Mold Spores - Do All Homes Have Them? House mold and types of mold growth indoors

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Q. Do all homes have some mold spores? In other words, is it possible to culture mold in a Petri dish but not have a mold problem in your home? We did a test for mold using Pro Labs home kit. The first time in January, nothing grew but in September we had growth. Does this mean we have mold in our house growing somewhere or could it have come from our dehumidifier, which has mold growing in it? - Tom, Pennsylvania
A. All homes will test positive when air samples for mold are collected. Mold is a common part of the environment as it and many similar organisms consume dead organic matter such as leaves, tree limbs, dead grass, etc.
Because mold is such a part of the environment, spores can be found in both indoor and outdoor air. Actually in most cases outdoor concentrations will be higher than those found indoors. Outdoor mold often dominates indoor sample concentrations when windows are open during the warmer months. Many of these spores will remain in the house and be re-suspended when surface dusts are disturbed.
Typically when one has an infestation problem in a house, indoor mold will reflect the mold types present. These are often distinctively different from those suspended in the air outdoors.
Mold concentrations indoors tend to vary with the season even when there are indoor sources. Mold grows best under warm, humid conditions, and less well in cooler, drier wintertime homes of our northern states. As such, seeing differences in test results in January vs. September can be expected.
I am not familiar with the Pro Labs home kit. Since you used a Petri dish, I assume you used the setting plate method. If that is the case, it is a relatively crude way of trying to determine if you have a mold problem or not. Many mold species associated with indoor air problems settle slowly and may not appear on settling plates. As such, this method is of very limited usefulness.
Mold growing in a dehumidifier or humidifier can be a significant source of mold exposure on a localized basis. As such these devices need to be maintained so that they are relatively clean of mold.
Mold air concentrations in residences and other buildings can only be adequately assessed by using dynamic sampling methods, that is techniques which use samplers with air pumps and an appropriate collection medium.