The question is usually asked as a list, and the regulations do not answer it as a list. COSHH defines a substance hazardous to health by what it does and where a limit applies to it, not by naming products, which is why a tin with a pictogram on it and a cloud of flour dust with no label at all can both be in scope. That definition is worth reading once, because it decides which assessments you owe, and because the substances people forget are almost never the ones on the shelf. They are the ones the work itself creates. This page sets out what the definition catches, what it deliberately does not, and how a quiz-style question about which of these substances COSHH covers is usually marked.
What the definition catches
A substance is hazardous to health where it is classified as hazardous under the chemical classification and labelling regime, where a workplace exposure limit is assigned to it, where it is a biological agent connected with the work, where it is dust of any kind in air at a substantial concentration, or where it creates a comparable hazard to health by its nature and the way it is used even though it falls outside all of those. That last limb is the one that matters in practice: it is a catch-all written so a firm cannot argue a substance is safe merely because nobody has classified it.
Four things it deliberately leaves alone
Asbestos, lead and radioactive substances are out of scope because each has its own regulations that go further, and treating them as ordinary COSHH substances would mean applying a weaker regime than the law requires. Substances hazardous only because they are flammable, explosive, corrosive at high temperature or under pressure are out as well: the harm there is a safety harm rather than a health harm, and it is controlled elsewhere. A COSHH register that quietly includes asbestos is a sign that nobody has read the regulation, and usually a sign the asbestos duties are not being met either.
The ones the process makes, which nobody buys and nobody logs
Wood dust from a saw, flour dust in a bakery, silica from cutting stone, welding fume, solder fume, mist from a machine tool coolant, diesel exhaust in a workshop, isocyanate vapour from a spray booth: none of these arrives with a delivery note, a supplier label or a safety data sheet, and all of them are substances hazardous to health arising from the work. Firms that build their COSHH register from the purchasing system get an inventory of what they bought rather than an inventory of what people breathe, and the gap between the two is where the occupational disease cases live.
How the quiz question is actually marked
Asked which of these substances COSHH covers, the answer that scores is the one that reads the definition rather than the shopping list: chemicals and products bought in and classified as hazardous, yes; fumes, dusts, mists, vapours, gases and nanoparticles generated by the work, yes; biological agents such as bacteria and other micro-organisms met at work, yes; germs used deliberately in a laboratory, yes; asbestos, lead and radioactive material, no, because each is covered by its own regulations. Working the definition is also what the assessment does, so the quiz and the job ask the same question.
Other questions people ask
What are substances hazardous to health covered by COSHH?
Substances classified as hazardous under the chemical classification and labelling rules, substances with a workplace exposure limit, biological agents connected with the work, dust of any kind in air at a substantial concentration, and anything else that creates a comparable health hazard by its nature and the way it is used. The last limb is deliberately open, so an unclassified substance is not automatically out of scope.
Which substances does COSHH not cover?
Asbestos, lead and radioactive substances, each of which has its own regulations that go further than COSHH, and substances hazardous only for safety reasons such as flammability or explosivity rather than for health reasons. Everything else that can harm health through exposure at work is in scope, including substances the work itself produces.
Does COSHH apply to substances we never bought?
Yes, and that is the common gap. Dust, fume, mist and vapour generated by a process are substances arising from the work, so the duty to assess and to prevent or adequately control exposure applies to them exactly as it applies to a drum on a shelf. A register built from purchase records will miss all of them.