What Is COSHH and When Is a COSHH Assessment Required?

COSHH — the Control of Substances Hazardous to Health Regulations 2002 — is the law requiring employers to control the risk from hazardous substances at work: chemicals, dusts, fumes, vapours, mists and certain biological agents among them. It does not apply to every substance a workplace happens to use, and it does not require a separate document for each one. What it requires is a suitable and sufficient assessment of the risk each hazardous substance actually presents, and control measures that reflect what that assessment finds. The Health and Safety Executive's own Approved Code of Practice and guidance, L5, is the fuller reference this guide draws on throughout.

What COSHH covers, and what it doesn't

A substance is generally treated as hazardous under COSHH if its packaging carries a hazard symbol, or if it is a dust, fume, vapour, mist or gas capable of harming health, or a biological agent such as the organisms responsible for legionnaires' disease or leptospirosis. That reaches a wide range of everyday materials: cleaning chemicals, adhesives, paints and solvents, dusts generated by cutting or sanding, fumes from welding or soldering, and process-generated substances that were never bought in as a product at all — the dust created by cutting a work surface is just as much a COSHH substance as the chemical used to clean it afterwards.

It is just as important to be clear about what COSHH does not cover. Lead, asbestos and radioactive substances each have their own dedicated regulations, separate from COSHH, precisely because the risks and the controls involved are different enough to need their own regime — a workplace handling any of those needs to work to that specific legislation, not to COSHH. COSHH is also a distinct regime from DSEAR, the Dangerous Substances and Explosive Atmospheres Regulations: DSEAR addresses the fire and explosion risk a dangerous substance presents, where COSHH addresses the risk to health from exposure to it. The same substance can sit under both at once — a flammable solvent, for example — assessed for two different reasons under two different sets of regulations, not folded into a single document.

Who COSHH applies to

The duty sits with the employer, and extends to the self-employed and to anyone who could be affected by the work — employees, contractors and visitors alike. It applies wherever a hazardous substance is present and someone could be exposed to it, which in practice reaches almost every sector: construction and manufacturing are the settings people associate with COSHH first, but an office running routine cleaning and using hand sanitiser, a school with a science department, a hotel kitchen, and a care home managing disinfectants and clinical waste are all just as squarely within scope. COSHH is not a construction-specific duty that happens to apply elsewhere; it is a general duty that happens to be more visible on construction sites because the range and quantity of substances involved is often larger.

What a COSHH assessment actually is

A COSHH assessment identifies the hazardous substances present, evaluates who could be exposed and how, and decides what controls need to be in place. It sits within the same broader framework as our guide to health and safety risk assessments: the general risk assessment is the umbrella exercise required of every employer, and COSHH is one of several hazard-specific assessments that sit beneath it, alongside fire, manual handling, display screen equipment, lone working and DSEAR. It is not a separate compliance regime running in parallel to general health and safety — it is the same underlying discipline, applied to a specific and legally defined category of hazard.

When a COSHH assessment is required

An assessment is required wherever hazardous substances are present in the workplace and people could be exposed to them, whether that substance was bought in as a product or generated by the work itself. That does not mean one document per substance. Similar substances, used in similar ways, by similar people, in similar areas can often be assessed together as a single, genuinely representative exercise — what matters is that the assessment is suitable and sufficient for the exposure it covers, not that a separate form exists for every product on the premises. As with the general risk assessment duty, employers with five or more employees must record the significant findings in writing; below that threshold there is no strict legal requirement to record them, though doing so remains good practice for exactly the same reasons it does for general risk assessment.

Identifying hazardous substances

Identification starts with what's on the label — the hazard symbols and the safety data sheet supplied with any product bought in — but it cannot stop there. Many of the substances that matter most in a COSHH assessment are generated by the work itself rather than purchased as a product: dust from cutting or sanding, fumes from welding, or a hazardous by-product created when two otherwise ordinary substances are mixed or come into contact with heat. A walk-round that only checks the chemical cupboard will miss exactly the exposures that tend to cause the most harm.

