The indoor:outdoor ratio is the indoor airborne spore concentration divided by a same-day outdoor concentration, used to decide whether the genera and counts found inside reflect normal outdoor air tracking in or an indoor mold source amplifying.
What is the indoor:outdoor ratio?
Outdoor air always carries spores, and indoor air normally mirrors it because doors, windows, and HVAC pull outside air in. So the question is not "is mold present indoors" (it always is) but "is more of it indoors than the outdoor baseline explains." Inspectors collect an outdoor control the same day, in similar weather, and compare. As a working rule, indoor concentrations at or below the outdoor sample, with a similar genus mix, read as no amplification; a much higher indoor count, or a genus common indoors but scarce outdoors, points to an indoor source (AIHA, Green Book). A common interpretive threshold for "no amplification" is an indoor:outdoor ratio under roughly 1.5, but it is a guide, not a legal line.
Why it matters to a mold inspection
The ratio is the backbone of air-sample interpretation, and it breaks if the controls are wrong. A missing or stale outdoor sample, or one taken in different weather, removes the baseline the whole comparison depends on. The EPA is explicit that sampling is not required to find a moisture problem and that results need professional interpretation, not a number plugged into a formula (EPA, Mold Remediation in Schools and Commercial Buildings). The depth version is in interpreting indoor:outdoor ratios, and why the same number reads differently by region is in normal spore count Phoenix vs Houston.
MoldMind keeps each indoor sample paired with its outdoor control and the computed ratio as structured fields, so an inspector never reports an indoor count without the baseline that gives it meaning.
Try MoldMind free, 3 jobs, no card.
Sources
- EPA, Mold Remediation in Schools and Commercial Buildings: sampling is optional, results need professional interpretation against outdoor air.
- AIHA, Green Book: indoor air normally tracks outdoor; amplification is judged by genus mix and concentration above baseline.