Issue Date 3/2021
Do not use emergency purge mode except for when the hood sash is fully closed and there is a spill in the hood. Working at the hood while it is in purge mode creates an unsafe condition for laboratory staff because the higher velocity creates turbulence in the hood. Purge mode also overrides the energy efficiency features of the hood.
This feature exists in variable-air-volume hoods (like in Chemistry). This is because when you close the hood sash all the way, the volume of air exhausted per unit time is reduced for energy efficiency. In the case of a spill, you'd want to override that efficiency feature to provide maximum ventilation. Increasing the velocity of air across the open sash while working in the hood can actually result in LESS containment!