Stachybotrys (Toxic Black Mold)
InvestigateStachybotrys chartarum · also called toxic black mold, black mould
This is the species behind the phrase 'toxic black mold', and the honest position is more measured than the marketing around it. Stachybotrys does produce mycotoxins, and it does require sustained saturation — days of standing water, not humidity — so finding it means you have a real and ongoing water problem. But public health bodies advise removing all indoor mould regardless of species, because the health effects that are well established (allergy, asthma aggravation, irritation) are not species-specific. Species identification does not change the remediation.
Quick facts
- Appearance
- Very dark greenish-black, and characteristically wet-looking or slimy rather than dusty. It grows in flat sheets or patches on paper-faced drywall, and dries to a sooty powder only when the water source is removed.
- Moisture
- Needs material soaked for days
- Smell
- Strong, musty and earthy.
- Colours
- black, green
- Texture
- slimy, velvety
- Surfaces
- drywall, wood, concrete
How to recognise it
- Wet, slimy appearance rather than the dusty or fuzzy look of common moulds.
- Needs material that has been genuinely soaked for days — usually paper-faced drywall after a leak or flood.
- Very dark greenish-black in flat spreading patches.
- Almost always accompanied by a discoverable water source. If you cannot find one, reconsider the identification.
What it tells you about the building
Sustained saturation. The important task is finding and stopping the water, not naming the fungus — because anything you clean will return within weeks if the source is still there.
What it gets confused with
One observation settles each of these.
Cladosporium is far more common, olive-green to black, and dry and powdery rather than slimy. It grows in ordinary humidity.
Grout mildew is superficial, wipes off, and grows on humidity alone. Stachybotrys needs soaked material.
Staining does not have texture and does not regrow after cleaning.
Where it turns up
The species matters less than you think
- Public health guidance is to remove all indoor mould regardless of type. The CDC and EPA do not recommend routine species testing for ordinary households, because the response does not change: find the water, stop it, remove the growth.
- “Toxic black mould” is a marketing phrase, not a category. Stachybotrys is real and does need soaked material to grow, so finding it tells you something useful about your building. But the health effects that are well established — allergy, asthma aggravation, irritation — are not specific to it.
- Mould without water does not persist. Cleaning growth while leaving the moisture source is why it comes back. The leak, the condensation or the ventilation is the actual problem.
Call a professional for areas larger than about 10 square feet, for anything in HVAC ductwork, after sewage or flood water, or if anyone in the household is immunocompromised or has a lung condition.