Alternaria alternata
The outdoor allergen that follows moisture indoors
What it tells you about the building
Alternaria is primarily an outdoor and agricultural organism whose large spores settle quickly. Indoors it colonizes chronically damp spots near outdoor-air pathways. Elevated levels indoors typically track either local dampness or heavy outdoor infiltration.
Why it matters in assessments
Alternaria is among the most studied fungal allergens. In building assessment it functions mostly as a dampness and infiltration indicator; its allergenic profile makes indoor amplification worth correcting even at moderate levels for reactive occupants.
What remediation requires
Clean colonized surfaces, correct the local moisture condition, and evaluate envelope sealing and filtration if infiltration is the driver. Its rapid settling means thorough horizontal-surface cleaning removes most of the settled reservoir.
Dark grey-brown to black woolly colonies; under magnification, distinctive club-shaped multi-celled spores.
- Window frames and door thresholds
- Under-sink cabinets
- Shower framing and damp textiles
- Dust in homes with outdoor-air-driven humidity
State editions
Where this species shows up depends on where local climate and housing stock meet its moisture requirement. Read the local picture:
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