Each cell of the damp wall carries a local Mold Growth Index M (0–6, the scale used in building-physics mold models). Growth depends on whether the local relative humidity exceeds a temperature-dependent critical threshold:
RH_crit(T) ≈ 65% + 0.55·|T − 24°C| (clamped to 65–98%)
if RH_local > RH_crit: dM/dt = k1 · (RH_local − RH_crit) · (1 − M/6)
else: dM/dt = −k2 · M (drying decline)
RH_local is highest at the simulated leak/condensation point in the corner and falls off with distance across the wall — the visible colony grid grows fastest right at the source. Once a cell's M passes a visibility threshold it releases spores into the room air:
Spore emission: E = e0 · Σ max(0, M_cell − 1)²
Well-mixed air: dC/dt = E/V − (ACH/3600 + f_HEPA) · C
The airborne concentration C is diluted by the fresh-air exchange rate (ACH, air changes per hour) and, when the HEPA toggle is on, by an added filtration removal term. Spores are rendered as particles that drift on the simulated ventilation current, spawn near active colonies, and are recycled when captured by dilution/filtration — so the particle cloud visually tracks the C readout.
- Humidity / temperature — raise RH above the local critical threshold (or push temperature away from ~24°C, which raises the threshold and slows growth) to start or stop colony growth.
- Ventilation (ACH) — higher air-change rate dilutes airborne spore concentration faster; it does not by itself dry the wall.
- HEPA filtration — adds a constant spore-removal rate to the air balance, independent of ACH.
Real-world relevance: this is the mechanism behind mold-risk assessment in building science (moisture control per ASHRAE/ISO guidance) and behind indoor-air-quality mitigation — keep sustained RH near a cold surface below roughly 70–80%, and use ventilation/filtration to control spore exposure once a colony exists.