The definition of a local exhaust ventilation (LEV) system is: an engineering control system to reduce exposures to airborne contaminants such as dust, mist, fume, vapour or gas in the workplace. Simply put it is something that sucks an airborne contaminant out of the workplace.

Supply air is admitted through outdoor air registers that may be fitted in window frames or in walls. There are self-regulating outdoor air registers that limit the air flow when the outdoor air temperature is very low. To enable the air to circulate inside the house, it must be able to flow through gaps at the bottom or top of doors.