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What kind of mould is this — and does it actually matter?
Almost everything written about household mould is published by companies that remove it, which is why so much of it is frightening. The useful version is shorter: find the water, stop it, remove the growth. The species rarely changes any of that — and 3 of the things on this site are not mould at all.
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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.
1.Where is it?
optionalLocation narrows things faster than appearance, because each part of a house has its own moisture problem and its own usual suspects.
2.What does it feel and look like?
optionalTexture is far more diagnostic than colour. Slimy, powdery and crusty point in three completely different directions — and crusty usually means it is not mould at all.
3.What is it growing on?
optionalMaterial matters. Nothing grows on bare masonry that also grows on bread, and a white crust on concrete is almost certainly a mineral deposit.
4.What colour is it?
optionalLast, and deliberately so. Dozens of unrelated moulds are black, and colour changes as a colony matures — it is the least reliable thing about mould and the thing people lead with.
- Black Dust Around VentsNot mouldSoot, dust and ghosting
- EfflorescenceNot mouldMineral salt deposit
- Pink Shower SlimeNot mouldSerratia marcescens
- AspergillusInvestigateAspergillus species
- Dry RotInvestigateSerpula lacrymans
- Stachybotrys (Toxic Black Mold)InvestigateStachybotrys chartarum
- AlternariaRoutineAlternaria species
- CladosporiumRoutineCladosporium species
- PenicilliumRoutinePenicillium species
- Surface MildewRoutineVarious surface fungi
- White MouldRoutineVarious, often early-stage colonies
Common situations
Each of these has more than one likely answer, and some of them are not mould.
Mould by location
What grows where, and what each location tells you about the building.
Six places the water comes from
Questions people ask
Is black mould always toxic?
No. Dozens of unrelated moulds are black, and most black growth in bathrooms is ordinary surface mildew on grout. Stachybotrys, the species called toxic black mould, needs material that has been soaked for days — it does not grow on humidity alone. Public health guidance is to remove all indoor mould regardless of species, so the identification does not change the response.
Do I need to get mould tested?
For most households, no. The CDC does not recommend routine sampling, because the recommended action is the same whatever the species: find and fix the moisture, then remove the growth. Testing is more useful in disputes, insurance claims, or where someone has a diagnosed condition and a doctor has asked for it.
Why does mould keep coming back after I clean it?
Because the water is still there. Cleaning removes the colony but not the condition that produced it. If growth returns within a few weeks, the cause is usually ventilation, condensation on a cold surface, or a slow leak behind the surface.
Is there an app that identifies mould from a photo?
Yes — Mold Identifier, and this site is its free web companion. Worth saying plainly, though: the species is rarely the useful answer. Public health guidance is to remove all indoor mould whatever it is, so what actually changes your next step is where the water is coming from, and no photograph tells you that.
Is this site free, and is it trying to sell me remediation?
Free, no account, no email, and no. Almost everything written about household mould is published by companies that remove it, which is why so much of it is frightening. This site has no remediation partner and no lead form; the app is a free download for photo identification and offline reference.
What is the best way to find out what mould I have?
Start with the location and the moisture source rather than the growth. Each part of a house has its own water problem and its own usual suspects, so where it is narrows the possibilities faster than what it looks like. Laboratory sampling exists and is worth it in disputes, insurance claims, or when a doctor has asked for it — but the CDC does not recommend it for ordinary households, because the response does not change.