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Source Sampling: When, Where and Why | Chaetomium species

Source Sampling: When, Where and Why

By Dr. Harriet Burge

Environmental investigators are often faced with the need to document with "numbers" that a problem does or does not exist. Attempts to do this with air sampling or casual bulk samples often do not provide the needed support. One approach that can maximize the chances of a successful outcome with a relatively limited sampling protocol is to use hypothesis-driven source sampling.

The aerobiological pathway
First, what are sources and what is their position in the aerobiological pathway (that is, the path between microbial growth and exposure). Organisms do not grow in air. They grow on surfaces, in water, and within bulk materials. If they would stay in these reservoirs, we could, perhaps, ignore them. They become aerobiological problems when conditions arise that lead to aerosols, exposure, and illness or discomfort. A source, then, is any reservoir that contains microbes or other biological material that could become airborne and lead to exposure.

Testing hypotheses using source sampling
Hypotheses not only contribute to the development of sampling protocols, but also are essential if data is to be interpreted. A source-related hypothesis can be as simple as "I think this black color on the wall is Stachybotrys" or as complex as "The Stachybotrys on this wall is becoming airborne in sufficient concentration to cause disease in those exposed". Both of these are legitimate and useful hypotheses that can guide protocol development and analysis and interpretation of the results.

So, how does hypothesis development guide sampling protocols? Consider the first hypothesis: "I think this black color on the wall is Stachybotrys: This is a yes/no type of hypothesis. Either the black color is the mold Stachybotrys, or it isn't. One can readily find out this answer by collecting a tape sample and examining it with a microscope. Possible outcomes are: the black color is not mold at all; the black color is mold, but not Stachybotrys; the black color is Stachybotrys. This is ALL the information you have gained. It is not sufficient information to extrapolate to aerosols, exposure, or disease.

The second hypothesis is more complex. Having documented that Stachybotrys is growing on the wall, one must determine whether or not Stachybotrys spores or other particles are becoming airborne, and, if aerosolization is occurring, what are the aerosol concentrations, and are these aerosol concentrations high enough to cause disease. The "whether or not" question seems straightforward: air sampling should tell us the answer. But how do we design an air sampling protocol that will actually convince us that we have the correct answer (i.e., not result in false negatives). This leads to a consideration of the factors that lead to aerosolization so that a determination can be made regarding conditions under which sampling should occur. In the case of documenting that a particular source can produce aerosols, one might use "worst case" sampling. This involves assessing the natural activities that go on in the space, choosing those that are most likely to lead to aerosols, and collecting samples during that time. This protocol leads to another yes/no outcome: Yes, aerosols can occur or No, aerosols are not likely. Note that you can never say aerosols do not occur, only that the probability of their occurrence is less than 100 percent.

The second question, "what are the aerosol concentrations" is answerable, but requires a more extensive sampling protocol that covers all the conditions in space so that one can calculate peak and average exposure levels. Actually, in the case of Stachybotrys, which has readily identifiable spores, one can use a continuous time-discriminating monitor such as the Burkard recording spore trap. If grab samples are to be used (for example, culture plate samples or air-o-cell cassettes, then many samples must be collected over a time that includes all the different conditions that might occur. At this point you have a picture of spore concentrations over the period of time you sampled, and for the specific areas from which samples were collected. You can compare this data with some "normal" database, and determine if the environment is unusual with respect to Stachybotrys. This data probably won't change your remediation recommendations (remember you already know that Stachybotrys is growing on the wall). However, if you do recover Stachybotrys, and overall levels are unusual, you may be able to use the data to support your recommendations. The contrary also applies, however. If you don't find Stachybotrys aerosols, you still are going to want to recommend remediation, and your data may be counterproductive. The third question is the real problem. We don't know for any of the fungal health effects how much exposure leads to disease, and in any case, exposures are likely to be disease specific. This, then, is a research problem, and should not be attempted except in a research setting.


Fungi of the Month IV: Chaetomium species

By Subramanian Thiagarajan

Chaetomium is a large fungal genus species of which are found worldwide on a variety of substrates including damp sheetrock, carpet, plant compost, soil and other substrates containing cellulose. It is also commonly found on deteriorating wood products. As measured by the EMLab database, Chaetomium is the third most frequently recovered fungus in indoor environments.

Currently, the genus includes more than 100 species, but the taxonomic data given by different authors varies greatly. Udagawa et al. characterized more than 200 species; Domsch et al. distinguished 160–180 species, while von Arx et al. accepted only about 80 species. Chaetomium globosum is the most common and widespread.

Chaetomium is an ascomycete that produces ascospores in dense, hairy fruiting bodies called perithecia. Perithecia are flask-shaped structures with a hole through which spores are extruded. The perithecial hairs can take a variety of forms depending upon the species. The ascospores are dark brown and often lemon-shaped. They collect in a dense mass just outside the perithecium. The perithecia, surrounded by the long hairs, can be as large as 1 mm, and are visible to the naked eye. Surfaces supporting Chaetomium growth with sporulation may superficially resemble old colonies of Stachybotrys.

In culture, colonies of Chaetomium are rapidly growing, cottony and initially white in color. As the colonies mature, they become gray to olive in color and from the reverse (i.e. from the bottom of the Petri dish), the color is tan to red or brown to black. Chaetomium species grow best at between 25 and 35° C.

Chaetomium species often occur together with other fungi that digest cellulose and require high water activity (e.g. Stachybotrys, Trichoderma, Acremonium etc.). In indoor environments, Chaetomium species have been recovered from kitchens, bathrooms, dry wall materials, wood, wallboard, carpets and window frames. Chaetomium species are commonly found in buildings with chronic water intrusion problems and are generally considered as one of the “marker” fungi when locating indoor environmental mold problems.

Chaetomium Microphoto
Chaetomium Microphoto

Chaetomium is a wood decay fungus, causing soft-rot. It is capable of degrading cellulose and hemi-cellulose and may partially digest lignin on wood substrates.


Udagawa S, Muroi T, Kurata H, Sekita S, Yoshihira K, Natori S: Chaetomium udagawe: a new producer of sterigmatocystin. Trans Mycol Soc Jap 1979, 20, 475-480.

Domsch KH, Gams W, Anderson T-H: Compendium of Soil Fungi. Academic Press, London 1993.

Arx JA von, Guarro J, Figueras MJ: The Ascomycete Genus Chaetomium. J. Cramer, Berlin 1986.

Barron MA, Sutton DA , Veve R, Guarro J, Rinaldi M, Thompson E, Cagnoni PJ, Moultney K, and Madinger NE, Invasive Mycotic Infections Caused by Chaetomium perlucidum, a New Agent of Cerebral Phaeohyphomycosis. Journal of Clincal Microbiolgy, Nov. 2003, p. 5302–5307 Vol. 41, No. 11


This article was originally published on September 2004.