Fume Control on a Galvanized Repair Job
A maintenance crew must repair a galvanized handrail inside a partially enclosed pump room. The room measures 6 m by 4 m with one 1 m by 1 m louvered opening and a door that stays propped. The welder will run about 1.5 m of 3 mm fillet using GMAW short-circuit transfer at 150 A. Air sampling on a similar job measured zinc oxide fume at roughly twice the 8-hour permissible exposure limit, and the welder reports a metallic taste and flu-like symptoms after prior shifts. The portable fan available is a 450 mm axial unit that can be set to blow across the work or to exhaust through the door. Management wants the job finished in one shift and asks whether a respirator alone is acceptable.
1. Identify the exposure controls you would put in place before welding, and justify their priority order.
Read the worked answer
Apply the hierarchy of controls rather than starting with PPE. First, substitute or remove the coating: strip zinc from the weld zone and 25-50 mm either side using a grinder with dust extraction, which eliminates most of the fume source and also improves weld quality. Second, engineer the ventilation: position the axial fan to exhaust air through the door opening and bring makeup air through the louver, so contaminated air moves away from the welder's breathing zone rather than across the arc into it. Add a movable extraction hood close to the arc if available. Third, administrative controls: rotate or limit time in the space and keep other workers out. Respirators are the last line, not the primary control; a half-face unit with P100 filters may be worn as added protection, but it does not replace ventilation in a confined, poorly ventilated room. Also verify the coating is zinc and not a lead-bearing paint before grinding.
2. The supervisor argues the fan should blow across the work to keep the welder cool. Explain why this is the wrong choice and what symptoms in the scenario support your position.
Read the worked answer
Blowing across the work pushes fume through the breathing zone and disperses it through the room, increasing exposure for everyone and defeating dilution. Cross-draft also disrupts shielding gas, causing porosity. The reported metallic taste and flu-like symptoms after shifts are consistent with metal fume fever from zinc oxide, which indicates the current exposure is already excessive. The correct arrangement is directional: exhaust at the source or at the low point of the room, with clean makeup air entering behind the welder, so the plume is captured before it reaches the face. Document the change and repeat air sampling to confirm exposure is below the limit; if it is not, add respiratory protection and reassess the ventilation design.