If your workplace uses chemicals, cleaning products, paints, adhesives, or any substance that could be hazardous to health, you have a legal duty to assess and control those risks. COSHH — the Control of Substances Hazardous to Health — is one of the most important risk assessment frameworks in occupational health and safety, yet many organisations still treat it as a paperwork exercise rather than a genuine protective measure.
This guide is written for safety managers, facilities coordinators, construction supervisors, and compliance officers who need to understand COSHH from the ground up. Whether you are setting up your first COSHH programme or auditing an existing one, this post provides a clear, structured framework covering hazard identification, the five-step assessment process, the hierarchy of controls, and the training and review requirements that underpin effective compliance.
By the end of this guide, you will understand what a COSHH assessment involves, which substances trigger the requirement, how to evaluate exposure risks, what control measures must be in place, and how to ensure your assessment remains current and enforceable in a real working environment.
What Is a COSHH Risk Assessment?
A COSHH risk assessment is a structured evaluation of the health risks arising from exposure to hazardous substances in the workplace. It requires employers to identify substances that could harm workers, assess the likelihood and severity of exposure, and implement appropriate controls to eliminate or reduce that risk to an acceptable level.
The legal framework in Great Britain is the Control of Substances Hazardous to Health Regulations 2002 (as amended), enforced by the Health and Safety Executive (HSE). Internationally, equivalent frameworks exist under ILO Convention No. 170 on Chemicals and various ISO and national occupational health standards. The duty applies to virtually all employers who use, store, produce, or process substances that are toxic, corrosive, irritant, carcinogenic, sensitising, or otherwise harmful to health.
A COSHH assessment covers not only commercially labelled chemicals, but also substances generated during work processes — such as welding fumes, wood dust, silica dust, and biological agents including mould and bacteria.
Key Hazards in COSHH Assessments
COSHH hazards are wide-ranging. Understanding the nature of the hazard — and the route by which it can harm people — is fundamental to designing effective controls.
Chemical and Physical Substance Hazards
- Toxic and very toxic substances: Industrial solvents, pesticides, and certain laboratory reagents that can cause systemic harm through inhalation, skin absorption, or ingestion — even at low exposure levels.
- Corrosives and irritants: Acids, alkalis, and bleach-based cleaning agents that damage skin, mucous membranes, and the respiratory tract on contact or inhalation.
- Carcinogens and mutagens: Asbestos fibres, respirable crystalline silica, hardwood dust, and certain chemical process by-products with long-latency occupational disease implications.
- Sensitisers: Isocyanates in spray paints, latex, and flour dust that can trigger occupational asthma — a permanent and irreversible condition once sensitisation has occurred.
- Flammable and explosive substances: While primarily covered under separate fire and DSEAR regulations, substances with dual hazard profiles must also be captured in a COSHH assessment where health effects are present.
Biological and Process-Generated Hazards
- Biological agents: Bacteria, viruses, fungi, and endotoxins present in wastewater, clinical environments, animal handling settings, and air-conditioning systems where Legionella risk applies.
- Process-generated substances: Welding fume, soldering flux vapour, diesel exhaust emissions, wood and metal dust, and cutting fluids — substances not supplied in a container but created during work activities.
- Nanotechnology and emerging substances: Engineered nanomaterials used in coatings, composites, and electronics manufacturing where toxicological data remains limited and the precautionary principle must apply.
The 5-Step COSHH Risk Assessment Process
The HSE's standard five-step risk assessment process applies directly to COSHH. Each step must be documented and tailored to the specific substances and tasks in your workplace.
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1
Identify the Hazardous Substances Compile a full inventory of all substances used, stored, or generated on site. Review Safety Data Sheets (SDS) for each product. Don't overlook substances produced during processes — for example, welding mild steel generates manganese fume, which carries neurological risk with repeated exposure.
