Metal Fumes
Metal & Process Fumes · Metal & Process Fumes overview
Metal fumes are the fine airborne metal-oxide aerosols generated when metals are heated to vaporisation during welding, cutting, brazing, gouging and high-temperature engineering work. In UK workplaces these aerosols are a primary route of occupational exposure for welders, fabricators and engineering operatives, and their composition determines how exposure is assessed against the Workplace Exposure Limits in HSE EH40.
What metal fumes are
Metal fume is not the same as general workshop dust. It is a freshly generated aerosol formed when metal is vaporised at high temperature — most often at the welding arc, but also during plasma and oxy-fuel cutting, thermal spraying, brazing and arc-air gouging. The vapour cools and condenses in the air into ultrafine metal-oxide primary particles, typically well below one micrometre, which then agglomerate into chain-like structures that remain suspended in the welder's breathing zone and deposit deep in the lung.
Because metal fume is formed from vaporised metal, its chemistry reflects the parent material, the consumable, and any coatings or contamination on the workpiece. A single welding bay can therefore produce very different metal fume profiles across a shift, depending on the joints, wires and surface conditions involved.
Where metal fumes are generated
Welding is the dominant source of metal fume in UK fabrication and engineering, but it is not the only one. Any process that takes metal to vaporisation temperature, or that violently disrupts a freshly molten metal surface, will release metal fume into the workshop air.
- MIG/MAG, MMA, FCAW and TIG welding of mild steel, stainless steel, aluminium and nickel alloys.
- Plasma and oxy-fuel cutting of plate and structural sections.
- Arc-air gouging and back-gouging of weld preparations.
- Brazing and soldering with cadmium-, silver- or zinc-bearing fillers.
- Thermal spraying, metallising and hard-facing of components.
- Grinding and abrasive work adjacent to fresh welds, where condensed fume deposits are re-aerosolised.
Visible fume, airborne particulate and exposure risk
Welders and supervisors commonly judge metal fume exposure by what they can see — the blue or grey plume rising from the arc. The visible plume is a useful prompt that metal fume is being generated, but it is a poor proxy for exposure. The fraction that reaches the breathing zone depends on torch angle, body position, head position inside the helmet, on-torch extraction performance, the position of any fume arm, and the wider workshop air pattern.
Equally, the absence of visible fume does not mean exposure is low. TIG welding produces relatively little visible particulate but still releases metal fume and ozone. Reliable judgement of exposure requires personal air sampling positioned in the welder's breathing zone, with laboratory analysis of inhalable metal fume mass and the specific metals relevant to the work.
Common metals and fume constituents
The constituents that matter most for occupational exposure assessment depend on the process and the material. Mild steel welding fume is dominated by iron oxide with a manganese fraction from the consumable. Stainless steel welding fume carries chromium and nickel from both the parent material and the filler wire. Aluminium welding contributes aluminium oxide and ozone. Coated steels add the coating chemistry — most commonly zinc from galvanising, which can drive metal fume fever.
- Iron oxide — mild steel MIG/MAG, MMA and FCAW.
- Manganese — mild steel and low-alloy welding consumables.
- Hexavalent chromium and nickel — stainless steel MIG, MMA and TIG.
- Aluminium and ozone — aluminium TIG and MIG.
- Zinc oxide — welding, brazing or cutting galvanised material.
- Copper, lead and cadmium — brazing, soldering and legacy coatings.
How metal fume exposure is assessed
Metal fume exposure assessment under COSHH typically combines a review of welding processes, metals and consumables in use; identification of similar exposure groups; personal pumped sampling in the breathing zone for inhalable particulate; and laboratory analysis for total inhalable metal fume mass plus the specific metals relevant to the work. Results are compared with the metal Workplace Exposure Limits in HSE EH40 and interpreted under BS EN 689 to give a defensible characterisation of exposure across representative shifts.
Sampling strategy matters as much as the analytical work. A single sample on a quiet day, or a sample taken outside the helmet on a welder using a powered respirator, will not characterise routine exposure. The detail of how, where and when sampling was carried out should be recorded and tied back to the COSHH welding risk assessment.
Why material, process, consumables and coating matter
Two welders working similar joints with different consumables, or with and without a paint primer in place, can record materially different metal fume profiles. Consumable manufacturers' safety data is a useful starting point, but does not substitute for breathing-zone measurement under actual working conditions. Where stainless steel, nickel alloys, coated steels or surface contamination are involved, the case for direct measurement strengthens because the assumed control regime may no longer be appropriate to the metals being released.
Control considerations
Engineering controls are the primary response. Where welding fume is generated indoors, the HSE expectation is that source capture — on-torch extraction or close-coupled local exhaust ventilation — is in place before reliance is placed on respiratory protection. Welding ventilation supports this by managing make-up air and dilution of residual fume. RPE remains an important supplement for sustained welding or tasks where source capture is impractical, but is no longer accepted as the primary control for routine indoor welding.
When to request a metal fume assessment
An exposure assessment is appropriate where a COSHH assessment cannot reasonably conclude that metal fume exposure is adequately controlled — for example after introducing stainless steel, nickel-alloy or coated-metal work; after a process or consumable change; where welders report respiratory symptoms or metal fume fever; where LEV is suspected of under-performing; or to verify ongoing control following the tightened HSE expectation around welding fume.
Frequently asked questions
What is the difference between metal fume and welding dust?
Metal fume is a freshly generated aerosol formed when metal is vaporised at high temperature and condenses into ultrafine metal-oxide particles. Workshop dust is generally coarser mechanical particulate from grinding, handling and surface work. Both can occur in the same workshop, but they differ in formation, particle size and how they are sampled.
How is metal fume exposure measured?
Through personal pumped sampling in the welder's breathing zone for the inhalable fraction, followed by laboratory analysis for total inhalable metal fume mass and the specific metals relevant to the work. Results are compared with the Workplace Exposure Limits in HSE EH40.
Does low visible fume mean low metal fume exposure?
No. Some processes such as TIG produce little visible particulate but still release metal fume. Exposure can only be characterised reliably by personal air sampling in the breathing zone.
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