The lab emails you two numbers: indoor 1,800 spores/m³, outdoor 2,400 spores/m³. A client wants to know if that is bad. The number that answers them is not on the report — you build it from the relationship between the two.
How do you interpret an indoor:outdoor mold spore ratio?
You compare the indoor total and genera to a same-day outdoor control. When indoor spore levels are similar to or lower than outdoors, with the same kinds of fungi in similar proportion, the air typically reads as no indoor amplification. When indoor exceeds outdoor — especially with genera scarce outside — that pattern suggests an indoor source (AIHA, Green Book).
There is no published numeric cutoff that turns the ratio into a verdict; the ratio is a reading, not a pass/fail. The EPA and CDC set no acceptable airborne mold concentration, so the comparison itself — not a threshold — is what you interpret (EPA, Mold Remediation; CDC, Mold: Basic Facts).
Does a ratio below 1 mean the building is clean?
Usually it points toward no amplification, but the genus breakdown can override the number. An indoor:outdoor total ratio below 1 means indoor air carried fewer total spores than outdoors, which generally argues against an indoor source. The total alone is not conclusive, because a low total can still hide a water-damage signal.
Watch the species composition, not just the sum. If the indoor air shows a genus that is rare or absent outdoors — Stachybotrys or Chaetomium, the classic water-damage indicators — that pattern flags a hidden indoor source even when the total ratio sits below 1. A favorable total with an indicator-species spike is a finding, not a clearance. The water-damage indicators connect to normal spore counts.
Why is there no single "safe" indoor:outdoor ratio?
Because outdoor spore levels swing by season, weather, and region, so any fixed cutoff would be right in one place and wrong in another. The ACGIH bioaerosol framework treats interpretation as multi-factor and relative — comparing affected to reference areas and to outdoors — precisely because no universal threshold holds across conditions (ACGIH, Bioaerosols: Assessment and Control).
This is also why the same-day control is mandatory. An outdoor sample taken on a different day, in different weather, is not a valid baseline; the outdoor population can shift dramatically between a dry afternoon and the morning after rain. Pair the indoor and outdoor samples on the same visit, same equipment, same volume, so the only difference being read is indoor versus outdoor air. The volume-matching detail is in air pump calibration.
What are the common mistakes in reading these ratios?
Three recur. First, citing a magic number — "indoor under 1,000 is fine" — when no standard publishes one (CDC, Mold: Basic Facts). Second, reading the total and ignoring the genus split, so a water-damage indicator hidden in a low total goes unflagged. Third, comparing to an outdoor control taken on a different day, which invalidates the ratio.
A defensible reading states the indoor and outdoor totals, names the dominant genera in each, calls out any indicator species that are elevated indoors relative to outdoors, and stops short of a numeric pass/fail. That framing survives scrutiny because it reports the relationship the standards actually support (AIHA, Green Book).
MoldMind computes the indoor:outdoor comparison per genus from the structured lab fields, surfaces water-damage indicators that are elevated indoors against the outdoor control, and writes the comparative reading into the assessment — so the report carries the relationship, not a bare count. See the sample report.
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Sources
- AIHA, Green Book — indoor:outdoor comparison and indicator-species logic.
- EPA, Mold Remediation in Schools and Commercial Buildings — no numeric airborne limit.
- CDC, Mold: Basic Facts — no established acceptable level.
- ACGIH, Bioaerosols: Assessment and Control — relative, multi-factor interpretation.