We all know it – the smell you get when you enter a dank cellar or crawlspace. It's a musty, stagnant smell that we immediately associate with mold and moisture. The question is – why? Why is this particular smell caused by fungal growth? The answer may surprise you a bit.
One may hasten to guess that the smell is caused by spores being released by the mold organism. This, in fact, is not the case. The smells given off by mold at certain stages of their growth and reproduction are called Microbial Volatile Organic Compounds (MVOCs). In other words, it is a mixture of various compounds (chemicals) that are being released by the organism as it eats, grows, and multiplies. In fact, some people have gone as far as to say that a good name for these gases would be "mold farts." It is no wonder that MVOCs can be the cause of occupant indoor air quality complaints.
The list of compounds produced by various molds is expansive; however, the following have been associated with active mold growth:
- 1-octen-3-ol - Odor has been described as green and moldy or meaty
- Geosmin - Distinct earthy flavor and aroma. Contributes to the scent following rain.
- 1-butanol – Alcohol fermentation smell
- borneol- Camphor-like smell
- 3-octanone – Herbaceous, nutty, fatty odor
- 2-hexanone – Acetone-like odor
Some identified MVOCs are known toxins; however, there is skepticism among the scientific community that the quantities of these MVOCs produced and released by molds would be far too small to have a major impact on building occupants.
Sampling for MVOCs can be helpful in certain situations. Many times the go-to sampling method of choice for many indoor air quality professionals is the spore trap cassette. Air is drawn through the cassette and impacted onto a slide, which is then analyzed by the laboratory for spore types and quantities. This sampling type can be useful when performed correctly and used under the right conditions.
However, it is important to note that fungi is not always sporulating – that is, mold is not always giving off spores. Spores tend to be released when the mold has dried up and has stopped growing. As a survival response, spores are produced, which can then remain viable for years or even decades. If the mold is actively growing, it is likely that the growth has a moisture source and is not dried to the point of sporulation. The useful thing to know here is that when mold is growing, it will much of the time release MVOCs.
In other words, spore trap sampling may not reveal active mold growth, while MVOC sampling will, and vice versa. It all depends on the stage of mold growth, along with other factors, such as air flow, disturbance, air pressurization, and the like.
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