What Is Mold?

Mold is a category of fungi — organisms that are neither plants nor animals. Unlike plants, mold cannot photosynthesize and must consume organic material for energy. It does this by releasing enzymes that break down organic compounds (cellulose, wood fibers, paper, cotton, leather) and then absorbing the resulting nutrients. This is why mold destroys the materials it grows on.

Mold grows in a network of thread-like filaments called hyphae, which collectively form a mass called mycelium. The visible mold colony — the fuzzy or powdery patch you see — is mostly mycelium and reproductive structures. By the time mold is visible, it has typically been growing for some time and has penetrated into the surface beneath.

The three requirements for mold growth are moisture, an organic food source, and suitable temperatures. Moisture is the limiting factor in most buildings — organic food sources (drywall paper, wood framing, insulation facing) are abundant, and temperatures in occupied buildings are always suitable. This is why moisture control is the fundamental strategy for preventing mold: eliminate water intrusion and condensation, and mold cannot establish itself.

Over 100,000 mold species have been identified, and the vast majority are harmless or beneficial — mold is essential for decomposing organic matter in ecosystems. Only a fraction of species colonize buildings, and of those, only a subset produce compounds that cause health problems. The most commonly found indoor molds are Cladosporium, Penicillium, Aspergillus, and (in severe moisture situations) Stachybotrys.