What is black mould? The common species explained
Black mould is not one species. It is an everyday name for any dark-coloured mould growing indoors. People usually mean Stachybotrys chartarum, but most dark household mould is really Cladosporium, Aspergillus or Penicillium. You cannot tell them apart by colour. Only a lab test of a swab or tape sample confirms which mould you have.
I started paying attention to this after a wet season in our Queensland house. There was a dark patch in the corner of the bathroom that kept coming back. I assumed it was the toxic black mould the internet warns about. It didn't matter how often we cleaned or what we cleaned with, it returned within a fortnight. What I worked out later is that the colour I was staring at told me almost nothing about what the mould actually was, and nothing about why it kept coming back.
Why is it called black mould?
Mould is a fungus. NSW Health groups it with organisms like mushrooms and yeast (NSW Health mould factsheet). Thousands of mould species exist, and a lot of them look dark to the naked eye, so "black mould" became shorthand for the ominous-looking stuff in the corner.
The name picked up its fearsome edge in the 1990s. A cluster of infant illness in Cleveland was linked in the media to a greenish-black mould called Stachybotrys chartarum, and the phrase "toxic black mold" entered everyday language. The current position of the US Centers for Disease Control and Prevention is more measured: it states that a possible association between Stachybotrys chartarum and that illness has not been proven (CDC, Facts About Stachybotrys chartarum). The label stuck anyway.
What does black mould look like?
Stachybotrys chartarum is genuinely dark. The CDC describes it as a "greenish-black mold" that grows on materials high in cellulose and low in nitrogen, such as fibreboard, gypsum board, paper, dust and lint (CDC). When it is actively wet it can look slimy and close to black.
The trouble is that plenty of other moulds look similar, and some look nothing like their reputation. Aspergillus niger, another mould routinely called black mould, starts out white and only turns black as its spore heads develop. Early growth of many species is pale and powdery, easy to mistake for dust. A dark stain on the wall could be several different organisms, and a mould that will eventually look black might not look black yet.
Can you identify mould by its colour?
Short version: no. Colour shifts with the species, the surface, the age of the colony and how wet it is. Aspergillus, Penicillium and Cladosporium often begin white or pale and only darken once their spores pick up pigment, so young colonies of completely different moulds can look identical.
Smell is a slightly better clue than colour, because the musty odour comes from compounds mould releases while it is actively growing, which is a topic on its own in why mould smells musty. But neither colour nor smell names the species. For that you need a laboratory.
What are the most common black moulds in Australian homes?
The reputation and the reality part ways here. Despite the fame of Stachybotrys, it is not the most common indoor mould. A 2015 study in PLOS One reported that Aspergillus, Penicillium and Cladosporium are the most predominant fungi in indoor environments (Segers et al., PLOS One, 2015). Cladosporium in particular is the dark mould most people are actually looking at: in that study the Cladosporium sphaerospermum species complex made up 44.7% of all isolates and 58.2% of surface-swab isolates, helped by its ability to grow on relatively dry surfaces, down to a water activity of about 0.82.
Stachybotrys is the opposite. It needs sustained, very high moisture. The Institut national de sante publique du Quebec (INSPQ) puts its requirement at a water activity of at least 0.94, with an optimum above 0.98, and notes it stays confined to materials that remain genuinely wet (INSPQ Stachybotrys chartarum fact sheet). In a normal home that is a narrower set of conditions than most people assume.
| Mould | Typical look | Where it grows | Moisture it needs | How common indoors |
|---|---|---|---|---|
| Stachybotrys chartarum | Greenish-black, slimy when wet | High-cellulose, low-nitrogen materials: plasterboard, fibreboard, paper, dust | Very high, sustained wetness (water activity ~0.94 or more) | Uncommon; the one people fear, not the one they usually have |
| Cladosporium | Olive-green through to brown or near-black, powdery | Many surfaces, including cooler and drier ones | Tolerates low moisture, down to about 0.82 water activity | The most common dark mould on indoor surfaces |
| Aspergillus (e.g. A. niger) | Starts white, darkens to black or brown as spores form | A wide range of damp organic materials | Moderate; less than Stachybotrys | Among the most common indoor moulds |
| Penicillium | Usually blue-green and powdery, not truly black | Damp building materials, fabrics, food | Moderate | Among the most common indoor moulds |
Prevalence and water-activity figures above come from the cited PLOS One and INSPQ sources. Appearance varies with the age of the colony and the surface, which is exactly why colour is a poor guide.
