Stachybotrys chartarum

Stachybotrys chartarum is a dark, slow-growing cellulose decomposer. Indoors that means the paper on gypsum board, cardboard, wallpaper, ceiling tile and wood — materials that are ordinary, and that become suitable substrate only once they are thoroughly and repeatedly wet.

It is the organism behind the phrase toxic black mold, which is a popular label rather than a taxonomic or clinical category. Several unrelated indoor fungi are dark in colour, and a dark colony on a wall is not an identification. That side of the subject — what the label means, what the health evidence supports, and what to do about a visible colony — is covered on the dedicated page rather than here.

Published minima sit in the region of 0.89 water activity, with growth described as requiring at least 0.94 for reliable establishment and an optimum above 0.98 — effectively free water.

That is the highest requirement on the water-damage panel, and it is what makes the species informative. There is no ambient-humidity route to those conditions. A material at 0.94 aw and above has been wet, not damp, and has stayed that way.

It is also a late coloniser in the succession sense. Other organisms establish on a wetted material first, and Stachybotrys follows once conditions stay at the top of the range. Work on moisture-damaged houses has examined how it interacts with the species already present, including Chaetomium globosum — which is one reason the two so often appear together in the same building.

Some isolates of this species produce macrocyclic trichothecenes, a class of potent mycotoxins. That is a real and well-characterised capability, and it is the origin of the organism's reputation.

Three qualifications travel with it, and all three are routinely dropped. Production varies by isolate: not every strain of the species produces them. Production varies by conditions: substrate, water activity and temperature all matter, so the same strain may produce none. And production on a material is not the same as airborne exposure at a dose relevant to a person.

This is why identifying the organism is not a health finding. The major evidence reviews on damp indoor environments did not conclude that a distinct species-specific syndrome operates at household exposure levels; they associated respiratory and allergic outcomes with dampness itself. A page that reports the species accurately has to report that too.

That something stayed wet, at that location, for a long time. The investigation it calls for is structural rather than microbiological: where did the water come from, is it still coming, and what else did it damage while it was there.

Because the species requires the most water of anything on the panel, it is also the strongest signal that the wetting was not a single dried-out event. The 48-hour window that remediation guidance is built around exists precisely because assemblies dried inside it do not go on to support organisms like this one.

And it is a reason to be sceptical of a result that reports Stachybotrys with nothing else. A building wet enough for this species has almost always supported the mid-range organisms too, and a lone high reading is worth confirming before anything expensive follows from it.