# Mould on walls: causes, health and how to get rid of it for good > Why mould appears, what the WHO and Portugal's DGS say about health, how to clean a small patch safely and why anti-mould paint is not enough. - Page: https://paredecerta.pt/en/guides/mould-on-walls - Language: English - Updated: 2026-11-03 ## Summary - Mould only grows where there is damp. Cleaning the patch without fixing the cause does not stop it coming back. - The WHO concluded that living in damp or mouldy homes raises the risk of respiratory symptoms and infections and of worsening asthma. - Small patches are cleaned with water and detergent, gloves, goggles and a mask, with the door shut and the window open. - Large areas, hidden mould or vulnerable people at home call for a firm and a diagnosis. - Anti-mould paint on a damp wall is not a repair: fix the cause first, then clean, dry, and only then paint. Black dots in a bedroom corner, a greenish shadow behind the wardrobe, a bathroom ceiling slowly darkening: that is mould, a fungus that grows wherever there is water. Wiping the patch away is easy. Stopping it from coming back means fixing the damp that feeds it. This guide brings together what the WHO, Portugal's health authority (DGS) and other public health bodies say about the risks, how to clean a small area safely, when to call in a firm, and why an anti-mould paint is not a fix on its own. *Updated 3 November 2026. General information, not a substitute for an assessment by a qualified specialist or advice from your doctor.* ## The essentials - **No damp, no mould.** The WHO says the amount of water in materials is the most important trigger for fungal growth, and that ordinary household dust provides enough food. - The WHO concluded (2009) that people living in damp or mouldy buildings face a **higher risk of respiratory symptoms, respiratory infections and worsening asthma**. - Small patches can be cleaned by the occupant, with protection. Large areas, mould that keeps returning or vulnerable people in the home call for **a firm and a diagnosis of the cause**. - **Do not paint over it.** The US Environmental Protection Agency (EPA) says not to paint or caulk mouldy surfaces. The Swiss Federal Office of Public Health (FOPH) opposes biocide paints in living areas. - Check the wall again **at least six weeks** after treatment, as the 2026 English guidance recommends. ## Why mould appears Fungal spores float in the air of every home. The WHO explains that microbes are everywhere and multiply quickly wherever water is available. The FOPH puts it simply: **materials that stay damp for a few days will very probably go mouldy**. The EPA advises drying anything that got wet **within 24 to 48 hours** to prevent it. The water may come from the air (condensation on cold walls), the ground, rain or a pipe. The DGS links excess humidity directly to condensation on walls and to mould. Before cleaning, it is worth finding the source: the guide on [the types of damp in walls](https://paredecerta.pt/en/guides/damp-walls-three-types) helps tell the cases apart. The FOPH also points to dirty cooker hoods, broken extractor fans and small windows, which cut ventilation, and to curtains over cold window reveals, which block air movement. ## Is mould bad for your health? The WHO guidelines on dampness and mould (2009) conclude there is **sufficient epidemiological evidence** that occupants of damp or mouldy buildings are at higher risk of: - respiratory symptoms; - respiratory infections; - exacerbation of asthma. The WHO also notes some evidence of higher risk of allergic rhinitis and asthma, and clinical evidence of rare mould-related conditions such as hypersensitivity pneumonitis. The WHO **sets no limit value** for fungi in the air: it recommends preventing damp and, when it occurs, putting it right. The DGS says poor indoor air can worsen symptoms in **people with chronic respiratory disease, cardiovascular disease and allergies**. The 2026 English guidance adds other groups: children, older people, pregnant women, people with weakened immune systems (for example during cancer treatment or after a transplant) and people who are bedbound. If someone at home has a lasting cough, shortness of breath or asthma attacks, talk to your GP (médico de família) or the SNS 24 health line. This guide does not diagnose anything. ## How big a patch you can clean yourself The sources use different limits, so we show each with its own figure: | Source | Clean it yourself | Call a professional | |---|---|---| | FOPH (Switzerland), 2021 | Traces up to 100 cm² (10 × 10 cm) in one spot; isolated patches up to 0.5 m² (70 × 70 cm) in one room | Patches over 0.5 m², mould in deeper layers, several spots or several rooms | | EPA (USA) | Less than about 10 square feet (about 0.9 m²) | Larger areas, a lot of water damage or contaminated water | The FOPH adds two rules: if **vulnerable people** live in the home (severe allergic asthma, cystic fibrosis, weakened immune system), the cleaning **should not be done by the occupants themselves**; and a **clear mouldy smell with no visible mould** points to hidden mould, which specialists should investigate. ## How to clean a small patch Based on the EPA, the FOPH and the English guidance: 1. **Protect yourself:** gloves, goggles without ventilation holes and a respirator mask (the EPA names an N95). Wear long sleeves. 2. **Shut the room door** and **open the window** during and after cleaning, so spores do not spread to the rest of the home. 3. **Scrub with water and detergent** and let it dry completely. The FOPH says small patches come off easily this way. 4. **Disinfecting is optional.