LEV That Is Running but Not Capturing
A fabrication shop employs 60 welders on gas metal arc welding, including galvanized and mild steel. Local exhaust ventilation was installed five years ago. A recent survey found capture velocity at the arc of 0.25 m/s; the hood is mounted 45 cm from the arc and the manufacturer specifies 0.5 m/s at 15 cm. The static pressure gauge sits in the red zone, and filters are changed quarterly. Welders report a metallic taste and two had metal fume fever symptoms last month. The production supervisor says the LEV is 'running' and respirators are mandatory. A new contract will raise throughput 20% and add a second shift. The safety manager must decide whether to approve the ramp-up.
1. Evaluate whether the LEV is an effective control and identify the evidence you would use.
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'Running' is not the test. The measured capture velocity at the arc is half the manufacturer's specified value, and the hood is three times farther from the arc than the specification assumes. Capture velocity falls rapidly with distance, so the combination of low velocity and excessive hood distance means contaminated air is likely escaping the hood and entering the breathing zone.
The static pressure gauge in the red zone indicates a flow problem: loaded filters, duct obstruction, fan degradation, or a closed damper. Quarterly filter changes do not prove adequate airflow.
The health symptoms are consistent with exposure to metal fume, particularly zinc fume from galvanized steel, which can cause metal fume fever. That does not by itself prove causation, but it raises the urgency.
Evidence to collect: smoke-tube visualization at the arc, capture velocity at the hood face and at the arc, static pressure and fan curves, duct inspection, filter differential pressure, and personal air sampling for the relevant metals across representative tasks and both base and galvanized welding.
2. Prioritize corrective actions and interim measures, and explain your ordering.
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Order the actions by the hierarchy and by the severity of the exposure signal.
- Restore airflow: replace loaded filters, inspect and clear ducts, verify fan operation, and measure static pressure against the design value. Until airflow is restored, the LEV should not be counted as a control.
- Fix hood position and design: move the hood to the specified distance, use a fume-extraction gun or a properly sized flanged hood, and re-measure capture velocity at the arc.
- Reduce the source: evaluate lower-fume wire, reduced galvanized welding until controls are proven, and process adjustments such as positioners that keep the welder's breathing zone out of the plume.
- Interim respiratory protection: if exposure continues above applicable limits, implement a full respirator program: medical evaluation, fit testing, training, and cartridge/particulate filter change schedules. Mandatory PPE without program elements is not an adequate interim control.
- Administrative controls: rotate tasks, schedule galvanized welding when ventilation is verified, and train workers to recognize metal fume fever symptoms.
The ordering is deliberate: engineering and source controls must be corrected first; respirators protect the individual but do not fix the workplace.
3. Design a verification plan to confirm the controls work before the second shift begins.
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Verification must combine control-performance checks and exposure measurements.
- Control performance: smoke-tube visualization at the arc, capture velocity at the arc and hood face, static pressure, fan amperage, and duct leakage inspection. Record baseline values and re-check after filter changes and hood adjustments.
- Exposure verification: personal sampling on the worst-case tasks, including galvanized welding, on multiple workers and shifts. Compare 8-hour and short-term results to the applicable occupational exposure limits. Use a statistical criterion such as an upper confidence limit below the limit, not a single sample below the limit.
- Health surveillance: track reports of metallic taste and fume fever symptoms before and after the change.
- Change control: re-verify after the throughput increase and after the second shift starts, because higher duty cycle can overwhelm the ventilation system.
Approve the ramp-up only when both control-performance checks and exposure results meet the criteria. Document the verification and set a re-test date.