Indoor:outdoor ratio calculator
Divide the indoor total by the same-day outdoor control to get the ratio an air sample is actually read by — then confirm the reading against the genus split.
Total indoor spore count from the air sample.
Same-day, same-equipment outdoor reference.
How to read the indoor:outdoor ratio
The ratio is indoor total spores/m³ divided by the same-day outdoor total. A value at or below 1 means indoor air carried fewer total spores than outdoors, which generally argues against an indoor source; a value meaningfully above 1 — especially with genera scarce outside — points toward an indoor source. There is no published numeric pass/fail: the EPA and CDC set no acceptable airborne mold concentration, so the ratio is a reading, not a verdict.
The total can mislead. A favorable ratio below 1 can still hide a water-damage signal if a genus that is rare outdoors — Stachybotrys or Chaetomium — is elevated indoors. Always read the per-genus comparison alongside the number. For the full interpretation logic, the indicator-species traps, and why the same-day control is mandatory, see interpreting indoor:outdoor ratios.
Related reading
- Interpreting indoor:outdoor ratios (full guide)
- What is a normal spore count?
- Air sampling vs. surface sampling
Sources
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