Is black mould in my home dangerous to health?

What this usually means

Mould is a living organism that releases spores, and some people react to those spores and to the substances mould produces. The recognised health effects include triggering or worsening asthma and other respiratory conditions, allergic reactions, coughing, wheezing, a blocked or runny nose, and eye and skin irritation. These effects are more likely and more serious for vulnerable groups — infants and young children, older people, and anyone with a respiratory condition, allergy or a weakened immune system — which is why persistent mould in a bedroom or a child's room is taken particularly seriously.

However, the visible mould is the end of a chain, not the start. Mould can only grow where a surface is repeatedly damp — from condensation on cold surfaces, a leak, or penetrating damp — so the mould is a sign that part of your home is too wet, usually because warm, moist air is meeting a cold surface that is poorly ventilated. This is why scrubbing it off, or painting over it with anti-mould paint, never lasts: the surface is still damp, so the mould simply grows back, and the underlying moisture problem — and its health implications — continues.

So protecting your health means doing two things. First, deal with the existing mould safely — cleaning small amounts carefully, and protecting vulnerable occupants — to reduce the immediate exposure. Second, and crucially, find and remove the moisture source that is feeding it: warming the cold surface (insulation, treating thermal bridges), improving ventilation to remove humid air, and fixing any leak or damp. Once the surface is no longer repeatedly damp, the mould cannot return, which is the only way to genuinely remove the health risk rather than managing it indefinitely. Diagnosing why the surface is wet is therefore the key step.

Common causes

Signs and symptoms

Mould germination and growth require liquid water or sustained high humidity at a surface, a temperature within a broad comfortable range, and a food source — which ordinary dust and finishes readily provide. The limiting factor in homes is almost always surface moisture: mould establishes where the surface relative humidity stays high for long enough, typically on cold surfaces where condensation forms or near a leak. The health concern arises because active mould releases spores and metabolic products into the indoor air, to which sensitised and vulnerable individuals react, so reducing exposure means reducing both the mould and the airborne spores.

Because surface dampness is the controlling variable, the durable solution is hygrothermal, not cosmetic. Where condensation feeds the mould, raising the surface temperature above the dew point (through insulation and treating thermal bridges) and lowering the indoor humidity (through ventilation) stops the surface reaching the conditions mould needs; where a leak or penetrating damp feeds it, the water ingress must be stopped and the fabric dried. Biocidal cleaning and anti-mould coatings reduce the visible growth temporarily but do not change the surface moisture, so growth resumes — which is why symptom-only approaches fail and the exposure recurs.

Managing the health risk therefore has an immediate and a root-cause component. Immediately, existing mould should be cleaned safely and vulnerable occupants protected to cut current exposure. Fundamentally, the moisture source must be diagnosed and removed so the surface can no longer support mould — confirmed by humidity logging, thermal mapping of the cold surfaces, and identifying any leak. This investigation-led approach, which treats mould as a symptom of a building-physics moisture problem, is what permanently removes both the mould and the associated health risk, in contrast to repeated cleaning that leaves the underlying dampness — and the exposure — in place.

How to protect your health from black mould

Deal with the existing mould safely to cut immediate exposure, then find and remove the moisture source so it cannot return — the only lasting protection.

1. 01

   Protect vulnerable occupants

   Reduce exposure for children, the elderly and those with respiratory conditions while you address it.

2. 02

   Clean existing mould safely

   Remove small areas of mould carefully to cut the immediate spore exposure.

3. 03

   Diagnose the moisture source

   Establish whether it is condensation, a leak or penetrating damp feeding the mould.

4. 04

   Warm the cold surface

   Insulate and treat thermal bridges so the surface stays above the dew point.

5. 05

   Improve ventilation

   Remove humid air with adequate ventilation so surfaces are no longer repeatedly damp.

6. 06

   Fix any leak and verify

   Repair leaks or penetrating damp, then confirm the surface stays dry and the mould does not return.

How to prevent it coming back

• Remove the moisture source rather than just cleaning the mould.
• Warm cold surfaces and ventilate to stop condensation.
• Protect vulnerable occupants from exposure promptly.
• Fix leaks and penetrating damp that keep surfaces wet.