Stachybotrys vs Aspergillus Field Decision Tree

Why this matters

Stachybotrys chartarum and Aspergillus or Penicillium are the two genus groups that drive scope decisions on most residential mold jobs. They look superficially similar to a homeowner, but they tell you very different things about water history, containment level, and clearance criteria. Misreading a Stachy job as an Asp/Pen job under-builds containment and risks a clearance failure or a homeowner medical claim. Misreading an Asp/Pen job as Stachy oversells the scope and burns trust. This article gives a field decision tree using visual cues, swab and tape lift results, and moisture history that aligns with IICRC S520 Condition 2 and 3 classification.

What each organism tells you about water

Stachybotrys is a tertiary colonizer. It needs prolonged free water on a cellulose substrate, typically 14 days or longer of continuous wetting. When you see Stachy on a wall, you are looking at a leak that ran undetected for at least two weeks, often months. The moisture source is almost never a single event. Common sources: long-running supply line drip behind the wall, failed shower pan, chronic roof leak, ice dam reflux, slab vapor drive into wall plates.

Aspergillus and Penicillium are primary colonizers. They germinate at lower water activity, around 0.78 to 0.85, and can fruit within 48 to 72 hours of elevated humidity. When you see Asp/Pen alone, the moisture trigger may have been a single short event, a humidity excursion, or chronic mild dampness with no liquid water. Common sources: a one-time HVAC condensate spill, basement summer humidity, crawlspace vapor without active leak, post-flood drying that ran too long below 60 percent RH but not dry enough.

Visual identification at the wall

Stachybotrys colonies are slimy when fresh, black to very dark green-black, often with a wet sheen. As they age and dry, they go matte black and powdery, and the colony edges look ragged. Hyphae are visible as a fine root mat on the substrate when you lift the surface growth. Stachy almost always sits on a paper-faced substrate: drywall paper, cardboard, ceiling tile facing, kraft-faced insulation, wallpaper backing. On wood framing alone, suspect Chaetomium or a different mold, not Stachy.

Aspergillus and Penicillium colonies are velvety to powdery from day one. Color varies by species: yellow-green for A. flavus, blue-green for P. chrysogenum, white-to-gray for many wall-cavity Aspergillus species. They colonize a much wider substrate range: wood, drywall paint, gypsum, dust films, paint chalking, OSB, vinyl wallpaper face, even glass with enough dust.

A wall showing both slimy black colonies on paper and powdery green colonies on adjacent paint is a long-duration leak that has gone through wet-to-dry cycling.

Sampling that confirms genus

Visual ID is presumptive only. Confirm with a swab or tape lift submitted to an accredited lab (AIHA EMLAP listed). Tell the lab to run direct microscopy with culture only when species ID matters for medical or legal claims.

• Tape lift on dry surface growth - cheapest, fastest, adequate for genus level.
• Swab on slimy or recessed growth - required when the surface is wet or textured.
• Bulk sample of cut drywall - best for cavity-side colonization confirmation.

Lab turnaround for direct microscopy is 24 to 48 hours. Culture is 5 to 10 days. For a Stachy claim that may go to litigation, request a chain-of-custody form and photograph the sample location with a scale reference.

Decision tree for scope

Build the scope from the worst-case organism present, not the average:

Step 1: Any Stachybotrys, Chaetomium, or Fusarium present (confirmed by lab). Treat as S520 Condition 3 over the entire materially affected area. Full Level 3 containment, negative air at 4 to 6 ACH minimum, full PPE including fit-tested half-face respirator with P100 cartridges per OSHA 29 CFR 1910.134, paper-faced substrate removed and bagged in 6-mil poly. Clearance requires both visual and air sampling.

Step 2: Aspergillus or Penicillium only, no tertiary colonizers, area under 10 square feet on a non-porous or semi-porous surface. Treat as S520 Condition 2 for the immediate surface and Condition 1 for the surrounding room. Level 2 containment (single-layer poly with zipper entry), negative air optional, HEPA vacuum and damp wipe acceptable, encapsulant on semi-porous substrates.

Step 3: Aspergillus or Penicillium only, area over 10 square feet OR area between 10 and 100 square feet with sensitive occupants. Move to Level 3 containment regardless of organism, because the spore load alone justifies it.

Step 4: Mixed Stachy plus Asp/Pen. Default to Step 1 protocol.

Clearance criteria that differ by organism

Stachybotrys jobs should clear by both visual inspection (no visible growth, no settled residue, moisture content of underlying framing under 16 percent on a Delmhorst BD-2100 wood-equivalent scale) and indoor-air sampling showing no Stachybotrys spores recovered in the work area when none are recovered in the matched outdoor control. Stachy spores are heavy and rarely show in air without active disturbance, so combine the post-remediation air sample with an aggressive sample (run air movers in the contained space for 5 to 10 minutes before the pump start) when the building owner or insurer demands a high-confidence clearance.

Aspergillus and Penicillium jobs clear by a comparison of work-area and outdoor air spore counts using a non-viable spore trap (Air-O-Cell or equivalent). The work area should show counts at or below the outdoor matched control for Asp/Pen genus. There is no zero-count standard. New York City DOH guidelines and ANSI/IICRC S520 both reject the "zero spores" premise because background Asp/Pen exists in all outdoor air.

References

• ANSI/IICRC S520-2024 Standard and Reference Guide for Professional Mold Remediation, Sections 10 (Inspection), 12 (Remediation), and 14 (Post-Remediation Verification).
• EPA Mold Remediation in Schools and Commercial Buildings (EPA 402-K-01-001), Chapter 2 on mold basics and Chapter 3 on remediation planning.
• New York City Department of Health and Mental Hygiene, Guidelines on Assessment and Remediation of Fungi in Indoor Environments, current edition, Sections 4 and 5.
• OSHA 29 CFR 1910.134 Respiratory Protection Standard for fit-testing and APR cartridge selection.
• AIHA Recognition, Evaluation, and Control of Indoor Mold (Green Book), Chapter 6 on sampling strategies.