Healthy Living Spaces


Do-It-Your-Self Kits May Not Detect Black Mold

Purpose, Techniques, Why not related to air borne exposure

Most of the time people who test their homes for mold using self-test kits end up confused by the results. They may think they have a mold problem when they don’t or think that their home is safe when there is in fact a mold problem.

Do-It-Your self test kits use petri dishes. There are some inherent limitations testing for mold with petri dishes and additional problems if you don’t have a vacuum pump to take the samples with. The typical instructions tell you to place the petri dishes out in your home for a certain amount of time and wait for mold spores to settle onto the dish. Then you send them to a lab where they count the number of colonies that grew and identify the type of mold.

Some mold spores are heavier than others and will settle out of the air faster. This will skew the results by detecting predominantly heavier spore types. Your outdoor molds such as Cladosporium are heavier than the potentially toxic Aspergillus and Penicillium types.

An inherent limitation with petri dishes is that less than 2% of all the mold spores will grow. Have you ever planted a seed and not had it grow? The reason Mother Nature has mold producing millions of spores and plants producing millions of pollen spores is that only a tiny percentage of them will germinate. The ones that do grow compete on the petri dish for the limited surface area with other molds and bacteria. Penicillium was discovered by a scientist who was upset that a mold spore had contaminated the petri dish he was trying to culture bacteria on.

A third limitation with petri dishes is time. Some common mold spores take only a few days to grow from a single spore into a fuzzy mass. Others, such as the potentially toxic Stachybotrys, may take a week or longer to grow. Common outdoor molds may over grow the petri dish before Stachybotrys even gets started. This makes petri dishes a poor choice to assess for the presence of Stachybotrys black mold.

Yet another disadvantage of Do-It-Your self test kits is that you don’t know the volume of air being sampled. You simply lay the dish out and expect mold to drop out of the air. Comparisons to outdoors are not done or are not accurate. This is important because a comparison is required to be made to the outdoors to determine if there’s a problem indoors. Mold is present in outdoor air in seasonal quantities. If you place a petri dish out in the open mold will grow just as if you left a piece of fruit out on the kitchen counter. Does that mean there is a mold problem in your house? Certainly not. But how do you know? There are no government standards regarding what level of mold spores indoors constitutes a problem. That is determined by comparing the level and types of mold indoors to outdoors. Do-it-your-self kits can not be used to make these comparisons because the same volume of air is not captured indoors and outdoors. You need to use a vacuum pump that is connected to the petri dishes during sampling. Self-test kits don’t come with a pump.

Speaking of standards, it’s a good thing that assessments are not made using absolute values for the maximum level of mold allowed indoors. If we did we would be in trouble. Some homes would be declared a hazard when they are not; others might be given a stamp of approval when they are infested with mold. For example, what would happen if the acceptable standard was based the level of mold spores normally present in the outdoor air in Florida as a threshold for testing done in Phoenix, Arizona. We would potentially miss detecting a problem indoors in Arizona. The level of mold outdoors varies greatly from day to day anywhere you go.

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Revised: July 05, 2017.
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