, either allergic, or invasive or a combination of the two. Aspergillus flavus produces the highly potent mycotoxin called aflatoxin which over the years has been reported as the cause of death in humans and animals. In 2004 aflatoxin killed over 100 people in an East African country who consumed grains infested by Aspergillus flavus. Many other species of Aspergillus such as Aspergillus clavatus, Aspergillus niger, and Aspergillus versicolor are also known to be toxigenic or allergenic. Instant action is required when growth of these moulds appear in occupied indoor environment.

Because of their ability to produce a large number of enzymes, species of Aspergillus are capable of utilizing an enormous variety of organic material for food. Therefore, under humid conditions, Aspergillus species grow on leather, cloth fabrics and books reducing their value and imparting musty odour to these items. Several species are frequently found on grains and exposed foodstuffs where they cause decay and subsequent loss of food.


Occurrence of Aspergillus species in indoor environment
Species of Aspergillus have been isolated from damp walls, wallpaper, PVC/paper wall covering, gypsum board, floor, carpet and mattress dust, upholstered-furniture dust , acrylic paint, UFFI, leather, HVAC insulations, filters and fans, humidifier water, shoes, leather, bird droppings, potted plant soil, plastic and decomposing wood.

Aspergillus fumigatus is frequently found in kitchens and bathrooms while in mattresses and carpets, Aspergillus versicolor and Aspergillus repens are common. Asperillus versicolor and Aspergillus fumigatus are also common in basements. Species of Aspergillus frequently found in flower pot soil are Aspergillus fumigatus, Aspergillus niger and Aspergillus flavus.

Generally the amount of airborne spores of Aspergillus spp. in indoor air is higher than outdoors at any given time. In the home, the amount of airborne spores is significantly increased when cleaning is carried out mechanically, for example, when carpets are vacuum cleaned.


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Mould spores are tiny structures produced by moulds for reproduction purposes. They are so small that we can hardly see some of them even when magnified 400 times. One cubic meter of outdoor air may contain as many as 1,000,000 spores. The volume of air we inhale at rest is estimated at 10 litres per minute. Therefore, in an hour we inhale close to 600,000 spores. The air in some working environments including mouldy buildings may contain up to 1,000,000,000 spores per cubic meter of air.

In indoor environments moulds grow on moist surfaces such as the drywall, wallpaper, carpet, baseboards, and heating, ventilation and air-conditioning systems (HVACs). As these moulds grow, a stage is reached when they produce spores. The spores become airborne after drying out or if disturbed.

Could airborne spores be dangerous to our health?
About 20% of the population is allergic to mould spores. Apart from being allergenic, spores of some mould species such as Stachybotrys contain toxic compounds called mycotoxins. Symptoms associated with mould spores may include allergy, headache and fatigue, running nose, sneezing, coughing, pneumonia and Asthma among other non-specific symptoms. Young children, the elderly and people undergoing medical treatment are particularly susceptible to mould spores.

How can we control indoor mould spores?
It is extremely difficult to get rid of spores completely. We can, however, reduce their numbers by controlling mould growth in our houses or offices. Mould growth is associated with moisture problem as a result of flooding, leaks in roofs or plumbing and condensation in case of poor ventilation or inadequate insulation. The key to controlling mould growth is keeping our houses or offices dry by maintaining low relative humidity (below 70%) thus eliminating or slowing the growth of most mould species. Any water leakage should be repaired immediately and the water dried out within 48 hours. Constant monitoring for mould growth in the kitchen, bathrooms, window frames, carpets and baseboards is recommended. The earlier the mould is discovered the cheaper and easier it is to get rid of the problem.

What should you do if you notice mould growing in your house or office?
Do not panic! Seek professional advice. Not all moulds are dangerous to health, but no mould should be allowed to grow in our dwellings. Also the health effects of indoor moulds depend on the amount of mould one has been exposed to, length of time of exposure, the types of moulds present and individual’s resistance. Therefore, presence of mould does not necessarily mean the occupants have been affected.


Which are the most dangerous moulds?
People have come to believe that black moulds are the most dangerous.
The truth is, colour does not determine whether a mould is of health concern or not. To know whether a mould is dangerous or not requires the mould to be identified by a qualified mycologist.


