HVAC SYSTEM

Commercial and public buildings have been the targets of terrorist attacks in the United States and abroad. Terrorists have used high explosives to destroy or damage the World Trade Center, the Pentagon, and the Alfred P. Murrah Building; U.S. embassies in Dar es Salaam, Tanzania, and Nairobi, Kenya; and the Khobar Towers in Saudi Arabia.1 In October 2001, terrorists sent biological weapons, Bacillus anthracis spores, through the U.S. mail to news media companies and to U.S. Congressional offices. Workers in the former Brentwood Post Office (renamed the Curseen-Morris Processing and Distribution Center); the Trenton, New Jersey, regional mail processing center in Hamilton Township; the Hart Senate Office Building; the American Media Inc. (AMI) Building; and Rockefeller Center were exposed to infectious spores when contaminated letters were processed or opened. Following these exposures, 22 people became ill, 5 of whom died.2 More than 30,000 people are estimated to have received antibiotics as a result of possible exposure to anthrax spores.3 Hundreds of millions of dollars were spent in decontamination and restoration of the attacked buildings and on hardening security in U.S. postal facilities and mailrooms in high-profile buildings throughout the country. The prospect of biological attacks is a growing strategic threat.4,5 Covert aerosol attacks inside a building are of particular concern.6 Given the fact that many Americans spend a great deal of their lives in commercial buildings, it is worth examining whether practical actions can be taken to reduce risk to commercial building inhabitants from an aerosolized biological attack. To this end, the Center for Biosecurity of the University of Pittsburgh Medical Center (UPMC) convened a Working Group to determine what steps should be recommended to reduce the risk of exposure of building occupants after an aerosol release of a biological weapon. The Working Group was composed of subject matter experts in air filtration, building ventilation and pressurization, air conditioning and air distribution, biosecurity, building design and operation, building decontamination and restoration, economics, medicine, public health, and public policy. The Working Group focused on functions of the heating, ventilation, and air conditioning (HVAC) systems in commercial or public buildings that could reduce the risk of exposure to biological aerosols following biological attacks.
This Working Group report provides practical recommendations intended to reduce the risk of building inhabitants to biological hazards. These recommendations are focused primarily on the use of currently available technologies whose applications would be neither prohibitively expensive nor require major renovations or retrofit. The report also includes a brief overview of HVAC systems for those not trained in the science, design, construction, or operation of HVAC systems. This Working Group report draws extensively on the findings and judgments made in a number of important reviews and guidance documents.7–12
Working Group Method Working Group members from the Center for Biosecurity compiled and reviewed evidence and recommendations from (a) literature published from January 1966 to June 2005; (b) guidance documents written by professional engineering societies and/or government agencies on reduction of building vulnerability to terrorism and on improvement of indoor air quality; and (c) reports and interviews with experts on building security and indoor air quality. Based on this review, core concepts and principles were drafted. The Working Group was convened on June 13–14, 2005, to discuss and critique the initial draft of the concepts and principles. Following this meeting, a report was drafted that incorporated the Working Group’s oral and written suggestions. This draft was circulated to the Working Group for critique in October 2005. The Working Group’s critiques of this draft were incorporated into a second draft, which was circulated in December 2005. The final report incorporates the critiques of the Working Group. All named authors of this report are in accord with the recommendations. Some Working Group members participated as ex officio members; those members have no position on the recommendations. Working Group Presumptions For the purposes of the recommendations in this document, the Working Group agreed to presume the following: Improvements in the performance of HVAC systems that reduce occupant exposure to airborne particles in the range of 1–3 microns in diameter could potentially reduce exposure not only to weaponized infectious agents (the size range of Bacillus anthracis spores) but also to naturally occurring infectious agents and allergens of similar size.6,13 In the future promising new and evolving technologies, such as ultraviolet germicidal irradiation and electronic filtration, might be definitively shown to play an important role in reducing risk to building occupants from deleterious agents in indoor air. However, these technologies have yet to be independently evaluated using standardized methods. The absence of