Exhaust fans may be running in your buildings to remove smoke, fumes, and contaminated
air. If you dont have a make-up air system - or a large enough make-up air system -
you have a negative air pressure condition.
Other building conditions such as temperature gradients can cause this phenomenon.
Negative air pressure means that air is drawn into the building to replace the
air removed by exhaust systems or other building conditions.
Negative air pressure creates many problems:
Drafts at the floor level
Back drafting down furnace stacks
Cold spots and wind chills in working areas
Exhaust fans starting to operate inefficiently
Strong air inrush when a door is opened..
This inrush of air brings with it fumes, insects and the outside climate. Make up air fans are used to offset negative air pressure.