COSHH · Mandatory · Pre-Construction
COSHH Assessment — Mould and Biological Agents
This COSHH assessment covers exposure to mould, fungi, bacteria, and other biological agents encountered during refurbishment, demolition, or work in damp buildings. It addresses respiratory sensitisation, allergic reactions, and infection risks. Control measures include RPE, containment, specialist cleaning, and appropriate waste disposal procedures.
Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. COSHH Regs 2002 remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.
| Mould spores | Aspergillus, Stachybotrys (black mould), Penicillium, Cladosporium — exposure causes respiratory sensitisation, allergic alveolitis, and occupational asthma. Stachybotrys chartarum produces mycotoxins. |
| Leptospirosis | Weil’s disease — caused by Leptospira bacteria transmitted through rat urine in contaminated water or soil. Can cause kidney failure, liver failure, and death. RIDDOR reportable. |
| Contaminated land | Phase 1 (desk study) and Phase 2 (intrusive investigation) assessment required under MHSWR 1999 and the Environmental Protection Act 1990. Biological hazards include anthrax spores, tetanus, and pathogenic organisms in former industrial land. |
| Bird and bat droppings | Cryptococcus neoformans (pigeon droppings) and Histoplasma capsulatum (bat droppings) cause serious respiratory and systemic infections. Dried droppings generate infectious dust when disturbed. |
| PPE and hygiene | Waterproof overalls, FFP3 respiratory protection, nitrile gloves, eye protection. Full decontamination procedures required before leaving the work area. No eating, drinking, or smoking in contaminated zones. |
1. Biological Hazards in Construction Work
Construction workers are exposed to a range of biological hazards that are frequently underestimated or overlooked entirely. Unlike chemical hazards, which are well-understood and routinely assessed under COSHH, biological agents are often treated as background risks rather than specific hazards requiring formal assessment and control. This is a significant gap in many construction health and safety management systems.
The COSHH Regulations 2002 define biological agents as micro-organisms, cell cultures, and human endoparasites that may cause infection, allergy, toxicity, or other health effects. Construction activities that disturb biological agents — including demolition of water-damaged buildings, excavation of contaminated land, work in sewers and drainage systems, clearance of bird and bat roosts, and refurbishment of properties affected by mould — all fall within the scope of COSHH and require a specific assessment.
Mould is the most commonly encountered biological hazard in construction, particularly in refurbishment, strip-out, and demolition of water-damaged buildings. Stachybotrys chartarum (black mould), Aspergillus species, and other fungi produce spores and mycotoxins that cause respiratory sensitisation, allergic alveolitis, and occupational asthma. Once a worker becomes sensitised to mould spores, even very low subsequent exposures can trigger severe allergic reactions. Sensitisation is irreversible.
Leptospirosis (Weil’s disease) is a bacterial infection transmitted through contact with water or soil contaminated by rat urine. It is a significant risk for workers involved in excavation, drainage work, canal and river works, demolition of derelict buildings, and any work in areas where rats are present. Leptospirosis can cause kidney failure, liver failure, and death. It is reportable under RIDDOR as a case of occupational disease.
Immunocompromised workers face significantly higher risk
Workers who are immunocompromised — due to medical conditions, chemotherapy, organ transplant medication, or HIV — face a significantly higher risk of serious illness from biological hazards that healthy workers may tolerate without symptoms. Aspergillus infection (invasive aspergillosis) can be fatal in immunocompromised individuals. Employers must consider the health status of the workforce when assessing biological risks and, where necessary, obtain occupational health advice on fitness for specific tasks. This is a specific requirement under MHSWR 1999 Regulation 3 (risk assessment must consider workers who may be especially at risk).