such data does not mean the technologies are ineffective; it means that there was no consensus in the Working Group on their performance characteristics, and, therefore, no recommendations about these technologies were made. New standards are being developed for many classes of devices. The Working Group could support their use in reducing the risk posed by bio-aerosols if their performance characteristics are documented by standardized methods. Aerosolized infectious particles fall out of suspension and settle on work surfaces, furniture, and floors; these particles also can stick to clothing and skin and present a risk from contact exposure. This type of risk would not necessarily be ameliorated by improved HVAC system functions. HVAC System Overview: Current Operations, Possible Changes HVAC systems are integral components of most commercial and public buildings.14 HVAC systems are intended to provide for the health, comfort, and safety of occupants by maintaining thermal and air quality conditions that are acceptable to the occupants15,16 through energy-efficient and cost-effective methods during normal conditions17 and, to the extent possible, to be responsive to hazardous exposures during extraordinary conditions.11 In principle, all HVAC systems have similar characteristics, but, in practice, they vary from a simple system serving a single thermostatic zone with a single air-handling unit to complex systems comprised of many air-handling units serving hundreds of thermostatic zones controlled by centralized energy management, lifesafety, and security systems. Furthermore, in some commercial and public buildings (e.g., hospitals), the HVAC systems must have the capability to remain operational in critical areas during emergency conditions. Buildings can be commissioned to ensure that building systems, including the HVAC system, are designed to function—and actually do function—according to specifications that address the preparedness and responsiveness requirements of the facility, including those of biological attacks. Building commissioning and re-commissioning are processes conducted by a team of experts and include design review, installation, performance testing, and balancing of systems according to intended design and applicable standards and codes.18 Well-commissioned buildings have efficient ventilation, pressurization, conditioning, and filtration functions. Air leakage into and out of buildings is low. These improved functions result in better-performing HVAC systems; the quality of indoor air improves, and operating costs are reduced (8–20%).18 Unfortunately, many buildings are neither commissioned nor re-commissioned. The design of an HVAC system is influenced by many factors, including but not limited to: the function, size, and configuration of the building; the selection of building materials and furnishings; construction methods; the budgets for HVAC capital equipment, maintenance, and operation; the air-quality requirements based on occupancy and use of the building; and the outside environment. Together, these factors determine the rates at which heat, water, and air contaminants have to be removed from the occupied spaces. Building renovations may change the initially designed airflow patterns and air supply route(s). The performance of an HVAC system is evaluated by two criteria: system capacity (i.e., size) and system control (i.e., regulation of rate changes). System capacity is determined by the HVAC system’s ability to provide sufficient heating, cooling, humidification, dehumidification, air dilution, and air cleaning to maintain the desired indoor conditions at design-specified ambient (i.e., likely peak environmental) conditions. System control is determined by the HVAC system’s ability to regulate the rates of these functions to maintain the desired indoor conditions during all ambient conditions.19 A typical HVAC system has three basic components, as shown in Figure 1.14 These components are: (a) outdoor air intake and air exhaust ducts and controls; (b) airhandling units (a system of fans, heating and cooling coils, air filters, controls, etc.); and (c) an air distribution system (air ducts, diffusers, and controls; return and exhaust air collectors; grilles and registers; return and exhaust air ducts and plenums).
HVAC systems perform multiple interdependent functions, including heating, humidification, cooling, dehumidification, ventilation, pressurization, and filtration/ cleaning. In most systems, several of these functions are performed simultaneously. These functions affect the occupants’ exposure to airborne contaminants, including aerosolized infectious agents. In the context of protection from biological attacks, HVAC systems can simultaneously perform three interdependent functions—ventilation, pressurization, and filtration—while providing the required temperature and humidity control. The Working Group considered each function to determine what changes to each might reduce the risks posed by biological attacks. The Working Group focused primarily on changes that might be made using currently available technologies.
