Black mould: what causes it, is it dangerous, and how to get rid of it

Black mould is the dark, speckled growth that appears in bathroom corners, round window frames and on the wall behind a wardrobe. It grows wherever a surface stays damp, and in a home that means condensation, a leak or damp getting through a wall. We cover what it is, how dangerous it is and for whom, how to remove a small patch safely, how to stop it coming back, and who fixes it in a rented home.

What black mould is

Black mould is not one species. It is the name people give to several moulds that grow dark green to black on damp walls, ceilings, sealant and window frames, among them Cladosporium, Alternaria, Aspergillus niger and Stachybotrys chartarum. Stachybotrys is the one the press calls toxic black mould.

Which species you have does not change what you do. Every one of them needs a damp surface to grow, comes off the same way and comes back for the same reason.

Mould spores germinate on a surface that stays damp for 24 to 48 hours, and a visible colony follows in 7 to 10 days. That is why a corner that is wet every morning can go black within a fortnight while the rest of the wall stays clean.

The signs in a house are easy to miss until they spread:

• Black or dark green speckles in the corners of outside walls, round window reveals and on the ceiling above the shower
• Grey or black lines in grout and along the silicone round the bath
• A musty smell in one room, or in a wardrobe
• Peeling wallpaper, blistered paint or a wall that feels cold and clammy
• Mould on the back of furniture, or on clothes and shoes kept against an outside wall

Is black mould dangerous?

Yes, and more for some people than others. The government's health guidance on damp and mould says they release allergens, irritants, spores and other toxins into the air. Because these are breathed in, they mainly affect the airways and lungs.

The respiratory effects can cause serious illness and, in the most severe cases, death. In 2020, two-year-old Awaab Ishak died of a severe respiratory condition caused by prolonged exposure to mould in a home with inadequate ventilation.

It is a common hazard, not a rare one. The same guidance estimates that 4% to 27% of homes in England have damp and mould, between 962,000 and 6.5 million households.

Who is most at risk from black mould

The guidance names the people for whom damp and mould are most harmful:

• Children and young people
• People with asthma, allergies, COPD, cystic fibrosis, or lung or heart disease
• People with a weakened immune system
• Pregnant women and new mothers
• Older people, and anyone bedbound or housebound
• People with mental health conditions

Is it safe to live in a house with black mould?

A house with black mould is not somewhere to leave it growing. The harm builds with how much mould there is, how long people live with it and who they are. A spot on the bath sealant is a job for this weekend. A bedroom wall going black where a baby or someone with asthma sleeps is a job for today.

Until the room is cleaned and the cause is fixed, keep anyone in the groups above out of it, especially overnight. If someone at home has a cough, wheeze or asthma that is getting worse, tell their GP about the mould.

What causes black mould

Mould needs water, and in a home the water comes from one of four places: condensation, penetrating damp, rising damp or a leak. Each one leaves black mould in a different place, and the pattern tells you which you have.

Condensation is the most common cause. Moisture from cooking, showers, drying laundry and breathing settles on the coldest surfaces in a room, and mould can grow once the air stays above 60% relative humidity. Stopping condensation comes down to less moisture in the air, more ventilation and warmer walls.

Mould that follows rain or sits low on a ground floor wall is not condensation, and airing the room will not clear it. A patch on the ceiling under a bathroom is a leak from the room above until proved otherwise, and one on a top-floor ceiling that darkens after heavy rain points to a leak in the roof. Stop the water before cleaning anything.

Black mould in the bathroom

Bathrooms make more steam than any other room, so mould shows first on the ceiling above the shower, in the grout and along the silicone round the bath and tray. In England, Approved Document F sets a minimum rate of 15 litres a second for a bathroom extractor fan. A fan only works when it runs, so leave it on through the shower and for an hour afterwards.

If there is no fan, or the one you have barely holds a sheet of tissue against its grille, a new extractor fan costs £60 to £300 supplied and fitted, in 3 to 4 hours. Refitting the bathroom is the time to fix the room for good, with a fan ducted to the outside, tanking behind the shower tiles and new anti-mould silicone.

Black mould in the bedroom

Bedrooms go black because of breathing. Two adults asleep breathe out up to half a litre of water a night, and with the door and window shut it settles on the coldest wall: the corner of an outside wall, the window reveal, or the plaster behind a wardrobe or bed pushed tight against it.

Pull furniture a few centimetres off outside walls, and open the window a little or leave the trickle vents open at night. Heat the room in cold weather even if nobody uses it by day. A spare room with a radiator that stays cold is often the first in the house to grow mould.

Black mould on windows and sills

Glass and frames are the coldest surfaces in most rooms, so condensation runs down them on cold mornings and mould grows in the rubber seals, on the sill and in the corners of the reveal. Wipe the water off first thing, before it soaks into the sill, and keep the trickle vents open day and night.

