Indoor environment factor · measured in spores/m³ (air sampling); usually assessed by moisture and visible growth
Mould
Also: mold, fungal growth, damp
Fungi that grow on damp surfaces and release spores, fragments and volatile compounds into the air. Indoors it is a symptom of moisture — condensation, leaks or poor ventilation — and a recognised cause of respiratory symptoms, allergies and asthma.
Why it matters indoors
Mould is not a pollutant that enters a building; it is one the building grows. Spores are everywhere, and they germinate wherever a surface stays damp long enough — a few days at high humidity is enough. Once established, a colony releases spores, hyphal fragments, allergens, mycotoxins and musty-smelling volatile compounds into the room air.
Where it grows indoors
Cold external walls and corners where condensation forms, window frames and reveals, bathrooms and kitchens without extraction, behind wardrobes and beds pushed against external walls, under sinks and around leaking pipes, in basements with rising damp, and in ventilation systems where condensate collects.
Health effects
Respiratory symptoms (cough, wheeze, shortness of breath), development and exacerbation of asthma, allergic rhinitis, and hypersensitivity pneumonitis in sensitised people. Children living in damp homes show significantly higher rates of respiratory illness. Some species produce mycotoxins; infections are rare and mainly affect immunocompromised people.
Guideline values
There is no health-based numeric limit for mould; spore counts vary too much by species and method. The WHO’s 2009 guidelines conclude that persistent dampness and microbial growth on indoor surfaces should be prevented or remediated, full stop. The practical control parameter is relative humidity: sustained values above 60–65 %, or surface temperatures below the dew point, are the risk conditions.
How it is assessed
Visual inspection and moisture measurement of building materials come first; air sampling for spores is used to compare rooms with outdoor air rather than against a limit. Continuous humidity and temperature monitoring identifies the rooms and times when conditions favour growth — the only approach that prevents rather than documents.
What the studies report
12 of 54 Findings — those with the strongest study design.
| Linked to | Effect | Population | Study design | Source | Checked |
|---|---|---|---|---|---|
| Asthma | — | students · school | prospective register-based cohort, 30 418 students in 110 school buildings, multilevel Cox models | F-0035 Pekkanen Inspection confirmed mold damage in schools and new use of d… (2025) | machine |
| Airway obstruction | not a dose-response: the highest risk was in buildings with limited damage, not the widest | students · school | prospective register-based cohort, 30 418 students in 110 school buildings, multilevel Cox models | F-0036 Pekkanen Inspection confirmed mold damage in schools and new use of d… (2025) | machine |
| Natural ventilation | mean 1001.30 | university students · classroom | comparison of thirty university classrooms in Brazil, 15 air-conditioned and 15 naturally ventilated: indoor CO2, temperature, relative humidity, wind speed, viable mould and airborne dust measured; values as mean ± standard error | F-0523 Jurado et al. Indoor air quality in Brazilian universities (2014) | machine |
| Air conditioning | mean 367.00 | university students · classroom | comparison of thirty university classrooms in Brazil, 15 air-conditioned and 15 naturally ventilated: indoor CO2, temperature, relative humidity, wind speed, viable mould and airborne dust measured; values as mean ± standard error | F-0524 Jurado et al. Indoor air quality in Brazilian universities (2014) | machine |
| Asthma | OR 2.4 (1.10–5.34) | social housing residents aged over 50 years · dwelling | cross-sectional study with face-to-face questionnaires of 302 residents of social housing in South West England: self-reported visible mould and mouldy musty odour against current asthma, wheeze and allergy, logistic regression adjusted for a priori covariates | F-0527 Moses et al. Exposure to Indoor Mouldy Odour Increases the Risk of Asthma… (2019) | machine |
| Asthma | OR 3.5 (1.37–9.08) | female social housing residents · dwelling | cross-sectional study with face-to-face questionnaires of 302 residents of social housing in South West England: self-reported visible mould and mouldy musty odour against current asthma, wheeze and allergy, logistic regression adjusted for a priori covariates | F-0528 Moses et al. Exposure to Indoor Mouldy Odour Increases the Risk of Asthma… (2019) | machine |
| Asthma | OR 1.4 (0.86–2.42) | social housing residents (adults) · dwelling | cross-sectional study with face-to-face questionnaires of 302 residents of social housing in South West England: self-reported visible mould and mouldy musty odour against current asthma, wheeze and allergy, logistic regression adjusted for a priori covariates | F-0529 Moses et al. Exposure to Indoor Mouldy Odour Increases the Risk of Asthma… (2019) | machine |
