Black Mould in the Home: Health Risks, Causes and What to Do

Black mould is one of the most common and most concerning problems reported by homeowners and tenants across the UK. It’s also one of the most misunderstood — both in terms of what causes it and what the health risks actually are.

This guide gives you the facts, separates myth from evidence, and explains what actually gets rid of it.

What Is Black Mould?

The term "black mould" is used loosely to describe mould growth that appears dark — usually black, dark green, or dark grey. In UK homes, the most commonly identified species in this category is Cladosporium, which is ubiquitous, and Aspergillus/Penicillium group species. The species frequently associated with the most serious health concerns — Stachybotrys chartarum — does occur in UK properties but is less common than media coverage suggests.

The colour of mould does not reliably indicate the species or the risk level. Laboratory analysis of air samples is the only way to identify which species are present and at what concentration.

What Causes Black Mould in UK Homes?

In the overwhelming majority of cases, black mould in UK homes is caused by condensation — moisture depositing on cold surfaces where it provides the water, nutrients, and temperature conditions that mould spores need to germinate and grow.

The ideal conditions for mould growth are:

• Relative humidity above 70% at the surface
• Surface temperatures between 5°C and 35°C (most moulds thrive between 15°C and 25°C)
• A nutrient source — dust, paint, wallpaper paste, plaster
• Time — mould colonies establish over days to weeks in suitable conditions

Cold spots in poorly ventilated rooms — corners, window reveals, behind furniture — provide exactly these conditions.

Less commonly, black mould results from penetrating damp (water entering through defective building fabric) or rising damp, both of which create persistently wet conditions in wall materials that support mould growth.

What Are the Health Risks?

Mould exposure is associated with a range of health effects, most significantly affecting the respiratory system. The evidence base includes:

Respiratory symptoms — coughing, wheezing, and worsening of asthma are well-established associations with indoor mould exposure. The WHO Indoor Air Quality Guidelines (2009) conclude that living or working in damp, mouldy buildings is associated with respiratory symptoms, asthma, and respiratory infections.

Allergic responses — mould spores are recognised allergens. In sensitised individuals, exposure causes rhinitis, conjunctivitis, and skin reactions.

Hypersensitivity pneumonitis — a serious inflammatory lung condition caused by repeated exposure to specific mould species. Less common but potentially severe.

Mycotoxin-related effects — some mould species produce mycotoxins — toxic secondary metabolites — that can cause systemic effects. Stachybotrys chartarum is the species most commonly associated with this, though the evidence for severe systemic mycotoxicosis in typical residential settings is more contested than media reporting suggests.

Vulnerable groups — infants, the elderly, those with pre-existing respiratory conditions, and immunocompromised individuals — face greater risk from the same exposure levels.

What Actually Gets Rid of It?

Step one: identify and resolve the moisture source. Mould cannot be permanently resolved without addressing why the surface is wet. Cleaning mould off a wall and repainting is a short-term cosmetic fix. Within weeks or months, in unchanged conditions, it returns. The moisture source — whether condensation, penetrating damp, or rising damp — must be identified and resolved first.

Step two: proper remediation. For minor surface mould (less than 1 square metre), careful cleaning with an appropriate biocidal wash, followed by HEPA vacuuming to remove spore debris, and repainting with a fungicidal paint is usually sufficient once the moisture source is resolved.

For larger areas of mould — particularly where it has penetrated into porous materials like plaster, plasterboard, or timber — professional remediation following ANSI/IICRC S520 standards is appropriate. This involves controlled removal of contaminated materials, HEPA filtration of the air during works, and post-remediation verification testing to confirm the environment is clear.

Step three: verify. Post-remediation air quality testing confirms that spore concentrations have returned to acceptable levels and that the remediation has been effective.

Do You Need Professional Air Testing?

For minor surface mould in a property with no specific health concerns in the occupants, professional air testing is not always necessary if the cause is clearly condensation and the remediation is straightforward.

Where there are health symptoms in occupants that may be mould-related, where significant mould growth is present, where vulnerable occupants are affected, or where legal or insurance matters are involved, laboratory air quality testing provides the documented evidence that supports decision-making and demonstrates due diligence.

Richard Bull MISSE holds IICRC AMRT certification and City & Guilds Level 3 in Professional Mould Remediation, and carries out mould surveys including laboratory air sampling across Derby, Nottingham, Leicester, and the Midlands. Call 07983 550 662 or use the contact form.