How to Write a COSHH Assessment

Under the Control of Substances Hazardous to Health Regulations 2002, any employer whose workers may be exposed to hazardous substances must carry out a suitable and sufficient assessment of the risks to health. The legal duty to assess risks sits in Regulation 3 of the Management of Health and Safety at Work Regulations 1999; COSHH then specifies exactly how that duty applies to hazardous substances. For construction, this means every material, process by-product, or cleaning agent that could harm a worker — not just the obviously dangerous ones — must be assessed before work starts.

Legal Basis

COSHH 2002 sets out eight principles of good practice in Schedule 2A. These require you to design and operate processes that minimise emission and exposure, apply controls in the correct priority order, and monitor their effectiveness. Crucially, COSHH does not stand alone: it runs alongside the Manual Handling Operations Regulations, DSEAR where flammable vapours are present, and CAR 2012 where pre-2000 buildings are being disturbed. If a building constructed before 2000 is being cut, drilled, or demolished, a refurbishment and demolition survey under CAR 2012 is legally required before any disturbance takes place — the COSHH assessment cannot proceed in isolation without knowing whether asbestos-containing materials are present.

Identifying Substances and Hazards

Start with the Safety Data Sheet (SDS) for every product brought to site. The SDS will flag the hazard classifications — carcinogen, sensitiser, respiratory hazard, skin corrosive — and give an indicative occupational exposure limit. Cross-reference all exposure limits against the current version of EH40 (Workplace Exposure Limits), published by HSE. Do not rely on figures quoted in older documents: limits are revised, and the legally enforceable figures are those in the current EH40 table. When citing a WEL in the assessment, note the time-weighted average (8-hour TWA) and any short-term exposure limit (STEL, 15-minute), and confirm you have checked against the current EH40 edition rather than a historic printout.

Two substances warrant specific attention on construction sites. Weld fume — including mild steel weld fume — is classified as a Group 1 human carcinogen. LEV (local exhaust ventilation) is required; relying on general ventilation and a dust mask is not sufficient. Isocyanate-based products (certain paints, foams, and adhesives) are respiratory sensitisers; once a worker is sensitised, any further exposure can trigger a severe asthmatic response. Where isocyanate spraying is taking place, air-fed respiratory protective equipment is required — not a half-mask with a filter. Identify the exposure route for each substance: inhalation, skin absorption, ingestion, or eye contact, as controls must match the route.

The Control Hierarchy

Controls must be applied in priority order. Eliminate the substance entirely if possible — specify a water-based product instead of a solvent-based one. If elimination is not reasonably practicable, substitute for a less hazardous alternative. Where a hazardous substance cannot be substituted, apply engineering controls: local exhaust ventilation, enclosed processes, on-tool extraction. Administrative controls — task rotation to reduce exposure time, restricted access, limiting numbers exposed — come next. Respiratory protective equipment is the last resort, not the first. Collective protection (LEV, enclosure) always takes priority over personal protection (RPE). This order is not optional; applying PPE first while skipping feasible engineering controls is a common ground for rejection.

Common Failings

Assessments are rejected for the following reasons: citing WELs that do not appear in EH40 or quoting outdated values; listing RPE as the primary control without demonstrating that higher-order controls are not reasonably practicable; failing to identify sensitisers or carcinogens despite them appearing on the SDS; omitting health surveillance — which is a legal requirement for workers exposed to sensitisers, carcinogens, and certain biological agents; and providing no monitoring plan to confirm that controls are actually keeping exposure below the WEL.

What the Document Must Include

A compliant COSHH assessment must contain: the substance name and SDS reference; hazard classification and exposure routes; the WEL confirmed against current EH40; the control measures in hierarchy order with reasoning where higher-order controls were rejected; RPE specification (type, fit-test requirement, assigned protection factor); health surveillance arrangements where required; emergency procedures for spillage or accidental exposure; and a review date or trigger (change of product, process change, or incident).