Phoenix Journal · LEV Testing
Dust, fume, mist, vapour and gas are five distinct states of airborne contaminant - and telling them apart is the first step in controlling exposure and testing your extraction properly.
Airborne contaminants explained
Dust, fume, mist, vapour and gas are not loose synonyms for "bad air" - they are five distinct physical states, and knowing which one you are dealing with decides how you capture it and how you test the control.
Under the Control of Substances Hazardous to Health Regulations 2002 (COSHH), all five sit inside the same duty: if a process gives off something hazardous to breathe, you must prevent or adequately control exposure. But the way each one behaves in the air is completely different. A solid particle thrown off by a cutting disc drops and settles; a solvent vapour spreads to fill a room and does not settle at all. A hood shape that captures one will miss the other entirely.
That is why the labels matter on the shop floor and not just in a textbook. Getting the classification right is the first step in a proper risk assessment, and it feeds straight into knowing what you are actually dealing with before you spend money on extraction. Below we split the five into the two families that matter for control - particulates you can filter, and the gaseous states you generally cannot.
These three are all aerosols - tiny solid or liquid bits suspended in air - and they share one crucial trait: they have mass, so they can be filtered, and a well-placed hood can pull them in before they reach the breathing zone.
Dust is solid particles thrown into the air by a mechanical action - grinding, sanding, cutting, sweeping, handling powders. Grain, flour, wood, stone and metal all shed it. HSE splits dust into two fractions that matter for testing: the inhalable fraction, which is everything you can breathe into the nose and mouth, and the respirable fraction, the finer particles small enough to reach the deep gas-exchange region of the lung. That distinction is not academic - it is the basis of two separate workplace exposure limits.
Fume is much finer. It forms when a material is heated so hard it vaporises, then the vapour condenses in the air into extremely small solid particles - classic examples being welding and soldering. Because fume particles are far smaller than mechanical dust, they hang in the air longer, travel further and slip deeper into the lung. HSE reclassified all welding fume as a human carcinogen in 2019.
Mist is the liquid version - fine droplets thrown up by spraying, plating, machining with cutting fluids or pressure washing. Metalworking-fluid mist and electroplating acid mist are common examples in engineering.
Where they are easier to deal with
Where they catch people out
Vapour and gas behave in a fundamentally different way. They are molecules, not particles - they have effectively no useful mass to grab, they mix into the surrounding air, and they will fill whatever space they are released into. You cannot filter them with a simple dust filter, and a hood that relies on catching heavy particles will not hold them.
Vapour is the gaseous form of a substance that is a liquid or solid at normal room temperature and pressure. It comes off anything that evaporates - solvents, thinners, adhesives, paints, degreasers, fuels. The tell-tale sign is a smell that spreads across a room from an open container. Because the source is an evaporating liquid, cutting vapour off often means containing or covering the liquid, not just extracting the air above it.
Gas is a substance that is already gaseous at room temperature and pressure - so it never has to evaporate first. Carbon monoxide, carbon dioxide (CO₂), chlorine, nitrogen dioxide (NO₂) and the products of combustion all fall here. Gases can be odourless and colourless, which is exactly what makes carbon monoxide so dangerous, and some are heavier or lighter than air, so they collect in pits or at ceiling level rather than dispersing evenly.
Where they are simpler than they look
Where they catch people out
Once you have classified the contaminant, everything downstream follows from it: the hood design, the duct velocity, the filter media and the way the system is examined. Local exhaust ventilation (LEV) is HSE's go-to engineering control for all five states, and the standard guidance, HSG258, is built around matching the capture method to how the contaminant travels. A slot hood sized for grinding dust is not a solvent-vapour control, and general room ventilation - HSE has been explicit since 2019 that it is not enough on its own for welding fume indoors.
The classification also sets the yardstick you test against. Workplace exposure limits in EH40/2005 (2020 edition) are quoted separately for the inhalable and respirable dust fractions, so the sampling method has to match the fraction. This is also why looking after people's lungs is as much a management task as an engineering one - it feeds directly into reducing the sickness absence that dust and fume cause.
Questions
They look similar but they are not the same thing physically. Fume is made of tiny solid particles - a metal or other material is heated until it vaporises, then that vapour condenses in the air into fine solids, as happens in welding. Vapour is the gaseous form of something that is a liquid or solid at room temperature, such as solvent evaporating from an open tin. The practical upshot is that fume can be filtered because it has particles, while vapour has to be contained, substituted or captured with specialist media.
Because each one behaves differently in the air and therefore needs a different control. Particulates like dust, fume and mist have mass and can be drawn into a hood and filtered, whereas vapour and gas are molecules that mix through the whole room and flow around a simple dust hood. Classifying the contaminant correctly is the first step of a COSHH risk assessment - it decides your hood design, your extraction rate and the way you test the system.
Inhalable dust is the whole fraction you can breathe in through the nose and mouth, while respirable dust is only the finer particles small enough to reach the deep, gas-exchange part of the lung. HSE sets separate workplace exposure limits for each in EH40/2005 - broadly 10 mg/m3 for inhalable and 4 mg/m3 for respirable general dust over an 8-hour day. Because the limits differ, air sampling has to be done with the method that matches the fraction you are measuring.
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