Mould Symptoms: Signs of Mould Exposure to Watch For

A dark patch in the corner of a room, a musty smell in the bedroom, a nose that runs whenever you are at home: mould rarely announces itself all at once. Usually several small signs add up, and each one on its own has other possible explanations.

This article sorts the typical signs: what you can observe on walls, windows and in the air, which complaints occupants report, and how to tell mould from harmless deposits. It draws on the mould guideline of the German Environment Agency (UBA), the German medical guideline on indoor mould exposure (AWMF), the WHO guidelines on dampness and mould and UK government guidance.

In short: visible spots, a musty smell and condensation are the most common signs of mould. Among occupants, irritated eyes, nose and airways and a worsening of allergies or asthma are the main effects – but these complaints are non-specific and often have other causes. Persistent complaints belong with a doctor; the growth itself has to be removed whatever its type or colour. Whether a room has a mould source is answered by an air sample with lab analysis.

Recognising mould at home: the 7 signs

Mould needs moisture, so the signs appear where walls are cold or water gets in: external-wall corners, window reveals, behind wardrobes, bathrooms and cellars. Look out for these seven clues:

1. Spots and specks. Small black, green, grey or whitish dots that merge into patches – in corners, on silicone joints or behind furniture.
2. Discolouration on walls and ceilings. Yellowish rings, dark shadows along the external wall, a grey haze on the wallpaper. Mould on the ceiling often shows first as a discoloured room corner.
3. A musty smell. An earthy, mouldy odour that returns after airing should be taken seriously – even when you cannot see anything.
4. Condensation. Fogged-up panes in the morning, wet window sills and damp corners show that humidity is condensing on cold surfaces – the precondition for growth.
5. Peeling paint, loose wallpaper. Blisters in the paint, wallpaper coming away or crumbling plaster point to moisture in the building element – the growth often sits behind it.
6. Damp patches and water marks. After a burst pipe, leaking joints or roof damage, material often stays damp for weeks – mould grows inside it before it shows on the surface.
7. Complaints tied to the home. Irritation and respiratory complaints that are worse at home and ease on holiday are a clue – not proof.

A single sign says little. Two or three together – a smell, condensation and a discoloured corner, say – justify a closer look.

Physical symptoms of mould exposure

Moulds release spores, cell fragments and metabolic products into the room air. According to the UBA and AWMF guidelines, allergic and irritant effects are the main established effects on health. Occupants of damp, mouldy homes mainly report:

• irritated, red or watery eyes
• a runny or blocked nose, sneezing, a scratchy throat
• a dry cough, wheezing or whistling when breathing
• a worsening of asthma, hay fever or other allergies
• more frequent airway infections
• in some people, skin irritation or eczema
• less often reported: headaches, tiredness, disturbed sleep

All of these complaints are non-specific. They occur just as readily with colds, pollen or dust-mite allergy, dry heated air or tobacco smoke. For conditions such as chronic fatigue or neurological disorders, the AWMF guideline finds no robust evidence of a link with mould in homes.

There is no threshold above which complaints start. How strongly someone reacts depends on their own sensitivity, the extent of the growth and the duration of exposure. Whether your complaints come from the mould is therefore a question only a medical examination can answer.

Who reacts more sensitively

Children, older people, allergy sufferers, people with asthma or other lung diseases and people with a weakened immune system – during chemotherapy, for instance – are considered more sensitive; UK government guidance lists these groups too. Infections caused by moulds are very rare according to the UBA guideline and affect almost exclusively people with a severely weakened immune system. For children with an allergic disposition, an increased asthma risk has been described where the bedroom or living room is damp.

Hidden mould: symptoms without visible growth

Mould is not always visible. The UBA guideline names growth behind wallpaper and panelling, in the impact-sound insulation under floor coverings, in cavities and in damp plaster – damage that often only stands out because of the musty smell. According to the WHO guidelines, occupants of damp or mouldy buildings are at increased risk of respiratory symptoms, respiratory infections and worsening asthma – whether or not the growth can be seen.

Typical situations for hidden growth:

• a past water leak that was “dried” without opening up the floor build-up
• wardrobes and beds pushed tight against cold external walls
• boxed-in plumbing, suspended ceilings and insulation layers in the roof
• a smell coming from sockets, skirting boards or gaps in the floor

This is where an air sample helps: according to the UBA guideline, assessing the spore concentration in indoor air serves above all as an indicator of hidden growth. The lab counts and identifies the spores and compares every room with an outdoor sample. If a room is clearly above the outdoor air or shows moisture-indicator genera, a source in the room is likely.

