Pollutant Pathways and Driving Forces

Identifying Pollutant Pathways

Airflow patterns in buildings result from the combined action of mechanical ventilation systems, human activity, and natural forces. Pressure differentials created by these forces move airborne contaminants from areas of relatively higher pressure to areas of relatively lower pressure through any available openings.

The HVAC system is generally the predominant pathway and driving force of indoor air movement in buildings. However, all of a building’s components (walls, ceilings, floors, penetrations, HVAC equipment, and occupants) interact to affect the distribution of contaminants.

Natural forces exert an important influence on air movement between zones and between the building’s interior and exterior. Both the stack effect and wind can overpower a building’s mechanical system and disrupt air circulation and ventilation, especially if the building envelope is leaky. This can lead to indoor air quality problems.

Stack effect is the pressure driven flow produced by convection (the tendency of warm air to rise). The stack effect exists whenever there is an indoor-outdoor temperature difference and becomes stronger as the temperature difference increases. As heated air escapes from upper levels of the building, indoor air moves from lower to upper floors, and replacement outdoor air is drawn into opening at the lower levels of buildings. Stack effect airflow can transport indoor air quality contaminants between floors by way of stairwells, elevator shafts, utility chases, or other openings.

Wind effect is transient, creating local areas of high pressure (on the windward side) and low pressure (on the leeward side) of buildings. Depending on the leakage openings in the building exterior, wind can affect the pressure relationships within and between rooms. This can allow an indoor air quality contaminant to move easily between rooms.

The basic principle of air movement from areas of relatively higher pressure to areas of relatively lower pressure can produce many patterns of contaminant distribution including:

  • local circulation in the room containing the pollutant source
  • air movement into adjacent spaces that are under a lower pressure
  • recirculation of air within the zone containing the pollutant source
  • movement from lower to upper levels of the building
  • air movement into the building through infiltration of outdoor air or reentry of exhaust air

The interaction between pollutant pathways and intermittent or variable driving forces can lead to a single source causing indoor air quality complaints in areas of the building that are distant from each other and from the source. Hence, indoor air quality complaints can be heard in one area of a building that is far removed from the actual source of the indoor air quality contaminant.