Issue link: https://hi.iaq.net/i/630729
10 Section 2 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 openings at the lower levels of buildings. Stack effect airflow can transport contami- nants between floors by way of stairwells, elevator shafts, utility chases, or other openings. Wind effects are 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. 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 lower pressure (Note: Even if two rooms are both under positive pressure compared to the outdoors, one room is usually at a lower pressure than the other.) ■ recirculation of air within the zone containing the pollutant source or in adjacent zones where return systems overlap ■ movement from lower to upper levels of the building ■ air movement into the building through either infiltration of outdoor air or reentry of exhaust air Air moves from areas of higher pressure to areas of lower pressure through any available openings. A small crack or hole can admit significant amounts of air if the pressure differentials are high enough (which may be very difficult to assess.) Even when the building as a whole is maintained under positive pressure, there is always some location (for example, the outdoor air intake) that is under negative pressure relative to the outdoors. Entry of contaminants may be intermittent, occur- ring only when the wind blows from the direction of the pollutant source. The interaction between pollutant pathways and intermittent or variable driving forces can lead to a single source causing IAQ complaints in areas of the building that are distant from each other and from the source. BUILDING OCCUPANTS The term "building occupants" is generally used in this document to describe people who spend extended time periods (e.g., a full workday) in the building. Clients and visitors are also occupants; they may have different tolerances and expectations from those who spend their entire workdays in the building, and are likely to be more sensitive to odors. Groups that may be particularly suscep- tible to effects of indoor air contaminants include, but are not limited to: ■ allergic or asthmatic individuals ■ people with respiratory disease ■ people whose immune systems are suppressed due to chemotherapy, radiation therapy, disease, or other causes ■ contact lens wearers Some other groups are particularly vulnerable to exposures of certain pollutants or pollutant mixtures. For example, people with heart disease may be more affected by exposure at lower levels of carbon monoxide than healthy individuals. Children exposed to environ- mental tobacco smoke have been shown to be at higher risk of respiratory illnesses and those exposed to nitrogen dioxide have been shown to be at higher risk from respiratory infections. 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.