Dust, fume and mist: knowing what you are dealing with

Look at how the contaminant is generated: mechanical work on a solid (grinding, sanding, cutting) makes dust; high heat vaporising a solid, usually metal (welding), makes fume; spraying, atomising or machining a liquid (paint, coolant) makes mist - so the process itself tells you which, and many workplaces make more than one. You tell whether your process makes dust, fume or mist by how the contaminant is generated - what the process is doing. Dust is solid particles from mechanical work: if the process is grinding, sanding, cutting, drilling, crushing or sweeping a solid, it makes dust (wood dust, flour dust, silica dust, metal grinding dust). Fume is very fine solid particles from heat: if the process uses high heat that vaporises a solid, usually a metal (welding being the classic case), the vapour condenses into fume, which is much finer than dust. Mist is fine liquid droplets: if the process sprays, atomises or machines a liquid (paint spraying, machining with coolant), it makes mist (paint spray mist, coolant mist, oil mist). So look at each of your processes and ask what it does - mechanical action on a solid (dust), high heat on metal (fume), or a liquid thrown into the air (mist). Many workplaces make more than one, and some processes make more than one. So the process tells you which - identify it per process. So look at what each process does to work out the contaminant. This is general information. This is general information.

Dust is solid particles from mechanically breaking up a solid (grinding, sanding), varying in size; fume is much finer solid particles formed when heat vaporises a solid, usually metal, and it condenses - like welding fume; both are solid, but fume is far finer and tends to rise on the process heat. Dust and fume are both solid particles in the air, but formed differently and different in size. Dust is solid particles thrown into the air by mechanical processes - grinding, sanding, cutting, drilling, crushing, sweeping - so it's bits of a solid material broken up small enough to float (wood dust, flour dust, silica dust, metal grinding dust). Dust particles vary in size, from coarse (settling quickly) to fine (staying airborne and reaching deep into the lungs). Fume is very fine solid particles formed when heat vaporises a solid - usually a metal - and the vapour condenses in the cooler air. In welding, the arc vaporises metal that condenses into welding fume. So fume is much finer than dust, making it easily breathed deep into the lungs, and it tends to rise on the thermal plume of the hot process rather than settling. A note: solvent 'fumes' are actually vapour, not fume - fume is specifically the fine solid particles from a condensed vaporised solid. So dust is mechanically broken solid; fume is condensed vaporised solid, much finer and rising. So dust is coarser mechanical particles; fume is fine, from heat. This is general information. This is general information.

Yes - many workplaces produce dust, fume and mist across different processes, and a single process can produce more than one (for instance one involving both mechanical action and heat); so identifying what you're dealing with means looking at each process and what it generates, not picking one label for the whole site. Yes - one process can produce more than one contaminant, and most workplaces produce more than one type across their operations. A workshop might grind (making dust), weld (making fume) and machine with coolant (making mist) - three processes, three contaminants. And a single process can produce more than one type: a process involving both mechanical action and heat could produce both dust and fume, for example. So identifying what you're dealing with isn't about picking a single label for the whole site - it's about looking at each of your processes and asking what each generates, and where. This matters because control has to be matched to each contaminant: different contaminants behave differently and need capturing differently, so a site with dust in one area, fume in another and mist in a third needs control suited to each, not a one-size approach. So the task is to map which contaminants arise across your operations, then control each appropriately. So yes - identify each contaminant across your processes. So yes; look at every process and what it makes. This is general information. This is general information.

Because dust, fume and mist behave differently - coarse dust settles, fine fume and mist float and follow air currents, and each is generated differently - so the LEV hood, position and airflow all depend on which you have; capture that works for one may fail for another, so the control follows from the identification. It matters which contaminant you have because dust, fume and mist behave differently, so they need capturing differently. Coarse dust is relatively heavy and settles fairly quickly; fine fume and fine mist are light, float, and follow air currents. And they're generated differently: grinding throws dust in the mechanical direction, welding sends fume up on a hot plume, spraying throws mist in the spray direction. So the LEV needed to capture each differs - the hood type, its position relative to the source, and the airflow all depend on which contaminant you have and how it's generated. This means capture that works for one type may fail for another: a hood and airflow sized for coarse dust may not capture fine fume or mist, and capture positioned for a settling dust may miss a rising fume plume. So mislabelling the contaminant risks control that looks right but doesn't actually work. This is why an LEV assessment starts by identifying the contaminant, and why the LEV is tested against the contaminant it's meant to control - the identification underpins the whole design. So it matters because the control follows from it. So because the contaminant determines how it must be captured. This is general information. This is general information.

Mist is liquid - fine liquid droplets suspended in the air, from spraying, machining or atomising a liquid (paint spray, coolant mist); it differs from dust and fume (both solid particles) but behaves somewhat like them, being fine particles suspended in air that can be breathed in, and fine mists can be almost invisible. Mist is liquid - it's fine liquid droplets suspended in the air, which distinguishes it from dust and fume (both solid particles). Mist forms by spraying or atomising a liquid (like paint spray from a spray gun), by machining with a liquid coolant (as in CNC machining), by splashing or agitation, or by a vapour condensing back into droplets. Examples: paint spray mist, metalworking-fluid (coolant) mist, oil mist, acid mist. Although mist is liquid rather than solid, it behaves in some ways like the solid particle contaminants: it's fine particles (droplets) suspended in air, so it has mass, can be projected in the spray or throw-off direction, and can be breathed into the lungs. As with dust, finer mists stay airborne longer and reach deeper into the lungs. Mist can be easy to overlook because fine mists can be nearly invisible - metalworking-fluid mist being a notable example (covered in the cnc-coolant-mist pages). So mist is the liquid-droplet contaminant, distinct from the solid dust and fume but behaving somewhat similarly in air. So it's liquid - fine airborne droplets from a liquid process. So mist is liquid droplets, not solid. This is general information. This is general information.

Yes - it's the foundation: the LEV hood, position and airflow are designed for the contaminant's size and behaviour, so correctly identifying dust, fume or mist leads to control that suits it, while mislabelling it risks extraction that looks right but doesn't capture the actual contaminant; the test then checks the LEV against that contaminant. Yes - identifying the contaminant is foundational to making the LEV work. The LEV hood type, its position relative to the source, and the airflow are all designed around the contaminant's size and behaviour - so correctly identifying whether you have dust, fume or mist leads to control that genuinely suits it. Correctly recognising that a process makes fine welding fume (which is light and rises on a plume), rather than coarse dust (which is heavier and settles), leads to a hood and airflow designed for that fume. Conversely, if the contaminant is mislabelled, you risk designing or accepting control that doesn't suit it - extraction that looks like it should work but doesn't capture the actual contaminant. So the identification underpins the design. It also underpins the testing: when the LEV is thoroughly examined and tested, it's tested against the contaminant it's meant to control, to confirm it captures it. So getting the identification right at the start supports both designing the control and verifying it. So yes - it's the necessary starting point for effective LEV. So yes; the whole design and test follow from it. This is general information. This is general information.