2. Mould Remediation Controls
Mould remediation in construction requires a systematic approach that prevents the spread of spores to unaffected areas and protects workers from inhalation exposure. The controls must be proportionate to the extent and severity of the contamination. For large-scale mould contamination (typically more than 1 square metre of visible growth, or where hidden mould behind walls or ceilings is suspected), full containment and specialist remediation methods are required. The following table sets out the key control measures.
| Control measure | Detail |
|---|---|
| Dampen surfaces before disturbance | Mist mould-affected surfaces with water before any disturbance to suppress the release of airborne spores. Never dry brush, dry scrape, or vacuum mould-affected surfaces with a standard vacuum cleaner. Dry disturbance generates extremely high concentrations of airborne spores that can travel throughout the building via air currents and HVAC systems. |
| Containment | For large-scale mould contamination, establish a containment enclosure around the work area using polythene sheeting sealed at all joints. Maintain negative air pressure within the enclosure using a HEPA-filtered air scrubber to prevent spore migration to adjacent areas. Seal HVAC openings within the containment to prevent spore spread through ductwork. |
| RPE — minimum FFP3 | All workers within the contaminated area must wear a minimum of FFP3 respiratory protection. For sustained remediation work or work involving Stachybotrys chartarum (black mould), a half-mask or full-face respirator with P3 filters is recommended. RPE must be face-fit tested to the individual wearer. Disposable masks must not be reused between shifts. |
| Protective overalls | Waterproof disposable overalls (Type 5/6) must be worn to prevent spore contamination of clothing and skin. Overalls must be removed and disposed of within the containment area before leaving the work zone. Work clothing contaminated with mould spores must not be taken home or worn outside the remediation area. |
| Decontamination | A decontamination procedure must be established at the exit from the containment area. Workers must remove and bag contaminated PPE, wash hands and face, and change into clean clothing before leaving the work zone. A designated dirty-to-clean transition area must be maintained at the containment boundary. |
| Waste disposal | Mould-contaminated materials must be double-bagged in heavy-duty polythene bags, sealed, and labelled before removal from the containment area. Bags must be HEPA-vacuumed externally before being transported through clean areas. Dispose of contaminated waste in accordance with the duty of care requirements under the Environmental Protection Act 1990. |
Address the moisture source first — remediation without moisture control is futile
Mould remediation is pointless if the underlying moisture source is not identified and eliminated. Mould will regrow within days if the conditions that caused it persist. Before commencing remediation, identify and rectify the source of moisture — whether it is a roof leak, rising damp, condensation from inadequate ventilation, plumbing failure, or flood damage. Remediation of the mould without resolving the moisture source wastes money and creates a recurring biological hazard.
3. Common Mistakes
No mould assessment before demolishing or stripping out water-damaged structures
Demolition or strip-out of buildings that have suffered water damage, flooding, or prolonged vacancy frequently disturbs extensive mould growth concealed behind wall linings, above suspended ceilings, and beneath flooring. Without a pre-work biological hazard assessment, workers are exposed to extremely high concentrations of mould spores without appropriate RPE or containment. This is a COSHH breach — biological agents require assessment in exactly the same way as chemical substances.
Not briefing workers on Leptospirosis risks during excavation and drainage work
Leptospirosis is transmitted through contact with water or soil contaminated by rat urine. Workers involved in excavation, drainage, canal works, and demolition of derelict buildings are at particular risk. Workers must be briefed on the transmission route, the symptoms (flu-like illness progressing to jaundice, kidney failure), and the importance of covering cuts and abrasions with waterproof dressings. A Leptospirosis information card should be issued to workers and carried at all times so that medical staff can be alerted if the worker presents with symptoms.
Dry brushing bird droppings without RPE or containment
Dried pigeon droppings harbour Cryptococcus neoformans, a fungal pathogen that causes cryptococcal meningitis — a serious and potentially fatal infection. Bat droppings may contain Histoplasma capsulatum, which causes histoplasmosis. Dry sweeping or brushing bird or bat droppings generates infectious dust that is readily inhaled. All bird and bat droppings must be dampened before removal, workers must wear FFP3 RPE and disposable overalls, and the waste must be bagged and disposed of appropriately. Never dry brush, sweep, or use compressed air to remove bird or bat droppings.
