Chemical Safety Assessment UK: How to Get COSHH Right on Site
If you use hazardous substances at work — solvents, adhesives, cement, cleaning products, paints, dust — you are legally required to carry out a chemical safety assessment before work begins. In the UK, this duty sits under the Control of Substances Hazardous to Health Regulations 2002 (COSHH), and it applies to virtually every trade.
This article explains what a proper COSHH assessment involves, how to work through it step by step, and what the HSE actually expects to see.
What Is a Chemical Safety Assessment Under COSHH?
A COSHH assessment is a structured examination of how a hazardous substance is used at work, what harm it could cause, and what measures will prevent that harm. It is not a box-ticking exercise — the Regulations require you to assess the risk and put adequate controls in place.
COSHH 2002 regulation 6 sets out the duty to assess. The assessment must be suitable and sufficient, which means it must reflect the actual conditions of the work, not a generic template copied without thought.
Which Substances Require a COSHH Assessment?
Any substance that is hazardous to health must be assessed. This includes:
- Chemicals with hazard labels — look for GHS pictograms (skull, flame, health hazard diamond, corrosion symbol, etc.)
- Dusts — wood dust, stone dust, silica (respirable crystalline silica is one of the most serious occupational health risks in construction)
- Fumes — welding fume, soldering flux, isocyanates in spray paints
- Biological agents — for example, legionella in water systems or leptospirosis risk in sewage work
- Substances without an obvious label — mould, engine oils, and even flour dust are covered
The starting point is always the Safety Data Sheet (SDS). Suppliers are legally required to provide one for hazardous substances. Section 8 of the SDS lists workplace exposure limits (WELs) and recommended controls — read it before you begin.
Step-by-Step: How to Carry Out a Chemical Safety Assessment
1. Identify the Substance and Its Hazards
Name the product, its form (liquid, vapour, dust, mist), and the hazards listed on its SDS and label. Note any Workplace Exposure Limits set in EH40 — the HSE's published list of WELs.
2. Decide Who Could Be Harmed and How
Consider your own workers first, but also other trades working nearby, members of the public, and anyone with a particular susceptibility (for example, a worker with asthma or dermatitis).
Identify the route of exposure: inhalation is the most common route, but absorption through skin and ingestion also matter — especially with solvents and pesticides.
3. Evaluate the Risk
How likely is exposure, and at what level? A quick spray job in a ventilated open area carries a very different risk to the same product used continuously in a confined basement. Consider:
- Quantity used
- Duration and frequency of work
- Degree of enclosure
- Whether existing controls are in place
4. Apply Controls — in the Right Order
The COSHH hierarchy of control is not optional:
| Priority | Control | Example |
|---|---|---|
| 1 | Eliminate | Don't use the substance at all |
| 2 | Substitute | Use a water-based product instead of solvent-based |
| 3 | Engineering controls | Local exhaust ventilation (LEV), on-tool dust extraction |
| 4 | Administrative controls | Limiting exposure time, rotating workers, permits to work |
| 5 | PPE | Respirators, gloves, goggles — last resort, not first |
PPE is the weakest control because it relies entirely on the individual using it correctly, every time. It should supplement engineering controls, not replace them.
5. Record the Assessment
If you employ five or more people, you must record significant findings in writing (COSHH reg 6(4)). In practice, even sole traders benefit from a written record — it proves the assessment was done and shows what controls were decided on.
6. Implement, Train, and Monitor
A written assessment is worthless if it sits in a drawer. Workers must be informed, instructed, and trained (COSHH reg 12) on the hazards they face and the controls they must use. Check that controls are working — LEV equipment, for instance, must be thoroughly examined and tested at least every 14 months where required.
7. Review When Circumstances Change
If you change product, change the task, or if a worker reports ill effects, review the assessment. It is not a one-off document.
Common Mistakes That Fail COSHH Assessments
- Using the SDS as the assessment — the SDS informs the assessment; it is not the assessment itself
- Generic assessments that don't reflect the specific job or site conditions
- Going straight to PPE without considering higher-order controls
- Ignoring dust — silica dust from cutting concrete, stone or sandstone causes silicosis, a fatal and irreversible lung disease
- No health surveillance where required — certain substances (isocyanates, chrome, hard wood dust) trigger a duty to provide health surveillance under COSHH reg 11
A Note on Welding Fume
Since 2019, HSE has classified all welding fume — including mild steel — as a human carcinogen. Adequate controls now means engineering controls as a minimum, typically LEV or on-tool extraction, plus respiratory protective equipment (RPE) where residual risk remains. General ventilation alone is no longer considered adequate.
Summary
A chemical safety assessment under COSHH is a practical, site-specific document that identifies hazardous substances, evaluates realistic exposure risks, and sets out the controls that will actually prevent harm. Get the hierarchy right, base it on the real conditions of the job, and make sure everyone doing the work understands what is expected of them. That is the standard the HSE applies — and it is the standard that protects people's health.