Introduction to the Mould-Safe Code
 

The Mould-Safe Code sets out the principles that a competent person follows when carrying out, specifying or overseeing work that may disturb indoor mould. It does not replace the standards and regulations; it condenses them into a form a working practitioner can keep in mind on site.
 

The five rules are the minimum. The ten principles that follow describe what competent practice looks like. Each is anchored to the document or regulation that gives it force. Where UKCMS expresses a view beyond the cited source, that view is marked as commentary.
 

The five rules
 

1. Protect yourself and others
 

Indoor mould is a biological hazard. Treat it as such until a competent assessor has determined otherwise. Use appropriate personal protective and respiratory protective equipment (PPE and RPE). Prevent exposure to occupants - particularly children, the elderly, and the immunocompromised.
 

References: COSHH Regulations 2002, Reg. 7 (hierarchy of control); HSE Approved List of Biological Agents; ANSI/IICRC S520 (2021) §12, "Safety and health."
 

2. Stop the spread
 

Many indoor mould species are listed in the HSE Approved List of Biological Agents as Human Pathogen Hazard Group 2. Some pest-associated species are Hazard Group 3. Both groups require containment and exposure control during work, regardless of the fact that no UK licence is required to undertake mould remediation.

References: HSE Approved List of Biological Agents; COSHH Reg. 7; ANSI/IICRC S520 (2021) §12.2 (containment); BS PAS 64:2013.
 

3. Remove, don't kill
 

Mould is contamination. Contamination is removed, not killed. Physical source removal of affected materials, followed by HEPA-filtered extraction and cleaning, is the primary intervention. Spraying, painting over or fogging mould growth without removing it is never sufficient.
 

References: ANSI/IICRC S520 (2021) §10, §12 ("Source removal is the primary principle"); IICRC R520; COSHH Reg. 7(3) hierarchy (elimination before control).
 

4. Clean thoroughly
 

HEPA-vacuum (filtration to H14 / EN 1822 is recommended) and damp-wipe all non-porous surfaces until visibly and microscopically clean. Clean before and after structural drying or remedial works where necessary to reduce risk to as low as reasonably practicable. Consider clearance verification before re-occupation.
 

References: ANSI/IICRC S520 (2021) §12, §13; BS PAS 64:2013; HSE guidance on local exhaust ventilation.

5. Fix the moisture
 

Mould is a symptom of excess moisture. Remediation that does not identify and resolve the moisture source will fail. The source may be a leak, condensation, cold-bridging, rising or penetrating damp, or inadequate ventilation. Identifying it is part of the remediation, not adjunct to it.
 

References: ANSI/IICRC S520 (2021) §11 (moisture control principles); BS 5250:2021 (control of condensation in buildings); BS 8102:2022 (protection from water ingress).
 

Ten principles of safe practice for indoor mould contamination
 

Indoor mould debris exists at low background levels in all buildings and forms part of normal household dust. Background is inevitable and is managed through routine cleaning and moisture control. Competent practice for mould contamination - active growth, concealed reservoirs, or dust concentrations well above background - recognises the following.
 

1. Indoor mould growth is a health hazard
 

Exposure to indoor mould growth is a health hazard. The degree of risk depends on individual vulnerability, the dose, duration and frequency of exposure, and the species or genera present. Even species considered "common" can cause harm under the right circumstances.
 

References: HSE Approved List of Biological Agents; WHO Guidelines for Indoor Air Quality: Dampness and Mould (2009); UBA Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (2024 revision).
 

2. All parts of mould growth are hazardous
 

Spores are not the only concern. Fungal fragments, hyphae, mycotoxins, β-glucans and microbial volatile organic compounds (MVOCs) all affect health. Growth that is active, dormant or desiccated remains hazardous.

References: WHO (2009); UBA (2024); UKCMS position paper Oxidative fragmentation hypothesis (draft).

3. Biocides cannot resolve a mould problem
 

Biocides may reduce viability or temporarily reduce surface growth. They do not remove contamination. They do not eliminate spores, fragments or settled debris, and they cannot, on their own, resolve a mould problem.

References: ANSI/IICRC S520 (2021) §10; UBA (2024), which states that "biocide applications are in most cases inappropriate for mould remediation and are used far too frequently in practice"; UKCMS position paper Biocide-induced hazard blindness hypothesis (draft).
 

4. Improper remediation makes contamination worse
 

Disturbing contaminated materials without controls aerosolises spores and fragments, raising exposure and spreading contamination to previously clean areas of the building.
 

References: ANSI/IICRC S520 (2021) §12.2 (containment); BS PAS 64:2013.
 

5. Effective remediation is based on physical removal
 

Source removal, HEPA-filtered extraction and cleaning are the primary interventions. Chemical treatments — biocides included — may be used only as secondary measures. They are never a substitute for removal.
 

References: ANSI/IICRC S520 (2021) §10, §12; BS PAS 64:2013.
 

6. Indoor mould falls under COSHH
 

Many indoor mould species are listed in the HSE Approved List of Biological Agents as Human Pathogen Hazard Group 2. Some pest-associated species are Hazard Group 3. Both groups require containment and exposure control during work. No UK licence is required to undertake remediation; the COSHH duty to risk-assess and control exposure applies regardless.
 

References: COSHH Regulations 2002; COSHH Approved Code of Practice; HSE Approved List of Biological Agents.

7. Excess moisture is the root cause
 

Unless the moisture source is identified and resolved, mould will recur. Remediation without moisture control is not remediation; it is surface cleaning.
 

References: ANSI/IICRC S520 (2021) §11; BS 5250:2021; BS 8102:2022.
 

8. Mould does not need liquid water to grow
 

Many indoor species grow at water activity (a_w) of 0.6 or higher. Early colonisers can germinate within 12 to 24 hours of favourable conditions. A surface does not have to be "wet" to support growth.
 

References: UBA (2024); WHO (2009); ANSI/IICRC S520 (2021) §11.

9. Visible growth implies hidden contamination

By the time mould is visible, a substantial biomass already exists. Significant reservoirs may remain hidden, and whole-room contamination can develop even where visible growth is limited.
 

References: ANSI/IICRC S520 (2021) §12 (condition 1/2/3 framework); IICRC R520.

10. Concealed reservoirs are common
 

Where penetrating, traumatic or rising damp is present, contamination may extend into voids, cavities, insulation layers, floor structures and other hidden locations. Concealed reservoirs can continue to contaminate the indoor environment long after surface cleaning unless they are identified and removed.
 

References: ANSI/IICRC S520 (2021) §12; BS 8102:2022; UBA (2024).