Building environment

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 
building. 
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