What the report shows is explained in Lab analysis; whether a DIY kit is enough is covered in DIY mould test or lab test?.

Mould or something else? Salt efflorescence, dirt, soot

Not every stain is mould. Three things are regularly confused with it:

If in doubt, a surface sample settles it: with a swab or tape lift from the surface tests, the lab checks whether the surface carries growth structures or just settled dust. Do not wipe suspect patches first – that spreads spores and makes the assessment harder.

Black mould: symptoms and signs

“Black mould” is a colour, not a species. Many different fungi form dark growth – Cladosporium, Alternaria, Ulocladium, Aspergillus niger or Stachybotrys chartarum, for instance. They cannot be told apart with the naked eye – and white, grey and green growth is mould too.

“Toxic black mould” usually refers to Stachybotrys chartarum. This fungus needs very wet material and, according to the UBA guideline, only occurs on thoroughly soaked material, after water damage for instance. For poisoning via the room air in homes, the AWMF guideline finds no robust evidence; nor do the guidelines describe any symptom pattern specific to black moulds.

For the assessment, colour is therefore secondary. What matters is the extent, the cause of the moisture, the duration and the sensitivity of the occupants. According to the UBA and the AWMF, mould growth indoors is a hygiene problem that should be removed regardless of species – even when nobody has complaints.

What to do if you notice symptoms

1. Have complaints checked by a doctor. For persistent complaints, see your GP, who can refer you to a respiratory or allergy specialist if needed – as the UBA guideline recommends. Note when and in which rooms the complaints occur and whether they ease away from home.
2. Find and fix the cause of the moisture. Too little ventilation and heating, thermal bridges, leaking joints or an old water leak: unless the cause is fixed, the growth returns. Small, superficial patches under about half a square metre you can clean yourself if you are not allergic, have no respiratory condition and are not immunocompromised – see Home remedies for mould. Tenants should report the growth in writing and document it: Mould in a rented home: rights and evidence.
3. Measure instead of guessing. If you can smell it or have complaints but see nothing, an air sample shows whether the room has a mould source. With a complete set you take one sample per room in five minutes plus an outdoor sample; the lab report with spores per genus against the outdoor air arrives within 48 hours of the samples reaching the lab.
4. Have larger damage remediated professionally. For areas over half a square metre, growth inside building elements or sensitive occupants, remediation belongs in competent hands. The UBA guideline describes the procedure.

Doctor and measurement complement each other: the medical examination clarifies what is going on with you, the measurement what is going on with the room.

Frequently asked questions about mould symptoms

How quickly do symptoms appear?
That cannot be predicted. In sensitised people, allergic reactions can occur within minutes; irritation of the mucous membranes may only develop with longer exposure. A dose–response relationship has not been established – which is also why there is no limit value.

Can mould cause complaints even though I cannot see any?
Yes. Hidden growth behind panelling, under floors or in cavities releases spores and odour compounds into the room air. A persistent musty smell is the most important clue; an air sample with an outdoor comparison shows whether a source is likely.

Is black mould more dangerous than white or green mould?
Colour allows no conclusion about the risk. Many moulds are black, and lighter growth releases spores and allergens too. Any growth should be removed – regardless of species.

Does a mould test prove that my complaints come from the mould?
No. An air or surface test shows whether and where there is a mould source. Whether it causes your complaints is a medical question.

Do the complaints go away after remediation?
In many cases, yes. The UBA guideline cites studies showing that reducing the moisture load can lessen asthma and respiratory allergies. If complaints persist, your doctor should look at other causes.

See complete sets   DIY test or lab?

Written by the AirTrap team. Sources: UBA guideline on the prevention, detection and remediation of mould in buildings (2017, updated 2024); AWMF S2k guideline “Medizinisch klinische Diagnostik bei Schimmelpilzexposition in Innenräumen” (161-001, 2023 version); WHO guidelines for indoor air quality: dampness and mould (2009); UK government guidance “Understanding and addressing the health risks of damp and mould in the home” (2026). This article is general information and does not replace medical advice.