Mold Species Identification Guide: The Fungi Actually Found in Homes

Homeowners searching for what’s growing in their home almost always start with color — “is it black, is it green, is it white” — but color is one of the least reliable ways to identify a mold species. Two very different species can look nearly identical to the eye, and the same species can vary in color depending on age, substrate, and lighting. Real identification requires laboratory analysis: microscopy for air and surface samples, or qPCR DNA analysis for dust samples (see our ERMI and HERTSMI-2 explainer for how that works). This guide explains what each commonly identified species actually tells an assessor.

Cladosporium

The most commonly identified genus in both outdoor and indoor air samples across North America. Appears green, olive, or black, typically as a surface growth rather than deep colonization. Found on window sills, tile grout, painted surfaces, HVAC vents, and fabric. Some species are allergenic; Cladosporium is not typically associated with mycotoxin production. Because it’s so prevalent outdoors, an elevated indoor Cladosporium count is only meaningful when compared directly against a simultaneous outdoor control sample — the whole reason air sampling always includes one.

Penicillium / Aspergillus (“Pen/Asp”)

Reported together in most lab results because their spores are difficult to distinguish reliably under standard light microscopy. Appears green or blue-green. Found on damp drywall, insulation, upholstery, wallpaper paste, and food. Some Aspergillus species (notably A. flavus and A. parasiticus) can produce mycotoxins called aflatoxins under certain conditions, though this is more relevant to food safety than typical residential air quality. An elevated Pen/Asp count indoors, especially well above the outdoor control, is one of the more reliable indicators of active indoor mold growth — it’s less common outdoors than Cladosporium, so an indoor spike stands out more clearly.

Stachybotrys chartarum

The species behind the “toxic black mold” label — the most publicly feared and most frequently misunderstood species in residential mold work. Appears slimy, black to dark greenish-black. Requires chronically wet, cellulose-rich material (drywall paper is close to ideal) and takes roughly 8 to 12 days of sustained wet conditions to establish, which makes its presence a reliable marker of a chronic leak or an inadequately dried flood — not a one-day event. It produces trichotecene mycotoxins under some growth conditions. Its spores are heavy and sticky and don’t aerosolize as readily as lighter species, which is exactly why Stachybotrys is frequently absent from standard air samples even when visibly present on a surface — surface sampling matters specifically when Stachybotrys is suspected. On the health question: the appropriate, evidence-supported framing is that mycotoxin exposure risk depends on concentration and duration, and it warrants professional remediation and a physician’s input for any health question — not a blanket claim that all exposure causes serious illness, and not a dismissal of it as media hysteria either.

Chaetomium

Brown to olive-black, with a distinctive woolly or cottony texture. An indicator species for chronic water intrusion specifically — it frequently co-occurs with Stachybotrys in drywall cavities and damp wood framing that’s been wet for an extended period. Its presence in a sample is a strong signal that a moisture problem is long-standing, not recent, even if no other visible sign points to that history.

Trichoderma

Green and fast-spreading, typically found on wet or decaying wood. Certain species produce cellulose-degrading enzymes, making Trichoderma relevant to structural timber integrity, not just air quality — a Trichoderma finding on framing lumber is worth flagging to a structural assessment, not just a mold remediation scope.

Alternaria

One of the most common outdoor allergen sources in North America (a major contributor to seasonal allergy symptoms), Alternaria also grows readily indoors in damp areas — shower stalls, window frames, and areas with condensation. Dark, with a fuzzy or velvety texture. Its ubiquity outdoors, similar to Cladosporium, means its indoor significance is read against the outdoor control rather than in isolation.

Serpula lacrymans (dry rot fungus)

Technically a wood-decay fungus rather than a mold in the strict sense, but relevant enough to flag: rusty-brown mycelium with strand-like growth patterns, capable of transporting moisture through its own mycelial strands into wood that wasn’t previously wet. Highly destructive to structural timber and requires remediation beyond a standard mold protocol — a structural assessment, not just surface cleaning.

Why the species matters more than the color

Two homes with what looks like the same “black mold” on the surface can require completely different responses depending on the actual species: a Cladosporium surface growth on a window sill is a routine cleaning matter; the same visual appearance from Chaetomium or Stachybotrys indicates a chronic moisture problem needing a full IICRC S520 assessment and remediation protocol (see our S520 standard guide for what that involves). This is exactly why a licensed assessor sends samples to an accredited laboratory rather than making a call based on appearance alone — and why we’d encourage any homeowner to ask what species was actually identified, not just accept “mold” as the full answer.

[Sources: species biology and remediation-relevance synthesized from IICRC S520 (Standard and Reference Guide for Professional Mold Remediation) and standard mycology/industrial-hygiene references used in AIHA-accredited laboratory reporting.]