Often, clients will posit the following question: can all of the mold in my home be eradicated?
The answer to this question, in short, is no.
Mold exists all around us in the outdoor environment, and inevitably in our indoor environments due to the oscillation and transference of air due to foot-traffic, open windows, etc. Outdoor mold is generally good, since mold breaks down and feeds on decaying, dead organic matter. This is vital to how nature works in relation to decomposition. Decomposition aids farmers, preserves the health of forests and also, most famously, aids in the process of biofuels.
After decomposition, carbon is released, which is the basis of life since living organisms capture this carbon and build new life; this is often referred to as the carbon cycle.
The reasons mentioned above are why outdoor molds are considered 'good'. When interpreting lab data, inspectors and microbiologists compare the potentially contaminated indoor air sample with the outdoor control sample to see if there is an issue in the property. When analyzing the data, discrepancies are registered. For example, if there are 5,000 spores m/3 of Cladosporium outdoors, and 4,000 spores m/3 indoors, then we can deduce that there isn't a mold issue in the property since the source is obviously the outside air.
Conversely, if there are 1,200 spores m/3 of Stachybotrys indoors (also referred to as 'black mold', even though there are numerous 'black molds'), but no spores outdoors, then we know that there is an indoor source for the mold and we have a mold issue within the property.
This isn't to say that outdoor molds can't cause an issue; they can in certain circumstances (in very high counts). People have reported the same side effects as we generally allergies to pollen.
Indoor molds, at high levels, is of concern since it usually suggests an indoor source (i.e. roof leak, high humidity, window condensation, slab leak, pinhole leak etc.). The reason one should be concerned with an indoor mold, even if it's a typical outdoor mold, is because if it's present in the indoor air at exponentially high levels, then it suggests that the outdoor air entering the property is actually reproducing inside and has found a moisture source to thrive in.
It is also of concern due to the concentrated health effects people report when exposed, the structural diminishing of a property, compromising the aesthetics of the structure, as well as rotting wood.
MOLD HEALTH EFFECTS BLOG AND RESEARCH