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Decide Who Might Be Harmed and How Identify all persons at risk: direct users, nearby workers, contractors, visitors, cleaners, and maintenance staff who may enter after processes have concluded. Consider vulnerable groups — those with pre-existing respiratory conditions, pregnant workers, and young persons with developing immune systems.
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Evaluate the Risks and Existing Controls For each substance, assess exposure frequency, duration, concentration, and route of entry (inhalation, skin absorption, ingestion, injection). Compare identified exposure levels against the Workplace Exposure Limits (WELs) set out in EH40 (HSE). Determine whether current controls are adequate or whether gaps exist.
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4
Record the Findings and Implement Controls Document your assessment in a format that is accessible to workers and supervisors. The record must detail the substance, the activity, the persons at risk, the risk rating, and the control measures to be implemented. A written COSHH assessment is a legal requirement under Regulation 6 where five or more employees are employed.
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5
Review and Update the Assessment A COSHH assessment is not static. Review it whenever substances change, processes are modified, monitoring data indicates elevated exposure, an incident occurs, or a defined review period (typically annual for high-risk substances) is reached.
Hierarchy of Controls for COSHH Hazards
The COSHH Regulations prescribe a hierarchy of prevention and control measures under Regulation 7. Employers must work through this hierarchy in order, using higher-level controls wherever reasonably practicable before relying on lower-level measures.
| Control Level | COSHH-Specific Application |
|---|---|
| 1. Elimination | Remove the hazardous substance entirely. Stop using a solvent-based cleaning agent if a water-based alternative achieves the same result. This is the most effective control. |
| 2. Substitution | Replace with a less hazardous substance. Switch from a category 1 carcinogen to a lower-hazard equivalent. Requires verification that the substitute does not introduce new risks. |
| 3. Engineering Controls | Local exhaust ventilation (LEV), enclosed processes, fume cupboards, and on-tool dust extraction. These capture or contain hazardous substances at source before they enter the breathing zone. |
| 4. Administrative Controls | Safe systems of work, permit-to-work procedures, reduced exposure time through job rotation, restricted access, COSHH training, and clear labelling and storage segregation. |
| 5. PPE | Respiratory protective equipment (RPE), chemical-resistant gloves, eye protection, and protective clothing as a final layer of protection where residual risk cannot be eliminated by higher-level measures. |
Specific Risk Considerations by Work Environment
Construction and Trades
Construction sites present complex, frequently changing COSHH exposure profiles. Respirable crystalline silica from concrete cutting and grinding, isocyanates from spray-applied coatings, and lead from paint disturbance on older structures are among the highest-priority hazards. Exposures are often short-duration but high-concentration, meaning TWA (time-weighted average) assessments may underestimate actual peak exposure risk. On-tool extraction and RPE programmes are typically required alongside substance substitution wherever feasible.
Cleaning and Facilities Management
Cleaning operatives are among the most frequently exposed workers in the UK, yet COSHH assessments in this sector are often inadequate. Common hazards include quaternary ammonium compounds (QACs), bleach-based disinfectants, multi-surface sprays with volatile organic compound (VOC) content, and mould-contaminated environments. Dermatitis and occupational asthma are the most significant outcomes. Pre-employment health surveillance and regular skin health monitoring are strongly advisable where exposure to sensitisers is routine.
Healthcare and Laboratories
Healthcare settings involve biological agents, cytotoxic drugs, anaesthetic gases, disinfectants, and fixatives such as formaldehyde. Laboratory environments add chemical carcinogens, reactive intermediates, and nanomaterials. COSHH assessments in these settings must incorporate biological agent risk assessment under the Control of Substances Hazardous to Health Regulations (Biological Agents) and align with ACDP (Advisory Committee on Dangerous Pathogens) guidance for containment levels.
