Phoenix Journal · LEV & Air Quality
Dust, fume and mist get lumped together as bad air, but they are three different things. They are made in different ways, they float and travel differently, and they lodge in different parts of the lung. Get the label wrong and you can end up controlling the wrong hazard, or controlling the right one badly.
The words describe how the contaminant is physically formed, and that is what decides how you catch it. Dust is solid particles thrown off when a solid is broken up: cutting, grinding, sanding, sawing. Fume is far finer, created when a material is vaporised and the vapour condenses back into tiny solid particles in the air, as happens in welding and soldering. Mist is airborne liquid droplets, produced by spraying or by high-speed machining that flings coolant and metalworking fluid into the air.
Particle size is the reason this matters. Larger dust may be caught in the nose and upper airways, but the fine respirable fraction, and almost all fume, travels deep into the lung where it does the most harm and clears the slowest. Because the three forms differ in size, buoyancy and behaviour, a hood and airflow that capture coarse dust well may let fume drift straight past. Naming the hazard correctly is the first step to controlling it.
Mist deserves its own mention, because it is the form people most often forget. The oil-based metalworking fluids used in machining are flung into the air as a fine mist that can cause occupational asthma, bronchitis and a lung condition called hypersensitivity pneumonitis, while spraying throws solvent and paint mists carrying their own sensitisers. All of these limits and named substances are published by the HSE in its EH40 list of workplace exposure limits, which is the reference an assessor works to rather than guesswork or a glance at the air.
The numbers that bite
Under COSHH, dust is treated as hazardous to health once airborne concentrations reach the workplace exposure limits of 10 mg/m3 for inhalable dust or 4 mg/m3 for respirable dust, averaged over an eight-hour day. Those are the general figures. For specific substances the bar drops sharply. Respirable crystalline silica, from cutting stone, concrete or brick, has a limit of just 0.1 mg/m3, a hundred times lower than general inhalable dust, and it is a recognised carcinogen. Isocyanates in spray mists sit lower still, around 0.02 mg/m3.
Fume rewrites the rulebook again. The HSE treats all welding fume as capable of causing cancer, so there is no comfortable threshold to aim at; the duty is to control exposure with extraction whatever metal is being welded, indoors or out. A practical test runs through all of it: if you can see a haze in the air, you are almost certainly over the limit already.
The limits are not targets to sit just underneath. For carcinogens and sensitisers the duty is to reduce exposure as low as reasonably practicable, so a workplace parked just below a limit and calling it done has missed the point of the law. Fume makes this concrete: because welding fume is treated as carcinogenic across the board, the honest target is not a number to sit under but the lowest exposure the job allows.
A worked example
A single job often generates more than one form. Dry-cutting a paving slab throws coarse dust, a respirable fraction and a plume of fine particles all at once, and the most dangerous part is the bit you can barely see. That is why the specifics matter so much, and why it pays to understand the health case behind a substance rather than treating all dust alike. The story of silica dust and the health risks every site should know shows how a familiar building material becomes one of the most tightly limited substances in the workshop, and why visible dust is a red flag rather than a nuisance. A single dry cut of a concrete block can generate silica levels many times the limit in seconds, which is why the HSE estimates around 600,000 workers are exposed to silica in Britain each year.
Where it ends up
Whatever the form, the principle is the same: capture it at source before it reaches the breathing zone, using local exhaust ventilation designed for that contaminant and its behaviour. But capture is only half the job. The air that carries dust, fume and mist away has to go somewhere, and poorly designed or badly maintained ductwork can recirculate contaminants or dump them back into the workspace. It is worth understanding how ductwork affects the air your staff breathe, because an extraction system judged only on the airflow reading, and never on whether it actually captures the plume, can look busy while protecting nobody.
Questions
Dust is solid particles from breaking up a solid, such as grinding or sanding. Fume is much finer solid particles formed when a vaporised material condenses, as in welding. Mist is airborne liquid droplets from spraying or machining. They differ in size and behaviour, so they need different controls.
Fume particles are extremely fine, so they bypass the body's upper-airway defences and travel deep into the lungs, where they do more harm and clear slowly. The HSE treats all welding fume as capable of causing cancer, which is why it must be controlled with extraction regardless of the metal.
Not necessarily. The most harmful respirable particles and fume are often invisible in normal light. Visible haze almost always means exposure is already over the limit, but clear-looking air is not proof of control. That is what air monitoring and proper LEV testing are for.
Phoenix Duct Clean · by the numbers
Dust, fume and mist each need extraction designed and tested for them. Phoenix examines and tests LEV to COSHH Regulation 9 and reports on real capture, not just airflow. UK-wide.