What Is Penicillium?

Penicillium is a large genus of molds most famous for producing penicillin — the antibiotic discovered when Alexander Fleming noticed Penicillium contaminating his bacterial cultures. Despite this beneficial connection, indoor Penicillium growth is a significant problem: it is one of the most common building-associated mold genera and a potent allergen for sensitized individuals.

Penicillium typically appears as blue-green to gray-green powdery growth, sometimes with white edges. It is particularly common on water-damaged wallboard, wallpaper, carpet backing, and wood. Unlike Stachybotrys, Penicillium can grow at relatively moderate moisture levels, making it one of the first molds to appear after water damage events and one that can persist in conditions that would not sustain more moisture-demanding species.

Some Penicillium species produce mycotoxins, most notably ochratoxin A (produced by P. verrucosum and P. nordicum) and patulin (P. expansum, often found on apples). At typical indoor building concentrations, these mycotoxins do not reach the levels demonstrated to cause systemic illness in laboratory studies, but long-term occupant exposure in severely contaminated buildings is a legitimate health concern.

Penicillium and Aspergillus are so frequently found together in indoor air samples — and their spores are so similar in appearance under microscopy — that laboratories often report them combined as "Penicillium/Aspergillus" in spore trap analyses. Distinguishing between the two genera requires culture-based testing or molecular methods. Elevated Pen/Asp counts in an indoor air sample relative to outdoor baseline strongly suggest active water-damage mold growth.