How to get rid of black mould

Clean black mould yourself when the patch is smaller than about 1m², sits in one room and comes from condensation. Anything bigger, mould in more than one room, or mould that comes back within weeks of cleaning is a job for a professional. If you have asthma, a lung condition or a weak immune system, have someone else do the cleaning.

Then work through it in this order:

1. Spray or wipe the cleaner over the patch and at least a metre beyond it, because spores sit in the wall around the growth you can see.
2. Leave it for the time on the label, usually 5 to 15 minutes.
3. Wipe it off with a damp cloth. Never brush or dry-scrape mould, because that lifts spores into the air you are breathing.
4. Bag the cloths and throw them away as you go.
5. Dry the wall fully. A dehumidifier running for 48 to 72 hours does it.
6. Vacuum the room once it is dry.
7. Repaint with an anti-mould paint once the plaster is dry.

Cleaning deals only with the mould you can see. The government's guidance is plain that removing surface mould will not stop it reappearing, so the clean buys you time to fix the moisture behind it.

Black mould on silicone and grout

On bath and shower sealant, a bleach-based mould gel left on for 1 to 2 hours lifts surface staining. Mould that has grown into the silicone itself does not come out. Cut the old bead out, let the joint dry fully and run a new bead of anti-mould silicone.

Grout is porous, so bleach lightens its colour without reaching the mould inside it. Grout that stays black after a mould cleaner needs raking out and replacing.

Mould on clothes, curtains and soft furnishings

Wash mouldy clothes, bedding and curtains at 60°C or hotter where the care label allows. Mould on shoes, bags and clothes in a wardrobe means the wardrobe stands against a cold outside wall or is packed too tight for air to move. Move it, or the clothes go mouldy again.

Does bleach kill black mould?

Bleach kills the mould it touches and lightens the stain, which is why it looks as though it has worked. On tiles, glass and other hard, sealed surfaces that is enough. On plaster, grout, wood and sealant it does not reach the growth below the surface, and the water in it gives the mould back some of the moisture it lives on.

The government's guidance recommends mould and mildew products in preference to bleach, for health and safety reasons. A 5-litre fungicidal wash costs about £11, and a mould removal kit about £25.

How to stop black mould coming back

Black mould returns wherever a surface stays damp, so preventing it means controlling moisture. Keep relative humidity at 40% to 60%. Above 60% mould can grow, and a hygrometer from about £8 shows where each room sits.

Where ventilation cannot keep up, as in a flat with nowhere outside to dry washing, a dehumidifier holds a room under 60%. Buy one that removes at least 5 litres of water every 24 hours. They start at around £100, and a mid-range machine costs about £220.

Steady heat matters as much as fresh air. A house left cold all day and heated in short bursts lets its walls cool right down, and cold walls are where condensation settles and mould grows.

When black mould needs a damp specialist

Call a damp specialist when:

• The mould covers more than about 1m², or it is in more than one room.
• It comes back within weeks of cleaning, with the fan working and humidity under 60%.
• It sits low on a ground floor wall, or gets worse after rain.
• Plaster is crumbling, or skirting boards, window sills or floorboards feel soft.
• Anyone living there has asthma, a breathing condition or a weak immune system.

Soft timber is the one not to wait on, because wood that stays wet for months is where dry rot takes hold.

A mould report or a full damp and mould survey costs roughly £120 to £300 including VAT, and it finds the cause before anyone prices a treatment. The bill for damp proofing follows that cause: condensation is fixed with fans, vents and heat, while rising and penetrating damp need building work.

Black mould in a rented home: who fixes it

In England, the landlord has to find and fix the cause of damp and mould. The government's guidance says it is unreasonable to blame damp and mould on a tenant's lifestyle, because cooking, bathing and drying washing all put moisture into the air. It ties the landlord's duty to the Housing Health and Safety Rating System under the Housing Act 2004 and to the Homes (Fitness for Human Habitation) Act 2018.

Social housing tenants in England have fixed deadlines under Awaab's Law. After the investigation, the landlord must send a written summary within 3 working days, start any further work within 12 weeks at the latest, and offer somewhere else to stay if the home cannot be made safe in time. If it misses a deadline, use its complaints process, then the Housing Ombudsman.

In Wales, the Renting Homes (Wales) Act 2016 makes the landlord keep the home fit to live in, and exposure to damp, mould or fungal growth is Matter 1 of the 29 things that can make it unfit. The landlord cannot ask the tenant to put right a problem that is not the tenant's fault. The fitness duty does not cover fixed-term contracts of 7 years or more.

Welsh tenants raise it with the landlord first, then get advice from Citizens Advice, Shelter Cymru or the council. Withholding rent is possible but risks arrears. In either nation, report mould in writing, keep a copy of every message and photograph the patch before you clean it.