Emergency and Rescue Planning
Where hazardous substances are present, the COSHH assessment must inform emergency response planning. Regulation 13 of COSHH requires employers to establish procedures to deal with accidents, incidents, and emergencies involving hazardous substances. This includes:
- Spill containment procedures: Appropriate absorbents, neutralising agents, and sealed waste containers appropriate to the substances held on site. Secondary containment bunding for liquid chemical storage.
- Emergency decontamination facilities: Emergency eyewash stations and drench showers within 10 seconds' travel time of areas where corrosive or highly irritant substances are used.
- First aid provision: First aiders trained in chemical exposure response. Relevant antidotes (e.g. calcium gluconate gel for hydrofluoric acid) stocked and accessible where the substance warrants it.
- Communication and evacuation: Emergency procedures communicated to all workers including contractors. Safety Data Sheets must be available to emergency services, and the site's chemical inventory should be disclosed to local fire authorities under COMAH or DSEAR thresholds where applicable.
Training and Competency Requirements
Under Regulation 12 of COSHH, employers must provide employees with suitable and sufficient information, instruction, and training in relation to the hazardous substances they may be exposed to. Competency requirements include:
- Understanding of the hazards and risks associated with each substance used in their role, including interpretation of Safety Data Sheets.
- Correct use, maintenance, inspection, and storage of PPE and RPE, including face-fit testing for filtering facepieces and tight-fitting respirators.
- Understanding of the COSHH assessment findings applicable to their work area and the control measures in place.
- Awareness of emergency procedures including spill response, evacuation routes, and first aid actions specific to the substances present.
- Awareness of the health surveillance programme and their right to access the results of any exposure monitoring data relevant to their role.
For specialist roles — such as those working with biological agents at containment levels 2 or above, or with COSHH-listed carcinogens — additional role-specific training and documented competency verification is required. Refresher training intervals should be risk-proportionate, with higher-risk environments warranting annual refreshers as a minimum.
Applicable International Standards
🌐 Globally Recognised Standards and Frameworks
- ISO 45001:2018 — Occupational Health and Safety Management Systems. Clause 8.1.2 requires management of change and Clause 8.2 covers emergency preparedness, both directly applicable to COSHH programme management.
- ILO Convention No. 170 (1990) — Chemicals Convention: Establishes international duties for labelling, safety data sheets, employer responsibilities for hazardous chemical management, and worker information rights.
- ILO Convention No. 139 — Occupational Cancer Convention: Requires periodic determination of carcinogenic substances and substitution or control to minimise occupational cancer risk.
- GHS / UN Purple Book: The Globally Harmonised System of Classification and Labelling of Chemicals, adopted into EU and UK law via CLP Regulation, underpins hazard communication referenced throughout COSHH documentation.
- REACH Regulation (EC) No. 1907/2006: The EU chemical registration and restriction framework; Safety Data Sheets produced under REACH directly feed into COSHH assessments across European supply chains.
- ISO 11933 / EN 481: Standards relating to measurement of workplace airborne particulate hazards — relevant to exposure monitoring requirements under COSHH Regulation 10.
Reviewing and Maintaining the COSHH Assessment
A COSHH assessment must be treated as a living document. It is not sufficient to complete the assessment at the point of first use and file it — the assessment must be actively maintained and reviewed in response to defined trigger points.
- Change of substance: Any substitution, reformulation, or introduction of a new chemical product requires a fresh or updated assessment before work commences.
- Change of process or task: Modifications to application methods, quantities, temperatures, or locations that could alter exposure profiles must trigger reassessment.
- Incident or near-miss: Any spillage, accidental exposure, or health effect reported by a worker associated with a COSHH substance must prompt immediate review of the relevant assessment.
- Health surveillance findings: Trends in health surveillance data indicating elevated biological exposure indices or emerging sensitisation warrant reassessment of control effectiveness.
- Routine scheduled review: As a minimum, all COSHH assessments should be reviewed on an annual cycle for high-hazard substances, and at least every two to three years for lower-risk profiles, unless a trigger point arises earlier.
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