Black Mould in Property, Causes, Health Risks and How to Treat It

What is Black Mould?

Black mould in properties is most commonly caused by Cladosporium species or, less frequently, Stachybotrys chartarum. Both appear as dark grey or black patches on walls, ceilings, window reveals, and soft furnishings. They grow where moisture is present and air circulation is poor, conditions typically created by condensation.

The UK Health Security Agency (UKHSA) recognises indoor mould as a significant public health concern. It is classified as a Category 1 hazard under the Housing Health and Safety Rating System (HHSRS) when present at a level likely to cause harm.

Health Risks Associated with Black Mould

The UKHSA has published guidance confirming that exposure to indoor mould is associated with:

• Respiratory symptoms including coughing, wheezing, and exacerbation of asthma
• Rhinitis (hay fever-like symptoms)
• Eye, nose, and throat irritation
• In vulnerable groups (infants, elderly, immunocompromised individuals), more severe respiratory illness

The inquest into the death of two-year-old Awaab Ishak in 2020, who died as a result of prolonged exposure to black mould in a social housing property, resulted in the introduction of Awaab's Law as part of the Social Housing (Regulation) Act 2023. Awaab's Law places strict time limits on social landlords to investigate and fix damp and mould hazards.

Awaab's Law, Key Requirements for Social Landlords

Awaab's Law applies to social housing providers. The Renters' Rights Act 2025 and existing obligations under the Homes (Fitness for Human Habitation) Act 2018 extend comparable, if less prescriptive, duties to private landlords.

Causes of Black Mould

Black mould grows where relative humidity exceeds approximately 70% at a surface. This is almost always caused by condensation, the result of moisture-laden air meeting cold surfaces. Common contributing factors include:

• Poor or absent mechanical ventilation in bathrooms and kitchens
• Thermal bridging at wall junctions, lintels, and window reveals
• Inadequate or inconsistent heating
• Overcrowding (more occupants produce more moisture through breathing and cooking)
• Draught-proofing without compensatory ventilation

Treatment

Surface mould can be treated with a diluted bleach solution (1 part bleach to 4 parts water) or with commercially available fungicidal wash products. The PCA recommends treating the affected surface, allowing it to dry fully, and applying a fungicidal paint or sealant.

However, surface treatment is not a permanent solution unless the underlying cause is addressed. Treatment without tackling condensation will result in mould returning.

Effective remediation requires:

• Improved ventilation, extractor fans on humidistat controls, or whole-house MVHR
• Raising surface temperatures through draught-proofing, insulation, and consistent background heating
• Identifying and repairing any associated penetrating damp

Mould in the Home-Buying Process

A RICS Level 2 or Level 3 survey will note the presence of mould and assess its likely cause. Buyers should ask surveyors to distinguish between surface condensation mould and mould associated with structural damp. Property Passport UK displays EPC data including wall and roof insulation ratings, which can help buyers assess a property's susceptibility to condensation before commissioning a full survey.