Dust, fume, mist, vapour and gas defined

Dust is solid particles thrown into the air by mechanical processes like grinding and sanding (bits of solid broken up small); fume is much finer solid particles formed when a vaporised solid, usually metal, condenses in cool air - like welding fume; both are solid, but fume is far finer than dust. Dust and fume are both solid particles suspended in air, but they form differently and differ in size. Dust is solid particles thrown into the air by mechanical processes - grinding, sanding, cutting, drilling, crushing, sweeping. It's bits of a solid material broken up small enough to become airborne (wood dust, flour dust, silica dust, metal 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 a vaporised solid - usually a metal - condenses in the cooler air. When a solid is heated enough to vaporise (as in welding, where the arc vaporises metal), the vapour cools and condenses into extremely fine solid particles. So fume is much finer than dust, making it easy to breathe deep into the lungs - the classic example being welding fume. A common misuse: people call solvent 'fumes', but that's vapour, not fume - fume is specifically the fine solid particles from condensed vaporised solids. So dust is broken-up solid, fume is condensed vaporised solid, and fume is much finer. So dust is mechanical particles; fume is condensed, much finer. This is general information. This is general information.

Both are genuinely gaseous in air, but differ in origin: a vapour is the gaseous form of something normally liquid (or solid) at room temperature, from evaporation - like solvent vapour; a gas is naturally gaseous at room temperature - like carbon monoxide; both mix readily with air and disperse. Vapour and gas are both genuinely in the gas phase (unlike solid or liquid particle contaminants), but they differ in origin. Vapour is the gaseous form of a substance that is normally a liquid (or sometimes a solid) at room temperature, formed by evaporation. When a volatile liquid evaporates, it gives off vapour - its molecules in the gas state, mixed into the air. The everyday example is solvent vapour, coming off liquid solvents in paints, thinners, degreasers and glues. Gas is a substance that is in the gaseous state at normal room temperature and pressure - it's a gas to begin with, not something evaporated or condensed. Examples: carbon monoxide (from combustion), chlorine, and carbon dioxide (from brewing and fermentation). So the practical difference is origin - a vapour comes from a normally-liquid substance (and would condense back to liquid if cooled), while a gas is naturally gaseous - but both behave the same way in air: they mix readily with it, spread out, and can't be captured by inertia. So vapour is from evaporated liquid; gas is naturally gaseous; both act as gases. So one is evaporated liquid, the other naturally gaseous. This is general information. This is general information.

Mist is liquid - fine liquid droplets suspended in the air, formed by spraying, splashing or a vapour condensing, like paint spray or metalworking-fluid mist; it differs from dust and fume (which are solid particles) but, like them, is fine particles suspended in air that can be breathed in. Mist is liquid - it's fine liquid droplets suspended in the air. This distinguishes it from dust and fume, which are solid particles. Mist forms by spraying or atomising a liquid (like paint spray from a spray gun), by splashing or agitation, or by a vapour condensing back into droplets. Examples: paint spray mist, metalworking-fluid (coolant) mist from machining, oil mist, and acid mist from some processes. Although mist is liquid rather than solid, it behaves in some ways like the solid particle contaminants (dust and fume): it's fine particles (droplets) suspended in air, so it has mass, can be projected, and can be breathed into the lungs. The droplet size matters 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 (as with metalworking-fluid mist). So mist is the liquid-droplet contaminant, distinct from the solid particles (dust, fume) and from the true gases (vapour, gas). So it's liquid - fine airborne droplets. So mist is liquid droplets in the air. This is general information. This is general information.

Because different contaminants behave differently in air and so need capturing differently - particle contaminants (dust, fume, mist) are projected and need the hood positioned and sized for how they're generated, while gases and vapours mix with air and must be captured right at source; so the type shapes the LEV's design. The type of contaminant matters for LEV because different contaminants behave differently in the air and so need capturing differently. Particle contaminants - dust, fume and mist - have mass and are often projected: grinding throws dust, welding sends fume up on the hot plume, spraying throws mist. So how a particle contaminant is generated (its speed, direction and plume) affects where the LEV hood must be positioned and how much airflow it needs to capture it before it escapes. And coarse, heavy dust (which settles) can be captured differently from fine fume or mist (which floats and follows air currents). Gases and vapours behave differently again: they mix readily with air and can't be captured by inertia (they have no useful particle mass), so they must be captured right at the source, before they disperse - because once mixed into the room air, they're very hard to remove. So the capture strategy differs by type: position and airflow tuned to a particle's generation, versus close source capture for gases and vapours. This is why an LEV assessment identifies the contaminant - the hood, position and airflow all depend on it, and capture that suits one type may not suit another. So the type determines how the LEV must capture it. So because each type needs a different capture approach. This is general information. This is general information.

Yes - welding fume is a genuine fume: very fine solid particles formed when the arc vaporises metal and the vapour condenses in cool air; it's the classic example of fume, and being very fine it's easily breathed deep into the lungs - unlike solvent 'fumes', which are actually vapour. Yes - welding fume is a genuine fume, and in fact the classic example of one. Fume is very fine solid particles formed when a vaporised solid - usually a metal - condenses in the cooler air. In welding, the arc's intense heat vaporises metal (from the workpiece, the electrode and any coatings); that metal vapour then cools in the surrounding air and condenses into extremely fine solid particles - which is welding fume (fine metal-oxide particles). So it fits the definition of fume exactly: fine solid particles from a condensed vaporised solid. Being very fine, welding fume is easily breathed deep into the lungs, which is part of why it's a serious health hazard (linked to lung and other diseases). This contrasts with the everyday misuse of 'fumes' for things like solvent vapour - solvent 'fumes' are actually vapour (the gaseous form of an evaporating liquid), not fume. So welding fume is a real, textbook fume, while many things loosely called 'fumes' are vapour. So yes - welding fume is the classic fume. So yes; it's fine solid particles from condensed metal vapour. This is general information. This is general information.

Yes - identifying whether you have dust, fume, mist, vapour or gas is part of designing and testing LEV that actually captures it, because the hood type, position and airflow all depend on how the contaminant behaves; capture that works for one type may fail for another, so the type must be known. Yes - knowing the contaminant type genuinely helps control it, because it's part of designing and testing LEV that actually captures it. The hood type, its position, and the airflow needed all depend on what the contaminant is and how it behaves. A particle contaminant (dust, fume, mist) that's projected from a process needs the hood positioned and sized for how it's generated - to catch the dust, fume or mist before it escapes; a coarse dust that settles is handled differently from a fine fume or mist that floats. A gas or vapour, which mixes with air and can't be captured by inertia, needs close capture right at the source before it disperses. So capture that works for one type may fail for another - a hood sized for coarse dust may not capture fine fume or a vapour. This is why an LEV assessment identifies the contaminant first: the whole design of the control follows from knowing what's being captured. And when the LEV is tested, it's tested against the contaminant it's meant to control, to confirm it suits it. So yes - identifying the type is a real, practical part of controlling it. So yes; the type shapes the whole control design. This is general information. This is general information.