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This is a list of some of the most common indoor moulds and their hazard classes. For an explanation of these hazard classes, read the May 24 post entitled "Indoor Moulds: Hazard Classes And Associated Health Effects". The list is not exhaustive. At least 150 moulds have been reported from buildings.

Acremonium spp (hazard class B).
Alternaria spp (hazard class B).
Aspergillus flavus (hazard class A).
Aspergillus fumigatus (hazard class A).
Aspergillus versicolor (hazard class A).
Aureobasidium pullulans (hazard class B).
Chaetomium globosum (hazard class C).
Cladosporium cladosporioides (hazard class B).
Cladosporium sphaerospermum (hazard class C).
Fusarium spp (hazard class A).
Mucor spp (hazard class A).
Paecilomyces spp (hazard class B).
Penicillium aurantiogriseum (hazard class B).
Penicillium chrysogenum (hazard class B).
Scopulariopsis spp (hazard class B).
Stachybotrys chartarum (hazard class A).
Trichoderma spp (hazard class B)
Ulocladium botrytis (hazard class C).
Wallemia sebi (hazard class C).

These moulds are commonly found on water damaged or damp surfaces in kitchens, basements, bathrooms, ceilings, and other drywalls. Occurrence of some of these moulds such as Stachybotrys, Fusarium, Ulocladium and Trichoderma in a building is an indication of serious water damage.

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The level of moisture (usually referred to as water activity) in building material determines not only whether mould will grow or not but also the types that colonize the material. Damp materials with a water activity value equal to or greater than 0.90 are usually colonized by strains of Aspergillus fumigatus, Trichoderma spp., Exophiala spp., Stachybotrys spp., Phialophora spp., Fusarium spp., Ulocladium spp., and yeasts such as Rhodotorula spp. Materials with a water activity value ranging from 0.90 - 0.85 are colonized by Aspergillus versicolor while those with water activity values of 0.85 or slightly less are colonized by Aspergillus versicolor, Eurotium spp., Wallemia spp., and Penicillium spp., such as Penicillium chrysogenum and Penicillium aurantiogriseum.

Water leakage through roofs, defective plumbing installations and condensation are the main sources for water damage with subsequent mould growth. The building materials most susceptible to mould attacks are water damaged, aged organic cellulose containing materials such as wood, jute, wallpaper, and cardboard. Moulds that are most frequently encountered in buildings are Penicillium (68%), Aspergillus (56%), Chaetomium (22%), Ulocladium (21%), Stachybotrys (19%), Cladosporium (15%), Acremonium (14%), Mucor (14%), Paecilomyces (10%), Alternaria (8%), Verticillium (8%), and Trichoderma (7%). These moulds are all known to cause different types of inhalation allergy.

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Hazard classes of indoor mould
Indoor moulds have been grouped into 3 hazard classes based on associated health risk. These classes are similar to risk groups assigned to micro-organisms handled in laboratory environments.

• Hazard Class A: includes fungi or their metabolic products that are highly hazardous to health. These fungi or metabolites should not be present in occupied dwellings. Presence of these fungi in occupied building requires immediate attention.
• Hazard class B: includes those fungi which may cause allergic reactions to occupants if present indoors over a long period.
• Hazard Class C: includes fungi not known to be a hazard to health. Growth of these fungi indoors, however, may cause economic damage and therefore should not be allowed.

Health Problems Associated With Indoor Moulds
Exposure to indoor mould has been associated with the following health problems:

• Lower respiratory symptoms such as coughing and wheezing;
• Respiratory infections such as aspergillosis;
• Allergic diseases, including allergic asthma and bronchitis;
• Non-inflammatory, unspecific symptoms, such as eye and skin irritation, fatigue, headache, nausea, and vomiting.

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Mould (also spelt as mold) has become a household name having been associated with a number of health effects, particularly allergic and respiratory problems. Some people, however, believe this is a creation of the media. In this blog we shall be looking at both the harmful and beneficial aspects of mould and bacteria. More information about mould and bacteria can be obtained at http://www.moldbacteria.com