natural ventilation, and mechanical ventilation. In a process
known as infiltration, outdoor air flows into the house through
openings, joints, and cracks in walls, floors, and ceilings, and
around windows and doors. In natural ventilation, air moves
through opened windows and doors. Air movement associated with
infiltration and natural ventilation is caused by air temperature
differences between indoors and outdoors and by wind. Finally,
there are a number of mechanical ventilation devices, from
outdoor vented fans that intermittently remove air from a single
room, such as bathrooms and kitchen, to air handling systems that
use fans and duct work to continuously remove indoor air and
distribute filtered and conditioned outdoor air to strategic
points throughout the house. The rate at which outdoor air
replaces indoor air is described as the air exchange rate. When
there is little infiltration, natural ventilation, or mechanical
ventilation, the air exchange rate is low and pollutant levels
can increase.
