Understanding Mould in UK Homes
 

Moulds appear in a wide range of shapes, sizes and colours, and although colour is often the first thing people notice, it’s not a reliable way to identify the type of mould or whether it poses a health risk. Proper identification normally requires laboratory analysis, as mould species are distinguished by microscopic features such as spore shape and growth patterns.
 

A single mould colony may display several colours or change colour over time. Factors influencing mould colour include:

• Food source (the material it’s feeding on)

• Humidity and moisture levels

• Exposure to light
   

Regardless of colour, any mould inside a home indicates an underlying moisture problem—commonly condensation, poor ventilation, penetrating damp, or leaks—and should be addressed promptly.

Below is a guide to the typical mould colours most commonly found in UK properties.
 

Common Mould Colours in UK Homes
 

Green Mould
 

Green mould is the most frequently observed in UK homes. Many species appear green during part of their growth cycle, particularly:

• Aspergillus

• Penicillium

• Cladosporium

Green mould often thrives on damp plaster, wallpaper, window reveals and household dust where condensation is present.
 

Black Mould
 

“Black mould” is a broad term and not all black moulds are the notorious toxic variety. However, black mould should always be taken seriously, especially if you suspect a persistent damp problem.
 

Stachybotrys chartarum (Toxic black mould)
 

This is the well-known “toxic black mould”, typically dark green-black and found in areas with long-term water saturation, such as:

• Leaking roofs

• Burst pipes

• Flooding

• Decaying timber or plasterboard
   

It produces mycotoxins, which can become airborne and cause headaches, coughing, rashes and other symptoms. Stachybotrys requires constant moisture, so eliminating the water source is essential. Professional remediation is recommended.
 

Alternaria
 

Usually an outdoor mould associated with soil and plants, but increasingly found in damp indoor areas such as bathrooms and window frames. While generally low-risk, it can aggravate asthma.
 

Aspergillus
 

A very common household mould; most people inhale small amounts daily. In vulnerable individuals, high exposure may cause respiratory problems.
 

Cladosporium
 

Often found on damp walls, insulation and carpets. It can cause skin irritation, sinus issues and eye discomfort.
 

Purple Mould
 

Uncommon in UK homes. When present, it is often a colour variation of Stachybotrys chartarum.
 

White Mould
 

White mould frequently appears in lofts, cellars and unheated rooms. Common culprits include:
 

• Alternaria – can appear white depending on environmental conditions.

• Chaetomium – thrives in dark, damp locations; often associated with a musty odour mistaken for “damp smell” in basements or lofts.

• Penicillium – may appear white early in its growth cycle before turning blue or green.
   

White mould often indicates prolonged humidity or condensation rather than sudden leaks.
 

Blue Mould
 

Commonly associated with Penicillium species. Frequently seen on damp walls, wallpaper paste, food items or on possessions stored in humid rooms.
 

Pink “Mould”
 

Often found as a pink film on shower curtains, tiles, and around baths.
This is usually not a mould, but a bacterium called Serratia marcescens, which thrives on soap residue.
Regular cleaning and improved bathroom ventilation help prevent recurrence.
 

Yellow Mould
 

Yellow moulds vary from pale yellow to brownish tones.
 

• Aspergillus – sometimes appears yellow.

• Serpula lacrymans – the fungus responsible for dry rot. Dangerous to building timbers rather than to health.

• Epicoccum nigrum – fast-growing and found on damp plaster, mattresses and timber.

• Geomyces pannorum – prefers cooler conditions such as unheated rooms, cellars and cold walls.
   

Bright Yellow Slime Mould
 

Bright, slimy yellow growth is usually a slime mould, not a true mould.
These organisms can be toxic—avoid touching or disturbing them.
 

Red Mould
 

Red mould is usually a transitional stage for other moulds rather than a standalone species.
On damp building materials, it is most often Aspergillus.
Red mould on food is typically Neurospora.
 

Orange Mould
 

Fast-growing, often found on damp or untreated timber. Like dry-rot fungi, it can damage wooden structures over time. Exposure may aggravate asthma but is generally low-risk otherwise.
 

Brown Mould
 

Brown mould can come from several species:

• Pithomyces chartarum

• Aureobasidium pullulans

• Stemonitis

• Taeoniella

• Cladosporium
   

Many brown moulds release a strong musty smell and can spread across hard surfaces.
 

Mucor
 

An important exception.
Mucor can be hazardous and has been linked to mucormycosis, a serious infection. It may appear brown, black, yellow, white or grey at different life stages, making visual identification unreliable.
 

Should You Test Mould?
 

Moulds are nature’s decomposers, but inside a home they can damage structures and affect indoor air quality.
Because colour alone cannot accurately identify mould, professional testing or a specialist survey is recommended when:
 

• The mould is widespread

• Health symptoms are present

• The source of moisture is unclear

• There is suspected dry rot or structural decay

• Vulnerable occupants (e.g., young children, elderly, immunocompromised) live in the property
   

Prompt identification and remediation can protect both the building and the occupants.