Ventilation Ventilation is the process of supplying air to or removing air from a space to reduce contaminant levels and to optimize humidity and temperature of the air within the space.16 For commercial and public office buildings, ventilation is usually achieved by exhausting some of the return air (recaptured indoor air) to the outside environment, replacing it with outdoor air, and mixing the outdoor air with the portion of return air that is being recirculated. After this mixture is filtered, it is conditioned (i.e., heated or cooled, humidified or dehumidified), and delivered to the occupied space as supply air (Figure 1). Improvements in three aspects of supply air might reduce the indoor concentrations of particles (including infectious particles that would be released during an attack): (a) the rate of air exchange (delivery of supply air and exhaust of return air); (b) the airtightness of the return air system; and (c) the effectiveness of the filtration and air cleaning processes (described later). Rate of Air Exchange There are two types of air exchange rates in HVAC systems: the supply air exchange rate, which is primarily determined by the thermal loads in the spaces, and the outdoor air exchange rate, which is primarily determined by the floor area and maximum number of occupants. Both of these air exchange rates are important for ventilation control. If the particulate concentration in the outdoor air is lower than in the indoor air, higher outdoor air exchange rates reduce the indoor particle concentrations by exhausting more of the particle-laden return air and diluting the recirculated return air with cleaner outdoor air. Thus, the indoor particle concentrations are decreased by exhausting and by diluting the return air; this process is referred to as dilution ventilation.6 Dilution ventilation requires that the system have the capacity for conditioning increased amounts of the outdoor air. Use of higher exchange rates to maximize the effect of dilution ventilation has advantages and disadvantages. Control strategies for protecting against external and internal releases of biological agents are different. If the particle concentration in the outdoor air is less than the indoor air, a high rate of air exchange will reduce the concentrations of indoor particles. In this case, as the quantity of outdoor air intake is increased, more energy is needed to condition the outdoor air. These processes increase operating costs. If the outdoor concentration of particles (if the aerosol attack involved external releases of biological agents) is higher than indoor concentrations, an increase in outdoor air exchange rate will increase the particles in the supply air. Dilution ventilation will not offer protection against external releases unless special filtration and air cleaning at the outdoor air intake is employed (shown in Figure 1).20 HVAC systems typically vary supply air exchange rates to control temperature and outdoor air exchange rates based on outdoor air temperature, or to reduce carbon dioxide concentratiion (see Figure 1). In principle, some HVAC systems may be able to control the rates of outdoor and supply air exchange and perform dilution ventilation specifically in response to increased concentrations of indoor air particles. Detection of increases in indoor and/or outdoor particle concentrations requires the use of devices that measure particle concentration in the air and a control system that could either adjust air exchange rates accordingly or generate a warning signal to indicate the need for manual adjustment.11,21 Particle counters are commercially available and are used to measure particle counts in industrial clean rooms. Furthermore, particle counters can be gated so that when the concentration of indoor and/or outdoor air particles in a particular size range increases beyond a certain point, signals can be sent to control devices to modify air exchange rates in zones in which particulate concentrations have increased. Particle counters can measure the concentration of particles of a given size (1–3-micron range) but are not specific for biological material. However, application of particle counters in commercial and public buildings is not widespread, and published literature on performance characteristics in typical commercial settings is limited. Therefore, use of these devices in conjunction with dilution ventilation is not recommended at this time. The Return Air System The return air system removes a portion of the supply air from the occupied zones and returns this air to the airhandling units for exhaust or recirculation (Figure 1). One of two methods is used to return air to the HVAC system: the ducted return or the plenum return (the plenum is the space between the finished ceiling and the floor slab above). Ducted returns collect air from each room or zone using return air devices in the ceiling or walls of the occupied spaces that are directly connected by ductwork to the air-handling unit (Figure 2A). The plenum return collects air from several rooms or zones through return air devices that empty into the negatively pressurized plenum. The air collected in the plenum is then returned to the air-handling unit by ductwork or structural conduits (Figure 2B). The effectiveness of the return air system plays a key role in indoor air quality since the HVAC system can only exhaust, filter, or condition indoor air that is returned to the handling unit. Regardless of whether the HVAC system has a ducted return or a plenum return, increasing the seal integrity of the return air system and air-handling units (Figure 1) will help to ensure that more air is returned to and reconditioned by the air-handling unit. This can be accomplished by improving the seam seals, recaulking and replacing failed gaskets, and sealing unlined structural conduits. Because return plenums draw air from openings into building cavities, return plenums are more difficult to seal than ducted returns.