| Asthma | OR 2.7 (1.46–5.02) | social housing residents (adults) · dwelling | cross-sectional study with face-to-face questionnaires of 302 residents of social housing in South West England: self-reported visible mould and mouldy musty odour against current asthma, wheeze and allergy, logistic regression adjusted for a priori covariates | F-0530 Moses et al. Exposure to Indoor Mouldy Odour Increases the Risk of Asthma… (2019) | machine |
| Asthma | OR 2.7 (1.32–5.58) | social housing residents (adults) · dwelling | cross-sectional study with face-to-face questionnaires of 302 residents of social housing in South West England: self-reported visible mould and mouldy musty odour against current asthma, wheeze and allergy, logistic regression adjusted for a priori covariates | F-0531 Moses et al. Exposure to Indoor Mouldy Odour Increases the Risk of Asthma… (2019) | machine |
| Allergy and allergic sensitisation | OR 2.2 (1.11–4.40) | social housing residents (adults) · dwelling | cross-sectional study with face-to-face questionnaires of 302 residents of social housing in South West England: self-reported visible mould and mouldy musty odour against current asthma, wheeze and allergy, logistic regression adjusted for a priori covariates | F-0532 Moses et al. Exposure to Indoor Mouldy Odour Increases the Risk of Asthma… (2019) | machine |
| Asthma | OR 2.4 (1.02–5.67) | female social housing residents · dwelling | cross-sectional study with face-to-face questionnaires of 302 residents of social housing in South West England: self-reported visible mould and mouldy musty odour against current asthma, wheeze and allergy, logistic regression adjusted for a priori covariates | F-0533 Moses et al. Exposure to Indoor Mouldy Odour Increases the Risk of Asthma… (2019) | machine |
| — | share-below 98 | households · dwelling | surface and activated air sampling of mould reserve quantified by β-N-acetylhexosaminidase (NAHA) activity in 140 residential rooms in England without visible water damage or mould growth, plus a few rooms with visible mould for comparison, to derive background benchmarks | F-0534 Normal background levels of air and surface mould reserve in… (2020) | machine |
Each row is one Finding — a sentence as printed in one study, recorded with its page in the Coridum Knowledge Base. Follow the Finding number for the exact wording and the citation. Checked says who verified the row: machine means every automatic check passed and the surrounding text was read; person means someone read the sentence in the document. Rows are re-checked by a person over time. The studies are curated in the Coridum Knowledge Base; the reading of them together is a Claim, shown separately.
How strong is the evidence
-
In schools, a single large Finnish cohort found no association between the extent of inspection-confirmed mould damage in the building and the onset of asthma in students, and no dose-response.
-
In dwellings, a single study suggests that house dust mites and visible dampness or microbial growth go together with more symptoms related to asthma in adults.
A Claim is Coridum reading the Findings together, not a quotation from one study — according to Coridum. The grade follows the study designs behind it: A is the strongest, D means a single study or a mechanism.
How Coridum measures it: Coridum devices do not detect mould directly; they detect the conditions that cause it — sustained relative humidity above 60 %, dew-point risk on cold surfaces and poor ventilation — and warn before growth starts.
Frequently asked questions
Is mould dangerous to health?
For most people it causes irritation and allergic symptoms; for people with asthma it can trigger attacks; for immunocompromised people some species cause infections. The WHO regards dampness and mould as a sufficient reason to act regardless of species.
How do I stop mould coming back?
Remove the moisture, not just the stain: fix leaks, ventilate kitchens and bathrooms, keep relative humidity under 60 %, and warm or insulate cold surfaces so they stay above the dew point. Cleaning alone fails because the conditions remain.
Can I have a mould problem without seeing any?
Yes — behind furniture on external walls, under floors, inside wall cavities and in ventilation ducts. A musty smell, persistent condensation or a humidity monitor showing long periods above 65 % are reasons to look.
Sources
- WHO guidelines for indoor air quality: dampness and mould. World Health Organization Regional Office for Europe, 2009.
- Tseng et al. Contribution of Visible Surface Mold to Airborne Fungal Concentration as Assessed by Digital Image Quantification (2021). doi:10.3390/pathogens10081032 CKB 0485
- Park et al. NIOSH Dampness and Mold Assessment Tool (DMAT): Documentation and Data Analysis of Dampness and Mold-Related Damage in Buildings and Its Application (2022). doi:10.3390/buildings12081075 CKB 0490
- Zhang et al. Indoor Air Design Parameters of Air Conditioners for Mold-Prevention and Antibacterial in Island Residential Buildings (2020). doi:10.3390/ijerph17197316 CKB 0484
CKB = document number in the Coridum Knowledge Base, our curated library of peer-reviewed studies. Follow the DOI for the original paper.
Cite this page
Coridum Research. Mould. Coridum Knowledge G-0006, 16 September 2026. https://coridum.com/k/G-0006/