17/10/2026
Common Types of Black Mould and Their Potential Health Risks
Different species of mould can appear black, dark green or nearly black. Some of the more commonly discussed types include the following.
1. Stachybotrys chartarum
Often referred to as toxic black mould, Stachybotrys chartarum is a dark greenish-black mould associated with persistently damp, water-damaged materials.
Potential health concerns: Exposure to mould-contaminated environments may contribute to respiratory irritation, coughing, wheezing, nasal congestion and allergic symptoms. The health effects of exposure to this particular species are not fully understood, and its presence does not automatically mean that a building is dangerous.
Where it grows: It is commonly associated with materials containing cellulose, such as plasterboard, paper and wallpaper, particularly when these materials remain wet for prolonged periods.
Important consideration: Some strains can produce mycotoxins under certain conditions. However, the presence of the mould does not, by itself, establish that harmful levels of toxins are present or that someone has suffered toxic effects.
2. Aspergillus niger
Aspergillus niger is a widespread mould that can develop a dark, almost black appearance as it matures. It occurs naturally in the environment and can also be found indoors.
Potential health concerns: Exposure may cause allergic reactions or respiratory symptoms in susceptible individuals. Certain Aspergillus species can cause more serious infections in people with significantly weakened immune systems or particular underlying lung conditions.
Where it grows: It can be found in soil, decaying plant matter, stored food and various indoor environments where moisture and suitable organic materials are available.
Important consideration: Not every strain produces harmful toxins, and the risks associated with Aspergillus niger should not be confused with those associated with every other species in the Aspergillus group.
3. Alternaria
Alternaria is a common genus of mould found both outdoors and indoors. Some species can appear dark brown to black.
Potential health concerns: It is a recognised source of airborne allergens and may trigger sneezing, a runny nose, itchy eyes and other allergic symptoms. In susceptible individuals, exposure may also aggravate asthma.
Where it grows: It is frequently found on decaying vegetation, soil and plant debris. Its spores can enter buildings through open windows, doors, ventilation systems and on clothing.
Important consideration: Although it is commonly encountered in the outdoor environment, indoor growth can occur when moisture and suitable surfaces are available.
Important Factors to Understand About Black Mould
Individual sensitivity and health
People do not all respond to mould exposure in the same way. Some may experience no noticeable symptoms, while others develop allergic reactions, respiratory irritation or worsening asthma.
Those who may be more vulnerable include:
People with asthma, mould allergies or other respiratory conditions.
Infants, children and older adults.
Individuals with weakened immune systems or certain chronic lung conditions.
Symptoms can vary considerably, and they cannot reliably identify a particular mould species or prove that mould is the cause.
The importance of moisture control
Mould requires moisture to grow. Common causes of indoor mould include leaking pipes, roof leaks, penetrating damp, condensation, poor ventilation and flooding.
Simply cleaning away visible mould without correcting the underlying moisture problem is unlikely to provide a lasting solution. If the source of dampness remains, mould can return and continue affecting the surrounding materials.
Addressing leaks, improving ventilation where appropriate, controlling condensation and drying affected materials are essential steps in preventing further growth.
Safe mould removal
Small areas of mould on suitable, non-porous surfaces may sometimes be cleaned safely using appropriate precautions. However, extensive growth, recurring mould, contaminated porous materials or mould associated with significant water damage may require professional assessment and remediation.
Avoid disturbing extensive mould growth, as this can release spores and other particles into the air. People with asthma, significant mould allergies or weakened immune systems should avoid carrying out mould removal themselves where possible.
If mould is widespread, repeatedly returns or is associated with a persistent damp problem, a qualified professional can help identify the moisture source and determine the appropriate course of action.
Can You Identify Dangerous Mould by Its Colour?
No. Black colour alone cannot tell you whether a mould is toxic or harmful.
Several unrelated mould species can appear black, and the same species may look different depending on its age, growing conditions and the surface on which it develops. Visual identification is therefore unreliable when determining the species or assessing potential health risks.
It is also important to understand that mould does not need to be black to cause problems. Green, white, grey and other coloured moulds can also contribute to allergies, respiratory irritation and other health concerns.
In many cases, the most important factors are the extent of the growth, the conditions that allowed it to develop, the potential for exposure and the health of the people occupying the building.
Final Thoughts
Not all black mould is deadly, and finding black discolouration in a property does not automatically mean that a dangerous or toxic species is present. Nevertheless, all visible indoor mould growth should be taken seriously enough to identify and correct the underlying moisture problem.
If you suspect mould in your home or workplace, avoid relying on colour alone to judge its safety. Investigate the source of dampness, take appropriate steps to prevent further growth and seek professional advice if the affected area is extensive, persistent or difficult to assess.
The priority should always be to control moisture, manage exposure and maintain a healthy indoor environment, rather than assuming that every black patch is toxic or that mould of another colour is harmless.
