Is black mould dangerous? The risks, and who they affect

What the risk actually is

Mould grows wherever moisture sits on a surface for long enough, and as it grows it releases spores and fragments into the air of the room. Breathing those in is the problem. The government's guidance on the health risks of damp and mould says the effects fall mainly on the airways and lungs, and that in the most severe cases they can be fatal.

The word dangerous needs some care. A few black spots on the sealant round a bath are not an emergency, and for most healthy adults they are a low risk. A bedroom wall that has been black behind the wardrobe for two winters, in a home with a baby or somebody with asthma, is a genuine health problem and deserves to be treated as one.

The symptoms it is linked to

The guidance groups the effects like this:

• Breathing: cough, wheeze and shortness of breath.
• Asthma: damp and mould can bring asthma on and make existing asthma worse.
• Infections: airway infections, including aspergillosis, a fungal infection that mainly troubles people whose immune system is weakened.
• Allergy and skin: sore or itchy eyes, allergic conjunctivitis, eczema and rashes.
• Mental health: anxiety and low mood from living with damp, spoiled belongings and waiting for repairs.

None of these is unique to mould, which is why it is so easy to miss. A cough that clears up on holiday and comes back at home, or that is worse for whoever sleeps in one particular room, is the pattern worth mentioning to a GP.

Who is most at risk

The same patch of mould does far more harm to some people than to others. The guidance names:

• babies, children and young people, whose lungs are still developing
• older people
• anyone with asthma, COPD or another lung or heart condition
• people with a weakened immune system, including those having cancer treatment, transplant patients and anyone on immunosuppressant medicine
• pregnant women and new mothers
• people who spend most of the day at home, including anyone bedbound or housebound

The case that changed housing law in England made the point. In December 2020 two-year-old Awaab Ishak died from a severe respiratory condition that the coroner found was caused by prolonged exposure to mould in his family's home. The rules that now carry his name are explained in the guide to landlord duties on damp and mould.

Is black mould worse than other colours?

Not in the way the phrase toxic black mould suggests. Colour says little about the type: several different moulds grow black, they look alike to the eye, and most of the black mould in British homes is ordinary mould feeding on condensation.

What decides the risk to health is how much there is, how long it has been growing, how much time people spend in the room, and who those people are. A large green or grey patch in a nursery is a bigger problem than a black line along a kitchen window that gets wiped every week.

What to do about it, in order

• Find the cause. Condensation is the most common source of damp: moist air from cooking, bathing and drying clothes meeting a cold wall or window. A patch that grows after rain, or sits under a bathroom on the floor above, points to water getting in instead.
• Clean off small patches. A small area on a hard surface can be cleaned by the householder with a mould and mildew product. The guidance says larger areas should be left to professionals.
• Change the conditions. Run extractor fans while cooking and showering, open a window for a while afterwards, dry washing outdoors or in a ventilated room, and keep furniture a few centimetres off outside walls.
• Report it if you rent. Tell the landlord in writing and keep a copy. In every part of Great Britain the law expects a rented home to be free of serious damp and mould hazards.
• Know when to replace things. The guidance warns that absorbent things such as carpets, mattresses and soft furnishings may have to be thrown away once mouldy, because cleaning does not always remove the growth completely.

When to see a doctor

Speak to a GP, or call NHS 111, if anyone in the home has breathing symptoms that are new or getting worse, if a child's asthma is becoming harder to control, or if somebody with a weakened immune system develops a cough, a temperature or chest symptoms.

Mention the damp. It changes what a doctor considers, and a letter from a GP or health visitor describing how the conditions affect a member of the household can support a repair request to a landlord or the council's environmental health team.

What a specialist treatment involves

A specialist mould treatment costs about £200 to £600 for a room and £600 to £1,500 for several rooms, with a damp survey at £150 to £400 where the cause is not obvious. The order matters more than the product. Treating a wall that is still wet buys a few weeks, while finding and fixing the moisture source is what stops the next bloom.

Ask anyone quoting to say what they think is causing the mould before they say how they will treat it. A thorough quote deals with the cause as well as the visible mould.

Once the cause is fixed and the walls are dry, a deep clean of the affected rooms, including the tops of wardrobes, inside cupboards and behind furniture where dust and spores settle, is a sensible last step before redecorating.