What type of ventilation should NOT be used while welding or cutting?

Using oxygen for ventilation during welding or cutting is inappropriate and potentially dangerous. Oxygen is crucial for the combustion process; thus, supplying more oxygen in the vicinity of welding or cutting can elevate the risk of fire and explosions. It's critical to maintain an atmosphere that prevents the accumulation of harmful fumes and protects the welder from exposure to dangerous gases, rather than enhancing the concentration of the fuel for combustion.

Natural and mechanical ventilation are both preferred methods for ensuring the workspace is safe and breathable. Natural ventilation allows for fresh air inflow, while mechanical ventilation actively manages air movement through fans and exhaust systems. Positive pressure ventilation, while generally used in specific controlled environments, might disrupt the intended airflow during welding, potentially increasing the risk of fire.

Thus, the selection of oxygen for ventilation would be contra-indicated, focusing instead on ventilation systems designed to effectively reduce harmful fume concentrations while maintaining a safe working environment.