Species Matter More Than Count
Why the Species Identified Changes Everything
The spore count alone tells you less than the combination of count AND species. Industrial hygienists evaluate both together:
Indicator Species: Any Count Is Concerning
Stachybotrys chartarum and Chaetomium are "indicator species" — they require prolonged, severe water damage to grow and are rarely found outdoors. Any detection of these species indoors, even a single spore, triggers investigation by professional industrial hygienists because their presence confirms active or past water damage, not merely coincidental outdoor entry.
Common Species: Context Matters
Cladosporium, Penicillium, and Aspergillus are common outdoors and will naturally appear in indoor samples. They only become problematic when indoor counts significantly exceed outdoor counts OR when found in unusually high concentrations in a specific room. See our guide on Aspergillus humidity thresholds by species for species-specific context.
The Indoor-to-Outdoor Ratio Approach
The EPA and most AIHA-trained industrial hygienists use this method: take simultaneous indoor and outdoor air samples, then compare counts and species. A healthy building's indoor air should reflect its outdoor air — similar species at lower or equal counts. Indoor counts that are higher, or that contain species absent from the outdoor sample, indicate an indoor source that must be found and remediated.
This approach also explains why sampling season and time of day for air sampling affects results significantly — outdoor baseline levels change dramatically by season, weather, and time of day.
