Why Indoor and Outdoor Air Comparisons Can Mislead

Aspergillus and Penicillium are not two molds. Each is a large group of several hundred species. A spore trap, the usual air test, cannot tell these species apart. Their tiny round spores look alike under the lab microscope, so the report lumps them into a single "Aspergillus/Penicillium-like" line.

That matters. The Aspergillus floating in your yard may be a completely different species from the one growing inside a damp wall, and spores found indoors need not have come from outside at all. Suppose a report shows 40 of these spores outside and 30 inside. The comparison says nothing about whether those 30 came from the yard or from a source in the basement.

She gave an example along these lines: one Aspergillus species found indoors at 100 spores per cubic meter, a different one outdoors at 200. Despite the higher outdoor number, she would suspect growth inside, because the indoor species had no outdoor match.

• An air sample captures only a few minutes. Spore levels outside rise and fall with season, weather, wind and nearby plants, so the outdoor number is a moving target, and one sample is a single snapshot.
• Spores sink into dust over time. If the indoor air was still when the sample was drawn, the reading can be low even when growth is nearby.
• Mold sealed inside a wall cavity often sends few spores into the room at all, so indoor air can look fine while growth continues.

• Compare rooms with each other, not with the yard. A clearly higher count in one area helps point toward a source.
• Read the raw counts, the spores actually seen, rather than the scaled-up per-cubic-meter figures.
• Look for types that should not be inside at all. Any Stachybotrys or Chaetomium suggests growth nearby.
• Treat Cladosporium differently. It is the one common mold whose indoor level may simply reflect outdoor air coming in. If it truly comes from inside, Aspergillus/Penicillium are usually raised too.
• Culture plate samples can be sent out for species identification when the question really matters.

The inspector still took an outdoor sample on most jobs. She did so mainly so that readers who trusted the comparison would not dismiss her report, and because it occasionally offered a useful hint, such as high outdoor Penicillium around a house surrounded by vegetation.

Evidence: Mixed or limited evidence.

Current guidance: Mainstream interpretation guides from occupational health bodies (AIHA, ACGIH) still use indoor/outdoor comparison as one line of evidence, alongside signs such as indoor types that differ from outdoor ones; her point about species is fair, but mainstream practice does not treat the comparison as worthless.

An outdoor number cannot clear a house, and it cannot condemn one. The question that matters is still where mold is growing indoors, and that is answered by looking at surfaces, rooms and the paths water has taken.

This page is for education only. It is not medical, legal or engineering advice. The inspector whose work this builds on was an environmental consultant, not a physician. Talk with your doctor about health concerns, and use licensed professionals for electrical work and large mold jobs. Products, prices and labs mentioned reflect the years they were recorded and may have changed.

Source: the field work of environmental inspector May Dooley, MS, MA, CMC

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