Mechanical ventilation under this section is either general mechanical ventilation or local exhaust. General ventilation must move enough air changes to keep welding fumes and smoke within the safe limits of subpart D; local exhaust uses freely movable hoods that the welder or burner places as close to the work as practicable, with capacity to capture fumes at the source and keep breathing zone levels within subpart D limits. Exhaust air is discharged to open air or away from intake air, replacement air must be clean and respirable, and oxygen may never be used to ventilate, to cool people, to blow dust off clothing or to clean the work area. Confined spaces: hot work in a confined space requires general or local exhaust ventilation meeting (a), except that where enough ventilation cannot be had without blocking access, workers inside wear subpart E airline respirators and a worker outside keeps communication and helps in an emergency. Where a welder enters through a manhole or small opening there must be a way to remove the welder quickly in an emergency; safety belts and lifelines used for this are attached so the body cannot jam in the opening, and an attendant stationed outside, with a rescue procedure planned in advance, watches the welder at all times and can put the rescue into effect. Toxic metals in enclosed spaces: zinc-bearing metals or zinc coatings, lead base metals, cadmium-bearing fillers, and chromium-bearing metals or coatings require general or local exhaust ventilation; metals containing lead (beyond an impurity) or lead coatings, cadmium-bearing or coated base metals, mercury-bearing coatings, and beryllium-containing metals require local exhaust or airline respirators, and beryllium work always needs both local exhaust and airline respirators. In the open air such work requires filter-type respirators, or airline respirators for beryllium. Other workers sharing the atmosphere get the same protection as the welder. Inert-gas metal-arc welding, with ultraviolet 5 to 30 times that of shielded metal-arc welding, may not proceed until chlorinated solvents are kept at least 200 feet from the exposed arc unless shielded and treated surfaces are dry, unscreened workers wear subpart E filter lenses, welders exposed to each other's arc wear filter goggles under helmets, hand shields are used when the helmet is raised, skin is fully covered, helmets and shields are free of leaks and reflective surfaces, and stainless steel work meets (c)(2) for nitrogen dioxide. Other hot work may normally proceed without mechanical ventilation or respirators, but unusual conditions causing unsafe contaminant buildup require them; all hot work requires subpart E eye protection.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.