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Mold indoor activity levels

The activity level of a particular environment at the time of sampling will
likely affect the data collected by that sampling. Indoor spore levels
usually average 30 to 80% of the outdoor spore level at the time of
sampling, with the same general distribution of spore types. Filtered air,
air-conditioned air, or air remote from outside sources may average 5 to
15% of the outside air at the time of sampling. These percentages are
guidelines, only. The major factor is the accessibility of outdoor air. A
residence with open doors and windows with heavy foot traffic may
average 95% of the outdoor level while high rise office buildings with
little air exchange may average 2%. In addition, dusty interiors may
exceed 100% of the outdoors to some degree, but will still mirror the
outdoor distribution of spore types.

Utilizing aggressive or semi-aggressive sampling methods typically
increases the number of spores on sampling media. While this may
help to provide an indication of past problems with mold growth by
agitating reservoirs of indoor spores, aggressive sampling compounds
problems with interpretation because the comparison between an
aggressively taken sample and the outdoor air is less meaningful.