Winter Welding Bay: Metal Fume Under a Closed Roof
Your workshop is a 6 m × 9 m × 3 m bay with a roller shutter at each end and a 900 mm roof vent. In December both shutters stay closed to keep the heat in. Two fabricators spend most of the day MIG welding galvanised handrail sections and some 3 mm mild steel sheet; the zinc coating is left intact at the weld preparation. At 14:30 both men report a metallic taste, headache, shivering and nausea, which ease about 20 minutes after they step outside. The bay has a portable 1 hp fan aimed across the bench, and both men wear disposable FFP1 dust masks because 'the welding helmet is protection enough'. A drum of anti-spatter solvent sits open 2 m from the bench and the only extinguisher in the bay is a 2 kg CO2 unit by the door. The supervisor wants the job finished before the Christmas shutdown, three days away.
1. Identify the most probable cause of the symptoms, the exposure route, and explain in detail why each current control is inadequate.
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Diagnosis. The pattern - metallic taste, headache, shivering, nausea, and resolution within roughly 20 minutes away from the bay - is the classic metal fume fever presentation. The source is the galvanised coating: zinc melts at about 419 °C and boils at 907 °C, so an arc at several thousand degrees generates zinc oxide fume as sub-micron particles that reach the alveoli. A polymer fume fever from overheated anti-spatter or paint is a secondary possibility and should be checked against the SDS.
Route. Inhalation is effectively the only significant route for fume; ingestion is secondary, via contaminated hands and face at break time.
Why the controls fail:
- FFP1 disposables. These are nuisance-dust masks, not respiratory protective equipment for welding fume. They leak at the face, are not fit-tested, and a welding helmet is not RPE - fume is drawn up under the shield into the breathing zone.
- The 1 hp fan. This is dilution, not extraction. Blowing across the bench stirs fume into the bay and can drive it through the breathing zone; it captures nothing at source.
- The building. The bay is 6 × 9 × 3 = 162 m³, and with both shutters closed a 900 mm roof vent gives negligible air change. The fume has nowhere to go.
- The solvent. An open drum of anti-spatter 2 m from the arc is a fire risk, and if its SDS shows chlorinated hydrocarbons, UV from the arc can decompose the vapour into phosgene and hydrogen chloride. That must be confirmed before the next shift.
- No assessment. There is no COSHH assessment covering welding fume, no fume monitoring and no health surveillance for the two men.
2. The supervisor proposes to hand out FFP3 masks and press on. Evaluate that proposal and set out, in hierarchy-of-control order, what must be in place before work restarts.
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Evaluation of the proposal. A fit-tested FFP3 is a genuine improvement on an FFP1, but offering it as the primary control is not acceptable. GMAW fume is classified as a substance that may cause cancer by inhalation, and the law and good practice both require exposure to be controlled at source; RPE is the last line, to be used only for residual exposure or short-term interim work. The men have already reported symptoms, which is a trigger to stop, reassess and involve occupational health, not to press on with better masks. RPE also brings its own failure modes: it depends on fit testing, clean-shaven faces, correct donning and stock control.
Hierarchy before restart:
- Eliminate/substitute. Specify uncoated steel handrail, or remove the hazard at source by grinding the zinc coating back 50–75 mm each side of every weld before welding. This is the single most effective step.
- Engineering. Fit on-torch extraction or an extracted welding bench with the hood within one duct diameter of the arc, ducted to outside air and subject to thorough examination by a competent person. Open both shutters and run the roof vent to give general ventilation; use fans only to supply make-up air, never to blow across the arc.
- Administrative. Move the solvent to a closed flammable store at least 3 m from hot work; introduce a hot-work permit; rotate the men out of the bay through the day; brief everyone on metal fume fever and its early symptoms; update the COSHH assessment; arrange health surveillance.
- PPE. Fit-tested FFP3 or half-mask with P3 filters as residual protection, plus the existing eye, ear and hand protection.
Emergency arrangements. The 2 kg CO2 is class B and electrically safe but gives poor cooling, so a solvent fire can re-ignite, and it does nothing for class A packaging; in a closed bay a CO2 discharge also displaces oxygen locally and the cylinder is under-sized for the risk. Provide a foam or dry-powder extinguisher at the exit, a fire blanket, clear escape routes and an alarm, and keep the CO2 specifically for electrical fires. No work restarts until extraction is commissioned and the supervisor signs off.