Safety data sheets: what they do and don't provide

A safety data sheet is an essential input to a COSHH assessment, and it is not the assessment itself. It tells you what a substance is, how it is classified, how to handle and store it, and what to do in an emergency — information about the product in isolation. It cannot tell you how that product is actually used in your workplace: by whom, how often, for how long, in what quantity, alongside what else, in what kind of space, or with what controls already in place. A COSHH assessment has to establish all of that. Filing a stack of safety data sheets is a reasonable first step towards identifying hazards; treating that stack as a completed assessment is one of the most common — and most easily corrected — mistakes made with COSHH.

Assessing exposure

Once a hazardous substance is identified, the assessment has to establish how people are actually exposed to it: by breathing it in, through skin or eye contact, by swallowing it, or in rarer cases through injection. It needs to consider how long and how often exposure happens, how effective any existing controls already are, whether a workplace exposure limit applies to the substance and whether exposure is likely to exceed it, and whether people are exposed to more than one hazardous substance at once, since combined exposures can matter even where each substance individually looks low-risk. Where health surveillance or monitoring has already been carried out, its results feed back into this part of the assessment too.

Choosing control measures, and the hierarchy that governs them

Controls follow a clear order of priority, and getting that order right is one of the most consistently misapplied parts of COSHH. The first question is always whether exposure can be prevented altogether — by eliminating the hazardous substance, substituting it for something less hazardous, or changing the process so it is no longer generated or used. Where prevention genuinely isn't reasonably practicable, the next priority is adequate control: engineering measures such as enclosure or local exhaust ventilation, and controlling how the work is actually carried out, through safe systems of work, supervision, and proper maintenance of the controls already in place. Personal protective equipment and respiratory protective equipment sit last in that order, not first. PPE is what's used where the measures above it cannot reduce exposure far enough on their own — a genuine and often necessary part of the control regime, but not a substitute for the engineering and procedural controls that should have been considered ahead of it.

Information, instruction and training

Controls only work if the people using them understand what they're working with and why the control matters, not just that a rule exists. That means training on the substances actually present in that workplace, on what the controls are for, on how to use them correctly, and on what to do if something goes wrong — proportionate to the substances and the work, not a generic induction slide repeated regardless of what's actually in use on site.

Health surveillance

Health surveillance is not a routine addition to every COSHH assessment — it applies only where a disease or adverse health effect is genuinely associated with the substance in question, where that effect is detectable, and where early detection could actually reduce the harm someone goes on to experience. Occupational asthma from certain respiratory sensitisers and skin conditions such as dermatitis are common examples where surveillance is likely to be relevant. Where none of those conditions is met, the assessment does not need to build in surveillance simply because a hazardous substance is present.

Monitoring exposure

Exposure monitoring — typically air sampling — is used where it's genuinely needed to check that exposure is staying below a workplace exposure limit, or to confirm that controls are actually working as intended, particularly where there's real uncertainty about the level of exposure from looking at the work alone. Like health surveillance, it is not a universal requirement of every COSHH assessment; whether it's needed depends on the substance, the way it's used, and how confident the assessment can otherwise be about the exposure involved.

Emergency arrangements

Where an uncontrolled release, a spill or an accidental exposure is a realistic possibility given the substances and quantities involved, the assessment needs to set out what happens if it occurs — containment, evacuation if relevant, first aid arrangements, and who is responsible for each. This should be proportionate to the actual hazard: a small quantity of a low-hazard cleaning product doesn't need the same emergency planning as a bulk chemical store, and the assessment should reflect that difference rather than applying a single template regardless of scale.

Reviewing a COSHH assessment

There is no fixed statutory interval for reviewing a COSHH assessment, in the same way there is none for the general health and safety risk assessment it sits beneath. Review is required when there is reason to think the assessment is no longer valid, when there has been a significant change in the work, or where monitoring or health surveillance results indicate it's necessary. Beyond those triggers, periodic review remains good practice — the right interval is a judgement the assessment should record explicitly, rather than an assumed fixed period the regulations don't actually set.

How this looks across different workplaces

In offices and commercial workplaces, COSHH is most often about cleaning chemicals, hand sanitiser, and occasional print or toner handling — genuinely lower-hazard in most cases, but still requiring an assessment rather than an assumption that "it's just cleaning products."

