The general public thinks that black indoor molds are toxigenic because of the publicity that Stachybotrys chartarum (a.k.a Stachybotrys atra in the old literature), a black mold, received in the recent years. However, the colour of the indoor molds does not determine whether they are toxigenic or not.

Growth of indoor molds 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 indoor molds are capable of growing over a wide range of a_w but there is always an optimal range.

Common Indoor Molds

The indoor molds commonly found on building materials 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 mold is commonly found together with Stachybotrys chlorohalonata on cellulose containing materials including paper, canvas and jute which are wetted to a water activity > 0.98.

In a study on indoor molds conducted in Denmark, Stachybotrys chartarum was found to produce a number of mycotoxins including macrocyclic trichothecenes, satratoxins and roridins when growing on building materials. However, only 35% of the isolates from buildings produced the extremely cytotoxic mycotoxins, the satratoxins.

This led to the conclusion that idiopathic pulmonary hemosiderosis in infants is possibly not caused by satratoxins but by other S. chartarum mycotoxins.The optimum temperature for growth for 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 of 0.89. To read more about Stachybotrys chartarum and S. chlorohalonata click here.

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 temperature 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 is 0.82 and an optimum at 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 at 6 and maximum at 47 ^oC with an optimum at 37 ^oC.

Aspergillus versicolor

Aspergillus versicolor is one of the common indoor molds. It 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 (<0.80). The minimum and maximum growth temperatures for A. versicolor(/i> are 4 and 40 ^oC with an optimum at 30 ^oC.Its optimal water activity is 0.95 with a minimum of 0.75.

To read more about Aspergillus species click Aspergillus. The university of Manchester, UK has detailed information on Aspergillus at https://www.aspergillus.man.ac.uk/

 Penicillium chrysogenum

Penicillium chrysogenum is one of the most common indoor molds. 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. The water activity requirement for C. globosum are a minimum of

 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 esters of it. Compared with other indoor molds such as Cladosporium, Aspergillus and Penicillium, Trichoderma is not very common indoors.

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