What Is a COSHH Assessment and When Do I Need One?
If you use chemicals, solvents, dust, fumes or biological agents at work, there is a good chance you already need a COSHH assessment — and may not have one in place. Here is a plain-English explanation of what the law requires, what a proper assessment looks like, and how to approach it practically on site.
The Legal Basis
COSHH stands for the Control of Substances Hazardous to Health Regulations 2002. Under Regulation 6, every employer (and self-employed person) must carry out a suitable and sufficient assessment of the risks created by work that is liable to expose anyone to a hazardous substance. This applies before the work begins.
The duty sits on top of the general risk assessment duty under Regulation 3 of the Management of Health and Safety at Work Regulations 1999, but COSHH goes further — it requires you to identify specific substances, evaluate actual exposure, and then apply a defined hierarchy of controls.
What Counts as a Hazardous Substance?
COSHH covers a broad range of substances, including:
- Chemical products — solvents, adhesives, paints, degreasers, cleaning agents
- Dusts — wood dust, silica dust (from cutting concrete, stone or brick), plaster dust
- Fumes — welding fume, isocyanates from spray painting, soldering flux fume
- Biological agents — legionella in water systems, leptospirosis risk in drainage work
- Substances generated by the work itself — diesel exhaust fumes, grinding sparks carrying metal particles
If the product has a Safety Data Sheet (SDS), or if the task clearly generates dust or fumes, COSHH almost certainly applies.
Note: asbestos, lead and radioactive substances are covered by their own regulations rather than COSHH, although the risk assessment principles are similar.
What Does a COSHH Assessment Actually Contain?
A COSHH assessment is not simply a copy of the Safety Data Sheet. It must address your specific work situation. A complete assessment covers:
- Substance identification — what it is, its form (liquid, dust, vapour), and relevant hazard classifications from the SDS
- Who is exposed and how — your workers, other trades nearby, members of the public, and the route of exposure (inhalation, skin contact, ingestion)
- Likely level and duration of exposure — is it a brief task or repeated daily? In an enclosed space or outdoors?
- Existing controls and their adequacy — ventilation, enclosures, local exhaust ventilation (LEV), safe systems of work
- Control measures you will apply — following the hierarchy (see below)
- Emergency arrangements — first aid, spillage, fire risk where relevant
- Health surveillance — required for certain substances such as isocyanates, wood dust and silica if exposure is significant
- Review triggers — when the assessment will be revisited
The Control Hierarchy Under COSHH
Regulation 7 of COSHH sets out the order in which you must apply controls. PPE sits at the bottom — it is a last resort, not a first response.
| Priority | Control approach | Example |
|---|---|---|
| 1st | Eliminate | Use a water-based product instead of a solvent-based one |
| 2nd | Substitute | Switch to a less hazardous substance that does the same job |
| 3rd | Engineering controls | Local exhaust ventilation, on-tool dust extraction, enclosed processes |
| 4th | Administrative controls | Limiting exposure time, rotating workers, restricting access |
| 5th | PPE | Respirators (correct FFP rating), nitrile gloves, eye protection |
A common mistake is jumping straight to handing out dust masks. If LEV or on-tool extraction is reasonably practicable, it must be used first.
When Do You Need a COSHH Assessment?
You need one before starting any work where employees (or others) may be exposed to a hazardous substance. In practice, this means:
- Any trade using chemical products — plumbers using flux, decorators using solvent-based paints, flooring contractors using adhesives
- Any task generating significant dust — carpentry, cutting masonry, sanding, demolition
- Any work producing fumes — welding, brazing, spray painting, bitumen application
- Maintenance work on systems carrying biological risk — water treatment, drainage, air handling units
For straightforward, low-risk tasks (such as using a small amount of a mild cleaning product in a well-ventilated space), a brief, proportionate assessment is acceptable. For high-risk substances — isocyanates, hardwood dust, respirable crystalline silica, welding fume — the assessment must be detailed and controls robust.
Practical Steps for Tradespeople
- Gather the SDS for every product before work starts. Your supplier must provide this free of charge.
- Walk through the actual task — think about how and where exposure occurs, not just what the label says.
- Record your assessment in writing. If you have five or more employees, this is a legal requirement; if you have fewer, it is still strongly advisable.
- Review it when the substance, method or work environment changes, and at regular intervals.
- Never rely on PPE alone if engineering controls are reasonably practicable.
A Note on RAMS
A COSHH assessment feeds directly into your method statement when you are producing a RAMS document. The method statement should describe the controls your COSHH assessment has identified — how substances will be stored, handled, used and disposed of — so that operatives on the ground know exactly what to do.
Getting COSHH right is not about paperwork for its own sake. Occupational lung disease, skin conditions and chemical burns are among the most common serious work-related illnesses in construction and the trades — almost all preventable with the right controls in place.