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

Species of Chaetomium are strongly cellulolytic moulds commonly found in soil, on paper, straw, cloth, cotton and other cellulose-containing substrates. They have caused problems in libraries, archives, and the food industry. Because of their bio-deterioration ability, several strains are used in testing materials for mould growth resistance.

Concentrations of Chaetomium spores in the air: why are they usually low?

Spores (ascospores) of Chaetomium are produced within structures (asci) contained in a flask-shaped fruiting body known as perithecium. On spore maturity, the walls of the asci dissolve releasing mucilaginous spores within the perithecium. The spores ooze out of the perithecium (plural perithecia) and get trapped by coiled or dichotomously branched hairs that grow around the neck of the perithecium.

Since the spores are cemented together by mucilage and also trapped by hairs, few become airborne until the mould has completely dried out or disturbed, say during renovations or mould remediation. It is therefore not uncommon to find low Chaetomium spore counts in pre-remediation mould test samples and relatively higher counts in post-remediation samples.

Chaetomium as a health hazard, indoor air biocontaminant and biodeteriorator

Some species of Chaetomium have been implicated in nosocomial infection of patients in hospital environment after bone marrow transplantation (9). Many species are also known to produce mycotoxins (6, 8) and are recognized human allergens (2). The most common species in water-damaged buildings is Chaetomium globosum. In many cases it occurs together with Stachybotrys chartarum and other hydrophilic moulds. It produces high quantities of biomass (up to 10 mg/cm²) on building materials (3).

It has been isolated from wallpaper, drywall, baseboards, carpets and window frames. It is a major cause of biodeterioration of paper and other cellulose containing materials. As a health hazard, Chaetomium globosum produces very high quantities of mycotoxins, especially chaetoglobosins A and C when growing on gypsum board (4, 6). It is a known agent of skin and nail infections in humans and is more rarely a cause of cerebral and systemic infections in immunocompromised individuals (1, 7).

Although Chaetomium globosum is reported to have type I & III allergens (5) the spores are not easily aerosolized and hence exposure to airborne spores may be rather limited. However, exposure to cytotoxic mycotoxins and also fine hyphal fragments released from dried mycelia could be a major concern.

References

1. Abbott, S. P., L. Sigler, R. McAleer, D. A. McGough, M. G. Rinaldi, and G. Mizell.1995. Fatal cerebral mycoses caused by the ascomycete Chaetomium strumarium. J. Clin. Microbiol. 33:2692-2698.
2. Liebeskind, A. 1972. Mycological problems in occupational allergies. J. Asthma Res. 10:71–73.
3. Nielsen, K.F, (2002), “Mould growth on building materials: Secondary metabolites, mycotoxins and biomarkers”, Dissertation, The Mycology Group, Technical University of Denmark.
4. Nielsen, KF, Gravesen S, Nielsen PA, Andersen B, Thrane, Frisvad JC. 1999. Production of mycotoxins on artificially and naturally infested building materials. Mycopathologia 145:43-56.
5. Pieckova, E. (2003). In vitro toxicity of indoor Chaetomium Kunze ex Fr. Ann. Agric. Environ. Med. 10(1):9-14.
6. Sekita, S., K. Yoshihira, S. Natori, S. Udagawa, T. Muroi, Y. Sugiyama, H. Kurata, and M. Umeda. 1981. Mycotoxin production by Chaetomium spp. and related fungi. Can. J. Microbiol. 27:766–772.
7. Serena, C. et al. (2003). In vitro activities of new antifungal agents against Chaetomium spp. and inoculum standardization. Antimicrob. Agents Chemother. 47(10):3161-3164.
8. Udagawa, S., T. Muroi, H. Kurata, S. Sekita, K. Yoshihira, and S. Natori. 1979. The production of chaetoglobosins, sterigmatocystin, O-methylsterigmatocystin, and chaetocin by Chaetomium spp. and related fungi. Can. J. Microbiol. 25:170–177.
9. Woods GL, Davis JC, Vasughan WP: Failure of the sterile airflow component of a protected environment detected by demonstration of Chaetomium species colonization of four consecutive immunosupressed occupants. Infect Control Hosp Epidemiol 1988, 9, 451-456.

Chaetomium is a cellulolytic mould commonly found in soil, air, and decaying plant material. There are about 80 species of Chaetomium. The most common ones are Chaetomium atrobrunneum, C.  funicola, C. globosum, and C. murorum.

In indoor environments the most common species of Chaetomium is Chaetomium globosum. C. globosum  is frequently isolated in water-damaged buildings and produces two mycotoxins called chaetoglobosins A and C when cultured on building material. Presence of Chaetomium species in indoor environment is a sign of serious water problem.

Chaetomium may occur together with other water-loving moulds such as Fusarium, Stachybotrys and Ulocladium. Species of Chaetomium are known to produce mycotoxins but to what extent these toxins contribute to poor indoor air quality or affect human health is not documented. However, injection of chaetoglobosin A in rodents has been shown to be fatal at relatively low doses. In medical literature some species have been reported to cause disease in immuno-compromised individuals.

Species that have been reported to cause invasive human disease grow well at 35 to 37^oC, and those with a predilection for the central nervous system often display growth at up to 42 to 45^oC.

Spores of Chaetomium are produced within structures (asci) contained in a flask-shaped fruiting body known as perithecium. Once the spores are mature, the walls of the asci dissolve releasing mucilaginous spores within the perithecium. The spores ooze out of the perithecium (plural perithecia) and get trapped by coiled or dichotomously branched hairs that grow around the neck of the perithecium.

Since the spores of this commonly found mould are cemented together by mucilage and also trapped by hairs, few become airborne until the mould has completely dried out or disturbed, say during renovations or mould remediation. It is therefore not uncommon to find low Chaetomium spore counts in pre-remediation samples and relatively higher counts in post-remediation samples