What’s the point of indoor-outdoor comparisons?
Depending on the geographic ecosystem, there is some natural level of mold and mold spores outside. It is pure common sense that one indication of a problem is when there is more mold inside than outside.
Naturally occurring biological materials such as mold, fungus, and pollen are not regulated by any Federal agency, and have no regulatory exposure limits. Because there are no Federal standards for mold spore count levels in residences, schools, or other buildings, current acceptable mold-industry guidelines are derived from the New York City resource, Guidelines on Assessment and Remediation of Fungi in Indoor Environments and the American Conference of Governmental Industrial Hygienists (ACGIH) and their book, Bioaerosols: Assessment and Control. Those guidelines specify that if the mold measured in indoor air is substantially different from the mold measured in outdoor air, the indoor air quality is considered to be degraded.
Airborne mold counts alone are not adequate indicators of whether air quality is degraded. In addition to the spore concentration, the mold types and degree of toxicity are also factors. If, when compared to outside air, the indoor air samples show elevated levels of mold spores, types of molds inconsistent with those present outside, or a known toxic genus, such samples are considered positive for mold. Positive samples indicate the need for additional investigation, supplemental testing, or corrective measures.