Not assessing soil for biological hazards before excavation on brownfield sites
Former industrial sites, agricultural land, and land that has been used for waste disposal may contain biological hazards in the soil — including anthrax spores (from former tanneries, wool mills, and bone-processing works), tetanus bacteria, pathogenic organisms from sewage works, and Leptospira from rat-infested land. A Phase 1 desk study should identify potential biological contamination from historical land use, and a Phase 2 intrusive investigation should confirm the nature and extent of contamination. Workers must be informed of the findings and provided with appropriate PPE and hygiene facilities.
Not providing hepatitis B vaccination for sewer and drainage workers
Workers who carry out construction work in sewers, drainage systems, and sewage treatment works are exposed to hepatitis B virus (and other bloodborne pathogens) through contact with sewage and contaminated water. The COSHH assessment for sewer work should identify hepatitis B as a biological hazard and recommend vaccination as a control measure. The employer should offer hepatitis B vaccination to all workers routinely exposed to sewage. While vaccination is not compulsory, the employer has a duty to offer it and to record the worker’s decision. Workers who decline vaccination must be informed of the residual risk.
5. Frequently Asked Questions
How is mould classified under COSHH?▾
Mould is classified as a biological agent under COSHH 2002. The COSHH Regulations define biological agents as micro-organisms, cell cultures, and human endoparasites that may cause infection, allergy, toxicity, or other health effects. Mould spores cause allergic sensitisation and respiratory disease, and certain species (notably Stachybotrys chartarum) produce mycotoxins that cause toxic effects. The employer must carry out a COSHH assessment for any work that is liable to expose workers to mould spores, determine the level of risk, and implement controls to prevent or adequately control exposure. The assessment must consider the species of mould present, the extent of contamination, the method of work, and the susceptibility of workers (particularly immunocompromised individuals).
What are the first aid priorities for suspected Leptospirosis exposure?▾
If a worker has been in contact with potentially contaminated water or soil and develops flu-like symptoms (fever, headache, muscle pain, nausea) within 2 to 30 days of exposure, Leptospirosis must be suspected. The worker must be referred to medical attention immediately and the attending doctor must be informed of the potential occupational exposure to Leptospirosis. A Leptospirosis information card carried by the worker serves this purpose. Early treatment with antibiotics is effective; delayed treatment can result in the disease progressing to Weil’s disease (icterohemorrhagic leptospirosis) with kidney and liver failure. Cuts, abrasions, and skin lesions should be covered with waterproof dressings before any work in contaminated environments, as broken skin is the primary route of entry for Leptospira bacteria.
When is air sampling required for mould remediation work?▾
Air sampling for mould spores is recommended before, during, and after large-scale mould remediation to establish baseline spore counts, verify that containment is effective during the work, and confirm that the area is safe for reoccupation after remediation is complete. Air sampling is not a mandatory legal requirement under COSHH but is considered best practice by the remediation industry and may be required by the building owner or insurer. For small-scale remediation (less than 1 square metre of visible mould), air sampling is generally not necessary provided that adequate RPE and control measures are in place. Where Stachybotrys chartarum (black mould) is present, air sampling is strongly recommended regardless of the scale of contamination due to the mycotoxin risk.
Are there HSE notification requirements for biological hazard exposures?▾
Yes. Under RIDDOR 2013, cases of certain occupational diseases caused by biological agents are reportable to the HSE. These include Leptospirosis, hepatitis (where contracted through occupational exposure), and any infection reliably attributable to work with a biological agent. An outbreak of Legionnaires’ disease is reportable as a dangerous occurrence. Occupational asthma caused by sensitisation to biological agents (including mould spores) is reportable as an occupational disease. The employer must report cases using the online RIDDOR reporting form. In addition, any incident resulting in the uncontrolled release of a biological agent that could cause serious human illness (such as a significant asbestos fibre release, though asbestos is classified separately) should be reported as a dangerous occurrence.
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