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Protecting Building Occupants from Exposure to Biological Threats

HVAC System Basics

To fully appreciate building occupants’ risk of exposure to airborne biological threats and what can be done to reduce it, it is necessary to understand how air enters and moves through buildings.


Building ventilation is the process of bringing outdoor air into a building, circulating it, and later purging it to the environment.17 The main purpose of ventilation is to provide acceptable indoor air quality by diluting and removing contaminants from the indoor air.17,18 Building ventilation is achieved by natural or mechanical means, or by a combination of the two.18

Natural ventilation: Natural ventilation is the use of differences in air pressure that exist between the inside of a building relative to the outside of it, across the building envelope, to ventilate a building.18,19 These air pressure differences are created by natural forces such as wind and temperature. Air moves into and out of naturally ventilated buildings through windows, doors, vents and other openings incorporated into the building design and via infiltration/exfiltration.

Mechanical ventilation: Mechanical ventilation is the use of mechanical air handling systems—commonly referred to as heating, ventilation, and air conditioning (HVAC) systems—to ventilate buildings.17,18 Most commercial buildings use mechanical ventilation, which is more controllable and responsive than natural ventilation in providing adequate indoor air quality. However, mechanical ventilation also can exacerbate infiltration/exfiltration, which can compromise indoor air quality.

Purpose, Design Basics, and Function of HVAC Systems

Purpose: An HVAC system provides adequate indoor air quality by: conditioning the air in the occupied space of a building in order to provide for the comfort of its occupants; diluting and removing contaminants from indoor air through ventilation; and providing proper building pressurization.[17]

Design: While there are many different HVAC system designs and operational approaches to achieving proper system functionality, and every building is unique in its design and operation, HVAC systems generally share a few basic design elements (figure 1):

  • Outside air intake
  • Air handling unit—a system of fans, heating and cooling coils, air-flow control dampers, air filters, etc.
  • Air distribution system
  • Air exhaust system.17

Function: In general, outside (“supply”) air is drawn into a building’s HVAC system through the air intake by the air handling unit (AHU). Once in the system, supply air is filtered to remove particulate matter (mold, allergens, dust), heated or cooled, and then circulated throughout the building via the air distribution system, which is typically a system of supply ducts and registers.17

In many buildings, the air distribution system also includes a return air system so that conditioned supply air is returned to the AHU (“return air”) where it is mixed with supply air, re-filtered, re-conditioned, and re-circulated throughout the building. This is usually accomplished by drawing air from the occupied space and returning it to the AHU by: (1) ducted returns, wherein air is collected from each room or zone using return air devices in the ceiling or walls that are directly connected by ductwork to the air-handling unit; or (2) plenum returns, wherein air is collected from several rooms or zones through return air devices that empty into the negatively pressurized ceiling plenum (the space between the drop ceiling and the real ceiling); the air is then returned to the air-handling unit by ductwork or structural conduits.1,17

Finally, some portion of the air within is exhausted from the building. The air exhaust system might be directly connected to the AHU and/or may stand-alone. 

Sample HVAC System. Enlarge.

hvac basics small

Building Pressurization

Building pressurization refers to the air pressure relationships that exist between the inside of a building relative to the outside of a building across the building envelope; it also refers to the pressure relationships that exist within different parts of a building relative to each other.[15,17] Building pressurization is used to limit infiltration, which can lead to indoor air quality problems because air that enters a building via infiltration bypasses the air handling systems and can introduce contaminants into a building and contribute to moisture problems.[15,31] Pressurization also can be used to control the movement of air contaminants within a building.[15,31]

Maintaining positive air pressure relative to outside air prevents contaminants in the outside air from entering a building by means of infiltration.15,31 Such an approach could reduce the risk of exposure to biological agents from a large-scale outdoor release, provided that air entering the building through the HVAC system is sufficiently filtered to remove contaminants. Whether a building can be pressurized depends upon the building’s geometry, HVAC system design, and envelope tightness as well as weather conditions. Building pressurization requires that the HVAC system be able to deliver more air to the occupied space than is being exhausted and lost due to exfiltration. It may not be possible to pressurize a leaky building without first addressing envelope leakage. 

Maintaining positive air pressure in one zone of a building relative to another can limit the distribution of an aerosolized biological agent released within that building by means of airflows created by pressure relationships that exist within different parts of the building relative to each other.[15,31] Such an approach can be used to isolate special-use spaces such as lobbies, parking garages, and mail rooms that may be more vulnerable to an internal release by maintaining them at negative pressure relative to adjacent parts of the building.

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*Note: The information that appears on the pages collectively known as "Protecting Building Occupants" was up-to-date and accurate when published in 2008; the materials have not been updated since original publication. Please be sure to check current resources for the most up-to-date information on this topic.



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