Act early:
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Ventilate more – especially the room where mould appears.
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Clean the affected area with appropriate cleaning products (there are specific anti-mould sprays in supermarkets and drugstores).
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Move furniture slightly away from the wall if mould is behind it.
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Check your habits – drying clothes, closed windows, lack of heating.
If:
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the mould comes back quickly,
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spreads despite better ventilation, or
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seems to be connected to a leak (wet patches, peeling paint, dripping),
then:
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take clear photos,
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inform the landlord in writing,
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describe since when you noticed it and what you already tried.
Early information makes it easier to find the real cause.