However, dilution ventilation does very little to reduce a welder’s exposure to weld fume. This is because the weld fume passes through their breathing zone in a concentrated plume right after it leaves the weld zone, well before it is diluted down.
Local exhaust ventilation (LEV) is the method of choice for welding operations. Weld fume is generated at a small point and rises in a plume of a relatively small diameter. These characteristics enable the weld fume plume to be easily captured before it enters the welder’s breathing zone. This is commonly accomplished with a moveable ventilation hood with flexible ductwork similar to the picture at right.
Proper positioning of the local exhaust ventilation hood is key to effective capture of the weld fume. The goal is to position the hood above the weld and slightly away from the welder to draw the fume away from their breathing zone. Maintaining an airflow rate around 100 feet per minute (fpm) just above the weld helps ensure the fume is captured efficiently without disturbing the shield gas.
One common misuse of this ventilation equipment is when the welder positions the hood directly above their head. This only helps to draw the weld fume up through their breathing zone, possibly even increasing their exposure. Large overhead hoods or canopies can cause the same problem. However, non-moveable backdraft hoods can be effective if designed properly and the work pieces do not disturb the airflow toward the hood opening.
A challenge for welders is to regularly reposition the ventilation hood so it is set correctly for each weld. If airflow rates around the plume drop much below 100 fpm then its effectiveness will decrease which allows increasing amounts of the fume to escape capture. As the welder progresses down a seam the local exhaust ventilation hood must be repositioned.
It is important to note that OSHA has specific guidelines for the use of ventilation, either dilution or local exhaust ventilation, in their welding standard. One of the most commonly applicable is 29 CFR 1910.252(c)(2)(i) which addresses welding in small production areas (low ceilings or small square footage) and 29 CFR 1910.252(c)(4) which addresses ventilation while welding in confined spaces. Refer to your state OSHA regulations if applicable.
Respiratory protection is likely the final option if local exhaust ventilation is not successful in reducing the welder’s exposure to below the applicable occupational exposure limits. Ensure a welder’s health is protected, such as through respirator use, until personal exposure monitoring confirms exposures are adequately reduced by local exhaust ventilation. If the local exhaust ventilation does not sufficiently control exposures then it would need to be supplemented by respirator use.
Kevin Sayler, CIH is a health and safety consultant who helps businesses increase profitability by reducing costs associated with workplace accidents, injuries and diseases. He specializes in the prevention of harmful exposures to employees. He can be reach at (360) 420-2985 or www.CascadeHealthSafety.com