Local exhaust ventilation (LEV)

LEV works on the principle that it is easier to remove a contaminant where it is released than to dilute it afterwards. Examples include a slot hood at a welding bench, a downdraft table for sanding, a snorkel arm at a mixing point and a hood over a tank. The closer the capture hood is to the source, the better it works.

Design matters. The hood must be shaped and positioned so the air movement draws contaminant away from, not across, the face of the worker. Cross-drafts from doors, fans or foot traffic can disrupt capture. The fan must supply enough airflow for the process, the ducts should be leak-free, and the discharge should be placed so that exhausted air does not reenter the building.

LEV needs to be inspected and tested regularly. Typical checks include looking for damage, measuring airflow or pressure at set points, confirming that gauges still read in the correct range and cleaning or replacing filters. A system that has fallen into poor condition can give a false sense of safety.

LEV is a form of Engineering controls and is preferred over relying on a Respirator. It is different from a Chemical fume hood, which is an enclosure, and from general ventilation, which dilutes contaminants throughout a room. Exposure measurements compared with limits such as the Permissible exposure limit (PEL) show whether the system is effective.