Is Black Mould Dangerous, and How Do You Get Rid of It for Good?

Quick answer: Black mould on walls and ceilings is a sign of persistent damp, and it can harm health. It mainly affects the lungs and airways, and children, older people, pregnant women and anyone with asthma or a weakened immune system are most at risk. You can clean small patches yourself with a mould-specific product, but mould keeps returning until the moisture causing it is fixed.

Black spots spreading across a bathroom ceiling, behind a wardrobe or around a window frame are among the most common problems in UK homes. They're easy to dismiss as cosmetic. They aren't. This guide explains what black mould is, how harmful it can be, how to deal with small patches safely, and what it takes to stop it coming back.

What "black mould" actually is

"Black mould" isn't one organism. It's an everyday name for dark-coloured mould of many different species that grow on damp surfaces. You can't tell which species you have, or how harmful it is, from its colour. The one sometimes called "toxic black mould", Stachybotrys chartarum, is mostly associated with materials that have been soaked for a long time, such as after a leak or flood. It's much less common than the moulds that grow from everyday condensation.

The practical point is the same whatever the species: visible mould means a surface has been damp for a long time. Treat it as a damp problem, not a cleaning problem.

Is black mould dangerous?

It can be. Government guidance for rented housing providers, produced with the UK Health Security Agency, says damp and mould mainly affect the airways and lungs, and that their respiratory effects can cause serious illness and, in the most severe cases, death.

That's not a theoretical risk. Two-year-old Awaab Ishak died in December 2020 after prolonged exposure to mould in his family's social housing flat in Rochdale. The coroner's findings led directly to Awaab's Law.

Exposure is linked to:

• Respiratory infections, coughs and wheezing
• Developing or worsening asthma
• Allergic reactions, including a blocked or runny nose and irritated eyes
• Effects on mental wellbeing from living with a persistent problem

People at higher risk include:

• Babies and children
• Older people
• Pregnant women
• Anyone with asthma, COPD or another respiratory condition
• Anyone with a weakened immune system, for example during chemotherapy

People in more than one of these groups are particularly vulnerable. If someone in your home has symptoms that seem linked to damp or mould, speak to a GP or pharmacist, and treat the mould as urgent.

Why it keeps growing

Mould needs three things: moisture, something to grow on, and time. Plaster, paint, wallpaper, grout, sealant, timber and fabric all qualify, so moisture is the factor that decides whether mould grows.

The moisture usually comes from one of four sources:

1. Condensation. Warm, moist indoor air meets a cold surface and condenses. This is the most common cause, and it's why mould clusters on external walls, in corners, around windows and behind furniture where air doesn't circulate. Building standards treat a surface relative humidity of around 80% over a sustained period as the point where mould growth becomes likely.
2. Penetrating damp. Water gets in from outside through defective gutters, pointing, render, roofs or window seals.
3. Rising damp. Ground moisture rises through walls with a failed or bridged damp-proof course. It's less common than often claimed.
4. Leaks. Plumbing leaks inside walls, under floors, from showers and baths.

Our guide to condensation vs rising damp explains how to tell them apart.

How to clean small patches safely

If the mould covers a small area and you're not in a higher-risk group, you can usually clean it yourself:

1. Protect yourself. Wear rubber gloves, eye protection and a well-fitting mask, ideally FFP2 or better. Open the windows and close internal doors so spores don't spread to other rooms.
2. Use a mould-specific product. Government guidance recommends mould and mildew products in preference to bleach, for health and safety reasons. Follow the label instructions.
3. Don't dry-brush or vacuum it with a normal vacuum cleaner. Both throw spores into the air. Wipe gently with a damp cloth, working outwards from the edge of the patch.
4. Bag up cloths and gloves and put them straight in the outside bin.
5. Dry the area thoroughly afterwards.
6. Deal with soft furnishings. Wash affected clothes and fabrics on a hot wash or have them dry-cleaned. Heavily affected mattresses, carpets and soft toys are often better thrown away.

What not to do:

• Don't just paint over it. Anti-mould paint can help once the cause is fixed, but painting over active mould hides it rather than solving it.
• Don't mix cleaning products, especially bleach with anything containing ammonia or acids.
• Don't let anyone vulnerable do the cleaning, and keep them out of the room while it's done.

When to call a professional

Get a specialist in if:

• The mould covers a large area, or several rooms
• It comes back within weeks of cleaning
• It's linked to a damp patch, tide mark, bubbling plaster or a leak
• There's a musty smell but you can't see the source. Mould may be behind units, under floors or inside walls.
• Plaster, plasterboard, timber or carpet is affected beneath the surface
• Anyone in the home is in a higher-risk group
• You rent your home. Damp and mould caused by the building is your landlord's responsibility.

What a professional actually does

A good mould specialist works through four steps.

1. Find the cause. An inspection or damp survey identifies where the moisture is coming from, using moisture readings, thermal imaging and sometimes humidity logging over several days. See how much a damp survey costs.
2. Remove the mould. They contain the area, remove badly affected materials, clean surfaces with HEPA vacuums and appropriate biocides, and sometimes use fogging treatments. See dry fog vs traditional mould removal.
3. Fix what caused it. That could mean repairing a leak or gutter, improving ventilation with better extractor fans, trickle vents, positive input ventilation (PIV) or MVHR, or improving insulation and heating.
4. Check the result. They confirm the area has dried out and the mould hasn't returned.

Typical costs range from around £150 for a small area of surface treatment to £3,000 or more for extensive remediation. Our mould removal cost guide breaks that down.

How to stop it coming back

Fixing the cause matters most. These everyday steps help the building cope with normal moisture:

• Keep the heating steady. A home kept at a moderate, even temperature has fewer cold surfaces than one heated in short bursts.
• Use the ventilation you have. Run kitchen and bathroom extractor fans while cooking and showering and for a while afterwards, and keep trickle vents open.
• Let air circulate. Leave a small gap between furniture and external walls, and don't overfill wardrobes on cold walls.
• Deal with water quickly. Wipe heavy condensation off windows in the morning and fix leaks straight away.
• Measure it. An inexpensive hygrometer shows indoor humidity. If it's regularly high despite these steps, the building probably needs better ventilation. Our guide to PIV vs MVHR vs extractor fans compares the options.

None of these replaces fixing a building defect. If mould returns despite reasonable care, something about the building needs to change.

If you rent your home

In England, your landlord is responsible for damp and mould caused by the building, and government guidance says damp and mould shouldn't be blamed on tenants' "lifestyle". Report it in writing with photos. Our guide to who is responsible for damp and mould in a rented property explains your rights and what to do if nothing happens.

If you're a landlord, act quickly. Social landlords already work to fixed Awaab's Law deadlines, and private landlords will follow once the law is extended to them. See our landlord's guide to dealing with mould.

Find a mould specialist near you

MouldPros lists verified mould removal, damp survey and ventilation specialists across the UK. Get up to three free quotes. Your details only go to the specialists you choose.