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Musty Odors | Microorganism of the Month: Ascospores

Musty Odors: Do They Always Indicate Mold?

By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board

Musty odors are caused by volatile organic compounds. While mold is a common source for these compounds, they can come from other sources. If there is no water anywhere in the basement, then musty odors are probably not caused by mold. If there is water, mold probably plays a role, but may not be the entire problem.

First, let’s talk about mold. Actively growing mold produces a wide range of volatile compounds, some of which can impart a musty odor. Many of these compounds can be detected by the human nose at very low concentrations. The mold colonies do not have to be sporulating to release these compounds, so that nearly invisible mold can be the source. When I see carpeting on a basement floor, I usually assume that it is the source for any musty odors that might be present, even if the carpeting looks new and dry and clean. Removal of the carpeting usually ends the odor problem. Mold does occur behind paneling if the wall is damp, and drying the wall is the only solution. Such drying may or may not involve removing the paneling. In both of these cases, airborne spore concentrations can be very low. This is an indication either that the mold producing the odors is not sporulating, or that any spores that are produced are trapped and not entering the air.

Wet concrete has an odor that could be interpreted as musty. While mold can grow on the surface of concrete, it isn’t always present, and just the dampness may be producing odors. Standing water usually doesn’t result in mold growth, but bacteria may grow and produce volatiles with "musty" odors.

Dehumidification is one means for controlling both mold and the damp concrete odors discussed above. I use a dehumidifier that keeps my entire 1000+ square foot basement odor free, and that drains directly into my stationary washtub. It could also drain into a floor drain. Otherwise, the reservoirs of dehumidifiers have to be emptied regularly, and if they are not, they could become odor sources in themselves.

Finally, some dry environments have a musty odor. One cause of this odor that I call the "Antique Shop Odor" is degradation of paper products and very old wood. Old waxes and polishes could also contribute. I am not aware of publications that discuss the specific compounds that cause these odors. I do know that some people find them objectionable and, for that reason, don't visit antique shops or even museums with antique collections.


Microorganism of the Month: Ascospores

By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board

Ascospores are sexual spores of the group of fungi known as Ascomycetes. Ascospore morphology is among the most variable in the fungi. Spores range from transparent and colorless to opaque black (Figures 1 and 2). They may be simple single cells or comprised of many cells either arranged in a single row or in a mass. In spite of this range of morphology, spores are rarely identifiable without the associated fruiting structures. Except for a few specific types, ascospores are generally reported as a group.

Ascospores are produced following mating between two specialized cells. Following nuclear fusion and meiosis, each of the 4 resulting daughter nuclei divide and eight spores are formed. Spore formation occurs inside of a fruiting body (ascocarp). There are three general types of ascocarps: completely enclosed balls that must disintegrate before spores are released (cleistothecia), more or less spherical structures with a pore through which spores are extruded (perithecia) and disc-shaped structures with the spore sacs lining the open surface (apothecia).

Many Ascomycetes have evolved forcible methods for releasing spores from the fruiting body. With changes in the amount of available water, the ascus swells and bursts, rocketing the spores into the air. Others extrude a gelatinous mass of asci and spores into the environment and rain splash causes the spores to become airborne. Thus, except for the types that form cleistothecia, rain or at least mist is necessary for spore release. This is why ascospores become so abundant in outdoor air on rainy days.

Many Ascomycetes produce ascospores following a winter dormant period, and the spores are released when the first warm rains occur. This means that ascospores are most abundant in outdoor air in the spring time. Subsequent rains will produce lower and lower spore concentrations as the winter spore supplies are depleted. However, there are always some ascospores in the air.

Although generally considered "outdoor" spores, there are Ascomycetes that grow well indoors on wet materials. Chaetomium is a perithecial Ascomycete that grows on wet cellulosic material. The fruiting bodies are covered with long hairs (Figure 1), and are sometimes visible as fuzzy dots on moldy paper. Spores are extruded in a slimy mass, and, indoors, active disturbance is necessary for their release. The spores are distinctive, and are generally listed separately from the catch-all ascospore category.

Chaetomium Microphoto: Fruiting bodies covered with long hairs The fruiting bodies of Chaetomium are covered with long hairs

Figure 1: The fruiting bodies of Chaetomium are covered with long hairs.
Source: EMLab P&K

Eurotium (Figure 2) is another ascomycete that is commonly found indoors, although usually in its asexual phase (Aspergillus). Aspergilli in the "glaucus" group produce Eurotium sexual stages, and the fruiting bodies (cleistothecia), ascospores, and asexual Aspergillus spores may be found in surface samples. Aspergillus nidulans also occasionally produces sexual fruiting bodies on indoor materials. This group of Aspergilli produce Emericella sexual stages. The spores of these are also distinctive, and are listed separately on reports. These ascospores are not forcibly discharged and require mechanical abrasion to become airborne.

Eurotium spores are transparent and colorless

Figure 2: Eurotium spores are transparent and colorless.
Source: EMLab P&K

There are several Ascomycetes that produce apothecia on indoor materials and forcibly discharged spores. Where rotting wood is present, species of the genus Peziza may form large (3-4inch diameter) fruiting bodies indoors. The spores of this genus are large and don't stay airborne long, but have been recovered from indoor sources. Another apothecia forming fungi that is found relatively often on ceiling tile is Pyronema, a tiny orange fungus. Each little disc is about 1 mm in diameter, and the overall effect on the ceiling tile is of pale orange freckles. The spores are large and oval, and should be recognizable on spore traps. If the Ascospore category on a report appears unusually high indoors, it may be worthwhile to check on the spore types present.

Ascospores are not readily produced in culture, and therefore are difficult to study with respect to allergenicity. Because they are so closely related to many of the common asexual fungi, it is probable that they are very important aeroallergens. Ascospores have been associated with epidemic asthma. Spores of the ascomycete Didymella exitialis are considered to play a role in thunderstorm asthma in the UK.


This article was originally published on September 2007.