Water activity (aw) is the fraction of water in a material that is biologically available, measured on a scale from 0 (bone dry) to 1 (pure water); it is the moisture metric that actually predicts whether mold can grow, not the total water content.
What is water activity?
A material can hold a lot of water yet still be too dry for mold if most of that water is bound and unavailable, which is why water activity, not raw moisture content, governs growth. Most common indoor molds need a water activity above roughly 0.8 to grow, and the more aggressive water-damage species generally need higher, sustained availability. This is the science behind the standard remediation rule of thumb: dry the material down so that available water falls below the growth threshold and growth stops, even if the material is not perfectly dry. ASHRAE 160 builds its moisture-control design criteria around keeping surfaces below the conditions that sustain mold (ASHRAE 160).
Why it matters to a mold inspection
Water activity explains why a quick dry-down within the first couple of days prevents a problem while a slow one does not: keeping available water below about 0.8 denies growth the resource it needs. It also explains why "the wall feels dry" is not the same as "the wall is safe," because a moisture meter reads bulk wetness, not availability. The EPA's 24-to-48-hour drying guidance is essentially a water-activity argument, dry fast enough that aw never climbs into the growth range (EPA, Mold Remediation in Schools and Commercial Buildings). See why dry-looking drywall comes out and relative humidity.
MoldMind stores moisture and material data per finding, so a drying decision is documented against the material that actually drives growth risk.