Ventilation can be mechanical or “natural”
(either passive or active).
The purpose of ventilation is to remove heat,
moisture, and contaminants or to reduce the
concentrations of contaminants.
Pressure differences drive ventilation in buildings.
Infiltration rates (ventilation through openings in
the building envelope) are heavily dependent on
pressure differences resulting from indoor-outdoor
temperature differences or wind induced positive
and negative pressurization of different sides of a
Stack effect results in relatively higher (positive)
pressure at the top and lower (negative) pressure
at the bottom of a building.
So, air leaks out of the top and in at the bottom.
Overall, the flows in and out through all pathways
and by all means must be balanced.
Within a building, pressure differences cause for
air (and, therefore, airborne contaminant) movement
from one location to another.
This is extremely important for the designer.
Knowing where pollutants will be generated or where
they will occur naturally and keeping such areas negatively
pressurized relative to the adjacent areas is important to
avoid unwanted distribution of the contaminants from one
location to another.
Locations of activities or equipment that will be strong sources
of contaminants should be provided with exhaust ventilation
or surrounded by spaces that are positively pressurized relative
to the source location. Sensitive areas should always be isolated
by pressure and fixed barriers where feasible