** The FOPH accepts bleach (sodium hypochlorite solution) or 70–80% ethyl alcohol. The EPA says bleach is not recommended as a routine step and that dead mould can still cause allergies, so it has to be removed. The English guidance prefers dedicated mould products to bleach. 5. **Never mix bleach** with products containing ammonia (EPA). Read the label of any anti-mould product and follow it. 6. **Porous materials** with mould, such as ceiling tiles or carpet, may have to be thrown away (EPA). 7. **Do not switch on a dehumidifier before the mould is removed**, the FOPH warns. Ventilate after cleaning. The FOPH warns that during repair work fungal particles in the air can rise sharply: one more reason for the closed door and open window. ## What about anti-mould paint? The FOPH gives the example of a damp wall painted with anti-mould paint: **it was not a proper repair**, because the damp and the mould stayed. The right sequence is to find and remove the cause, take off all the mould, let the wall dry, and only then redo the finish. The FOPH also says anti-mould products **do not replace repair work**. The EPA adds that paint applied over mould tends to peel. If your paint is already failing, see [peeling paint, blisters and stains](https://paredecerta.pt/en/guides/peeling-paint-stains). To choose a paint for bathrooms and kitchens once the cause is fixed, see [which paint for each room](https://paredecerta.pt/en/guides/which-paint-for-each-room), and work out the quantity with the [calculator](https://paredecerta.pt/en#calc). ## Keeping mould away FOPH recommendations unless stated otherwise: - **Air the home with a through-draught** at least three times a day, for 5 to 10 minutes, with doors and windows open. Avoid leaving tilt windows permanently ajar while the heating is on. - **Keep relative humidity between 30% and 50%** in the heating season (the EPA says below 60%, ideally 30–50%). - **Use the extractor fan** when cooking and showering, and keep it clean. - **Keep furniture 10 cm away** from outside walls. - **Do not put off heating** in old, poorly insulated homes. The FOPH suggests about 20 °C in such cases. - **Renew the sealant** around the bath and shower when it loses elasticity. If you rent, tell the landlord in writing, with photos and dates. Who pays for the repair depends on the cause and the contract: see [painting before renting or selling](https://paredecerta.pt/en/guides/painting-before-renting-or-selling) and, if the water comes from another flat, [leak from upstairs: who pays](https://paredecerta.pt/en/guides/leak-from-upstairs-who-pays). ## What we could not confirm - **A DGS or SNS 24 page with instructions for cleaning mould at home.** We found none. We used official guidance from Switzerland, the USA and England, which may not reflect Portuguese practice. - **How many Portuguese homes have mould.** We found no recent official statistic. The WHO estimates damp in 10–50% of indoor spaces across several countries, but gives no figure for Portugal. - **How effective and safe each anti-mould paint or product is.** We did not test brands. Read each product's technical and safety data sheets. - **Prices for mould removal.** We have no reliable public source. ## Sources (read on 3 November 2026) - World Health Organization, Regional Office for Europe, *WHO guidelines for indoor air quality: dampness and mould*, 2009 ([publication page](https://www.who.int/publications/i/item/9789289041683); executive summary, pp. XIV–XVI, read in PDF). - Direção-Geral da Saúde (DGS), [Qualidade do Ar Interior](https://www.dgs.pt/paginas-de-sistema/saude-de-a-a-z/ar-interior/qualidade-do-ar-interior.aspx) (Indoor air quality: humidity, mould, vulnerable groups). Page undated. - Swiss Federal Office of Public Health (FOPH), [Bolor em espaços habitáveis: informações para locatários, inquilinos e senhorios](https://res.cloudinary.com/adminch/image/private/s--IEfWBxCL--/v1719952980/Bundespublikationen/311_311_por.pdf) (Mould in living spaces: information for tenants and landlords), Portuguese edition, February 2021 (full text). - U.S. Environmental Protection Agency, [A Brief Guide to Mold, Moisture and Your Home](https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home) (cleanup and prevention sections). - Department of Health and Social Care (UK), [Understanding and addressing the health risks of damp and mould in the home](https://www.gov.uk/government/publications/damp-and-mould-understanding-and-addressing-the-health-risks-for-rented-housing-providers/understanding-and-addressing-the-health-risks-of-damp-and-mould-in-the-home--2), updated 1 April 2026 (health effects, groups at risk, cleaning, follow-up visit). ## FAQ ### Is mould on the wall harmful? The WHO (2009) concluded that occupants of damp or mouldy homes face higher risk of respiratory symptoms, respiratory infections and worsening asthma. If you have symptoms, see a doctor. ### Can I clean mould with bleach? The Swiss FOPH accepts bleach or 70–80% alcohol after washing with water and detergent. The EPA does not recommend it as routine and warns never to mix it with ammonia. ### Is anti-mould paint enough? No. The FOPH cites a damp wall painted with anti-mould paint as an improper repair, because the damp and mould stayed. The EPA says not to paint over mould. ### How big a patch can I clean myself? The FOPH accepts isolated patches up to 0.5 m² in one room if no vulnerable people live there. The EPA gives about 0.9 m². Beyond that, call a firm. --- Source: https://paredecerta.pt/en/guides/mould-on-walls. Independent information, not legal or tax advice.