How do you know if it's Stachybotrys chartarum, and do you need a lab test?
You confirm it in a laboratory. Under a microscope, the shape and surface texture of the spores and the spore-bearing structures separate Stachybotrys chartarum from its relatives and from unrelated dark moulds (Update on Stachybotrys chartarum, peer-reviewed review). That is a level of detail no phone photo or naked eye can reach.
There is a practical wrinkle with Stachybotrys specifically. Its spores are rarely airborne, so air sampling tends to miss it. INSPQ notes it is usually identified from direct swab or sticky-tape lift samples taken off the surface and analysed in a lab, rather than from an air test (INSPQ).
Stachybotrys chartarum has been studied for a long time. It was first described in 1837 from wallpaper in a house in Prague, it once carried the name Stachybotrys atra, and the literature records it under roughly 132 synonyms accumulated over the years (peer-reviewed review). It can produce mycotoxins, a group of compounds that includes the macrocyclic trichothecenes called satratoxins, which is part of why it has been researched so heavily (review). A mould being able to produce a toxin is not the same thing as the stain on your wall being poisonous, and it is not something you can read off the colour.
Do you need to identify the mould before you deal with it?
For most households, no. The CDC's guidance is blunt: it says it is not necessary to determine what type of mould you may have, because the response is the same for all of them (CDC). You remove it and you fix the water.
That is the part the scary articles tend to skip. Naming the species rarely changes what you do next. Whether it is Cladosporium, Aspergillus or Stachybotrys, the mould is there because a surface stayed damp long enough for airborne spores to settle and grow, which is the whole reason how mould spores travel through a home matters more than the label on the stain. Deal with the damp and you deal with the cause. Leave the damp and any of them will come back.
What actually gets rid of black mould?
Moisture control. Not a stronger colour-coded warning, and not a single miracle wipe. The US Environmental Protection Agency states plainly that it is impossible to get rid of all mould and spores indoors, and, verbatim, that "the key to mold control is moisture control" (US EPA, A Brief Guide to Mold, Moisture and Your Home). Victoria's Better Health Channel says the same for Australian homes: there is no practical way to keep a home free of every mould spore indoors, and the way to control it is to control the source of moisture (Better Health Channel, mould and your health).
In practice that means finding why the surface is wet and drying it. If water-damaged materials are dried out within 24 to 48 hours, mould usually does not get established in the first place. Past that window it settles in, and cleaning the surface without fixing the damp just resets the clock. If you want the underlying physics, from where the spores come to why a wiped patch returns, it is worth reading how mould actually works in the home. The same principle explains why the patch in my bathroom kept coming back: I was cleaning the stain and ignoring the reason the corner stayed wet.
Frequently asked questions
Is all black mould toxic?
No. "Black mould" is a colour description, not a toxicity rating. Most dark indoor mould is Cladosporium or Aspergillus rather than Stachybotrys, and even Stachybotrys is described by the CDC in terms of what it is and where it grows, with the possible link to the illness that made it famous still unproven (CDC). Colour cannot tell you whether a mould can produce a toxin.
Black mould versus regular mould: what is the difference?
There is no scientific line between them. "Regular mould" and "black mould" are both everyday phrases for fungi that differ by species, not by a safe-versus-dangerous split you can see. The useful difference is moisture: some moulds, like Cladosporium, cope with drier surfaces, while Stachybotrys only appears where something has stayed very wet.
What is the black mould in my bathroom?
Most often it is Cladosporium or a similar surface mould feeding on soap film, sealant and damp grout, not Stachybotrys. Bathrooms get wet and then dry out between uses, which suits the drier-tolerant species more than the ones that need constant saturation. If you want certainty, a lab swab is the only way to name it.
Do I need a lab test to identify black mould?
Only if you specifically need the species named, for example for an insurance or tenancy dispute. For cleaning it up, the CDC says identification is not necessary, because you treat all mould the same way: remove it and fix the moisture (CDC). A lab test tells you the name; it does not change the fix.
Related guides
- How essential oils work on mould: the chemistry
- How mould spores spread through your home
- What causes the musty mould smell? MVOCs explained
Written by Ben, founder of Aurala Naturals. Last reviewed July 2026.
