Outdoor air enters a home or building in three ways:
- Natural Ventilation: When air enters through open windows and doors.
- Infiltration: Outdoor air can flow through openings, joints, and cracks in walls, floors, ceilings, and around windows and doors.
- Mechanical: When air enters through heating, ventilation, air conditioning, or the HVAC system. Sometimes, outdoor-vented fans can withdraw air from a single room, such as a bathroom and kitchen.