How Do You Calculate the Outdoor Air Mold Count?


Quick Answer

Healthy Building Science describes two methods of air sampling that result in mold spore counts: sporetrap and petri dish testing. A small known volume of air is drawn into a pump across a greased slide or a petri dish. The spores that stick to the slide or grow in the dish are counted and divided by the number of air units per cubic meter to get the mold count in units of spores per cubic meter.

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Full Answer

Airborne mold spore counts may be differentiated by species or "generic," meaning a cumulative total of all mold species present, according to Inspectapedia. Sporetrap sampling is the most common method of mold testing listed by Healthy Building Science. The disadvantage of this method is that some common mold types such as Penicillium and Aspergillus cannot be differentiated by visual inspection and reported separately, and heavier spores like Stachybotrys are less likely to be airborne and may be undercounted depending on the sampling location.

Petri dish sampling, which cultures spores to maturity, allows more accurate identification of individual mold species than spore trapping, which Healthy Building Science notes often only identifies genera of mold. This is important because some genera have hundreds of species that vary widely in toxicity. However, Inspectapedia claims that only about 10 percent of all molds will grow in any growth medium under all lab conditions and considers using a single test medium to be a questionable practice for mold screening. Multiple cultures using different growth media are required for the most accurate results with this sampling method.

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