In schools and education settings, science department chemicals, design and technology dusts and fumes, and cleaning products used across the site all fall within scope, often assessed by different people for different parts of the same building.

In healthcare and care settings, cleaning and disinfecting agents, clinical chemicals, certain medicines, and biological agents encountered through clinical waste or patient care are typically the dominant COSHH concerns, frequently alongside infection control arrangements that sit outside COSHH itself.

For cleaning operations, COSHH is often the single most significant health and safety consideration in the work: concentrated chemicals, the risk of harmful reactions when products are mixed, and repeated daily exposure all make a genuinely substance-specific assessment more important than in most other settings.

In hospitality, kitchen cleaning and catering chemicals, and occasionally pest control products, are the substances most likely to require assessment, usually alongside the food hygiene and fire considerations already present in that environment.

In warehousing and industrial premises, the range is typically wider — a broad mix of stored chemicals, dusts and fumes generated by processes on site, and materials such as batteries and fuels — often warranting several linked assessments rather than one.

Maintenance work, wherever it happens, regularly involves solvents, adhesives, paints, fuels and welding fumes, and is easy to under-assess precisely because it's occasional rather than part of the core business of the site.

On construction sites, COSHH is the regime most people already associate with the sector — silica dust, cement, solvents and adhesives are common and well-documented hazards — but construction's higher profile shouldn't be read as meaning COSHH is a construction-only duty; it's the same regulations applied to a setting where hazardous substances are simply more numerous and more visible.

COSHH assessments and general health and safety risk assessments

A general health and safety risk assessment is the umbrella exercise every employer has a duty to carry out. A COSHH assessment is one of several hazard-specific assessments that sit beneath it — alongside fire, manual handling, display screen equipment, lone working and DSEAR — each following its own regulations and its own detail, but each a specific application of the same underlying principle: identify the hazard, evaluate who's affected and how, and decide what controls are needed. Our guide to health and safety risk assessments covers that general process in full and explains how these hazard-specific assessments relate to it.

Frequently asked questions

Does every hazardous substance need its own COSHH assessment?

Not necessarily as a separate document. Similar substances, used in similar ways in similar areas, can often be assessed together, provided the result is genuinely suitable and sufficient for the exposure it covers.

Is a safety data sheet the same as a COSHH assessment?

No. A safety data sheet supplies information about a substance in isolation — its hazards, handling and emergency information. A COSHH assessment goes further, considering how the substance is actually used in that workplace and what controls are needed as a result.

Does COSHH cover asbestos, lead or radioactive substances?

No. Each has its own dedicated regulations, separate from COSHH, and needs to be assessed against that specific legislation rather than treated as a general COSHH substance.

What's the difference between COSHH and DSEAR?

COSHH addresses the health risk from exposure to a hazardous substance. DSEAR addresses the fire and explosion risk from a dangerous substance. The same substance can fall under both, assessed for two different reasons.

How often does a COSHH assessment need to be reviewed?

There's no fixed statutory interval. It needs reviewing when there's reason to doubt it's still valid, when the work changes significantly, or when monitoring or health surveillance results indicate it — and periodically as good practice even where nothing obviously has changed.

Do I need health surveillance for every hazardous substance?

No. Health surveillance applies only where a disease or health effect associated with the substance is detectable and where early detection could genuinely reduce the harm involved — not as a default addition to every assessment.

When to seek competent health and safety advice

Many COSHH assessments can be carried out competently in-house, particularly where the substances involved are familiar and well understood. Specialist input is worth bringing in where the substances or processes are unfamiliar, where the assessment needs to consider health surveillance or exposure monitoring and that expertise doesn't exist internally, where an incident or near miss has raised doubts about existing controls, or where a client, insurer or regulator is asking for assurance the organisation can't currently provide. As with any COSHH assessment, the person carrying it out needs to be a genuine competent person for the substances and processes involved, not simply available.

Our Health & Safety Consultancy covers COSHH and wider workplace risk assessment across offices, education, healthcare, retail, hospitality and warehousing, and our Construction Health & Safety service covers the substances and controls specific to site-based work. Get in touch if you'd like to discuss what a COSHH assessment should cover for your workplace, or arrange for one to be carried out.