What’s the point of indoor outdoor comparisons?
Depending on the geographic ecosystem, there is some natural level of mold and mold spores outside (lower of course, somewhere like the North Pole compared to an area in the tropics). There is no practical way to eliminate all mold and mold spores in the indoor environment; and while there is no official standard, it is pure common sense that one indication of a problem is that there is more mold inside than out. Airborne mold counts alone are not adequate indicators of air quality. Type and degree of toxicity and concentration are also factors.
The National Allergy Bureau publishes these concentration standards:
|1 – 6499||Low|
|6500 – 12999||Moderate|
|13000 – 49999||High|
High concentrations tend to bother most individuals sensitive to mold.
Because there are no Federal standards for mold spore count levels in residences, schools, or other buildings, the Industry guidelines are derived from NYC (New York City Guidelines) and ACGIH (American Conference of Governmental Industrial Hygienists).
Naturally occurring biological materials such as mold, fungus, and pollen are not regulated by any government agency, and have no regulatory exposure limits. Current acceptable industry practice as explained in the previous paragraph, provides 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. If the indoor air samples show elevated levels of mold spores, or inconsistent types of mold, such samples are considered positive for mold. Positive samples indicate the need for additional investigation, supplemental testing or corrective measures. The theory is that any mold in the house comes from outdoors. If there is more indoors, it must come from indoors.