A COSHH assessment is the decision an employer makes about a hazardous substance used in or produced by their work: what harm it can do here, who is exposed and how much, what will prevent or control that exposure, and how you will know the control is still working. It is a judgement about your workplace, which is why no supplier and no template can make it for you. This page sets out the four things it has to decide and the substances people are surprised to find in scope.
The four decisions
What the substance can do to health, taken from the supplier's information and from what the process does to it. Who is exposed, doing what, for how long, and at what likely level. What will prevent exposure or, where prevention is not reasonably practicable, adequately control it, working down from elimination and substitution through engineering controls and ways of working to protective equipment as the last resort. And how you will check the control keeps working: maintenance, testing, monitoring and, where required, health surveillance.
Which substances are in scope, including the ones nobody buys
The obvious half is what you purchase: chemicals, cleaners, adhesives, fuels, coatings. The half firms miss is what the work creates: wood dust from sawing, silica dust from cutting stone, welding fume, solder fume, flour dust in a bakery, oil mist from machining, and biological agents in some settings. None of those arrives with a label or a sheet, and several of them are among the most damaging exposures in ordinary workplaces. An inventory built from a purchasing report will not contain them.
Control measures are a hierarchy, and the order is the point
Do not use the substance if the job can be done without it. Use a less hazardous one or a less hazardous form of it, powder to pellet, spray to brush. Enclose the process or extract at source. Change how or where the work is done to cut exposure time and the number of people exposed. Only then reach for respiratory protective equipment, which depends on fit, training, maintenance and the person actually wearing it every time. Assessments that go straight to gloves and a mask have skipped four cheaper and more reliable controls.
What the assessment is not
It is not the safety data sheet, which describes the substance in general and knows nothing about your ventilation. It is not a one-off: the duty includes reviewing regularly and whenever there is reason to think it is no longer valid. And it is not a document that discharges anything by existing; where it identifies further action, the action is the duty and the assessment is only the record of having decided on it.
Questions people ask about what is a coshh assessment
What substances does COSHH include?
Substances hazardous to health used in or arising from work: purchased chemicals, and also dusts, fumes, mists, vapours, gases and biological agents produced by the process. It does not cover asbestos, lead or radioactive substances, which have their own regulations, and it treats some things such as certain dusts by reference to concentration in air.
Are nanomaterials covered by COSHH?
Where a nanomaterial is a substance hazardous to health used in or arising from work, the same duties apply: assess, prevent or adequately control exposure, and review. The practical difficulty is that the hazard information for some engineered nanomaterials is thinner than for conventional forms, which argues for a more precautionary control rather than for treating the gap as an absence of risk.
What are COSHH control measures?
Everything you put in place to prevent or adequately control exposure, in a hierarchy: eliminate the substance, substitute a safer one, enclose or extract at source, change the way the work is done, and only then rely on personal protective equipment. Maintenance, examination and testing of engineering controls are part of the control measure, not an optional extra.