Damp, mould and structural
Black mould: how dangerous is it and how to get rid of it
Black mould is a fungus that grows on damp surfaces, usually as black or dark green patches in corners, around windows and behind furniture. The NHS advises that damp and mould can affect breathing and are riskier for babies, older people and those with asthma. Removing it without fixing the moisture only delays its return.
- Also known as
- Aspergillus and Cladosporium moulds, damp mould
- Easily confused with
- mildew (usually flatter and grey or white), soot or dust staining, efflorescence salts (white, not black)
- How serious
- Moderate: a health concern, especially for the vulnerable, and a damp signal
- Typical cost
- Usually no cost for DIY control
How to identify black mould
Black mould shows as clustered black or very dark green spots that spread into patches, often slightly furry or slimy, with a musty smell. It favours cold, poorly ventilated spots: window reveals, the corners of external walls, behind wardrobes and in bathrooms.
It is almost always a sign of condensation or another damp source rather than a structural fault. If it returns within weeks of cleaning, the underlying moisture has not been addressed.
How serious is it?
Moderate: a health concern, especially for the vulnerable, and a damp signal
The NHS states that living in a damp and mouldy home can cause or worsen respiratory problems, allergies and asthma, and that the very young, the elderly, and people with existing breathing or immune conditions are most at risk.
A small patch of mould in a bathroom is a maintenance and ventilation issue you can manage. Widespread or recurring mould, or mould affecting a vulnerable person, is more serious and the damp behind it should be diagnosed and fixed properly.
How to fix it yourself
- Protect yourself: open a window, and wear rubber gloves and an FFP-rated mask if you are sensitive or the area is large.
- Clean small areas of mould off hard surfaces with a proprietary mould remover or a diluted solution following the product label. Do not brush dry mould, which sends spores into the air.
- Reduce condensation, the usual cause: ventilate when cooking and bathing, dry washing outdoors or in a vented space, and heat the home more evenly to keep wall surfaces above dew point.
- Move furniture a little off external walls so air can circulate, and wipe down window condensation in the morning.
- If mould keeps returning, look for a leak or persistent cold spot rather than cleaning repeatedly.
When to call a professional
- Mould covers a large area (the NHS and many councils suggest professional help for extensive growth) or keeps returning despite ventilation changes.
- Someone in the home has asthma or another respiratory or immune condition, or you cannot find the moisture source.
Who to call
- A damp specialist or surveyor (a Property Care Association member) to diagnose the moisture source.
- Your local council environmental health team if you rent, as landlords have legal duties on damp and mould.
Frequently asked questions
Is black mould dangerous?
It can be. The NHS says damp and mould can cause or worsen respiratory problems and allergies, with babies, children, older people and those with asthma or weak immune systems most at risk. A healthy adult may notice little, but it should still be removed and the cause fixed.
What kills black mould permanently?
Nothing kills it permanently while the damp remains. A mould remover clears the visible growth, but it returns unless you cut the condensation or fix the leak feeding it. Lasting control means better ventilation, even heating and removing the moisture source, not stronger sprays.
Why do I keep getting black mould in the same corner?
External corners and window reveals are the coldest spots, so warm, moist indoor air condenses there first and feeds mould. Improve ventilation, keep furniture off that wall, and warm the room more evenly. Persistent mould in one spot can also point to a hidden leak.
Can I just paint over black mould?
No. Painting over mould traps it and it grows back through the paint, because the damp is still there. Clean the mould off first, fix the moisture cause, let the surface dry, then redecorate, ideally with a breathable or anti-mould paint.
Sources
Editor, HomesAndHedge
Oliver leads HomesAndHedge's editorial coverage of home and garden problems. He researches and writes the plain-English explainers on pests, invasive plants, damp and mould, drainage and wildlife, drawing on guidance from bodies such as the Property Care Association, the RHS and the NHS, and is clear about when a job needs a qualified professional.
Last reviewed: 8 June 2026