Design and Maintenance of Hospital Ventilation Systems and the Prevention of Airborne Healthcare-Associated Infections
Andrew J. Streifel
A building ventilation system is expected to supply air at a comfortable temperature and humidity level (1, 2 and 3). In the hospital setting, heating, ventilation, and air-conditioning (HVAC) systems must often provide specially conditioned air to protect the health of patients and staff. Certain patients (e.g., bone marrow transplant [BMT] recipients) are particularly vulnerable to infection from airborne pathogens (4). Others, such as tuberculosis patients, are potential sources of airborne infection, which may put those around them at risk. To design a proper hospital ventilation system, one must be familiar with both the physical and biologic characteristics of airborne agents causing healthcare-associated infections. Knowledge of ventilation strategies and equipment used to reduce the potential for airborne transmission of disease requires an understanding of airborne particle management for contamination control (5).
The science of aerobiology began with Louis Pasteur’s discoveries in the middle of the 19th century. By this time, investigators had made great strides in characterizing airborne flora and fauna and in developing methods for accurate quantitative sampling of these populations. During the 1930s, William Wells published on the infectious capacity of droplets and droplet nuclei. He also studied the air-sterilizing properties of ultraviolet (UV) light. By the 1960s, investigators were reporting on the airborne transmission of a variety of infections, including tuberculosis, influenza, smallpox, and measles. From particle science and fluid dynamics has evolved the study of bioaerosols, which quantitatively describes the generation and dispersal mechanisms that dictate the behavior of airborne microorganisms (6). By applying an understanding of these biologic and physical principles, the hospital can provide a ventilation system that can help protect against the spread of healthcare-associated or occupationally acquired airborne infection. Although these principles are well known, the challenge on a global scale is for the world’s medical facilities to keep up with the advances in medicine. These advances are producing mainstream treatments for life-threatening diseases such as leukemia or solid organ failure. Medical schools can deliver trained personnel to utilize advanced medical treatments, but the buildings where patients are being treated cannot keep up with the demands for specialized treatment areas.
BIOAEROSOLS AND INFECTION
For an object to remain airborne, it must be small enough so that the viscosity of the air impedes its fall in response to gravity. Lewis Stokes (7) developed an equation that predicts the falling velocity of a particle as a function of its diameter. Stokes’s law for determining the sedimentation velocity (Vs) of particles from 1 to 100 µm in diameter is as follows:
V = (2gr2)(d1 − d2)/9µ where V is the velocity of fall (cm sec-1), g is the acceleration of gravity (cm sec-2), r is the “equivalent” radius of particle (cm), d1 = density of particle (g cm-3), d2 = density of medium (g cm-3), and µ = viscosity of medium (dyne sec cm-3).
Gregory published a table of experimentally observed falling velocities for a number of microorganisms. It can be readily observed that many particles ranging in size from 1 to 5 µm have falling velocities in still air on the order of 1 yard an hour. Many spores, such as those of Aspergillus fumigatus, have roughened surfaces that tend to further enhance their buoyancy. Such particles can stay airborne almost indefinitely and can ride on air currents for thousands of miles from their point of origin (7) (Table 84-1).
The removal of these infectious particles is essential for ventilation efficacy. Newer concepts provide an understanding of the age of air (AOA) as a direct measure of ventilation performance. The principles concerning AOA (or the movement of particles around and eventually out of the rooms) are important for airborne infectious disease management.
Often, the focus for indoor air complaints is the inference that infections are caused by poor ventilation. We do, of course, want to ensure that ventilation is moving air approximately at design specification, because we also know that stagnant air and infectious aerosols prolong exposure potential and disease depending on
circumstances. This excuse of poor ventilation as a cause of healthcare-associated infection is a continuing challenge, and determining the AOA will be useful for proving or disproving ventilation issues (8). It is important to realize that if such small particles were entrained in a patient’s respiratory airstream, they would be of the size most likely to elude the cilial and mucosal defenses of the upper respiratory tract and to deposit in the alveoli of the deep lung (Fig. 84-1). Since the early 1970s, investigators have enhanced the understanding of the respiratory fate of small particles as a function of their Stokes diameter.
circumstances. This excuse of poor ventilation as a cause of healthcare-associated infection is a continuing challenge, and determining the AOA will be useful for proving or disproving ventilation issues (8). It is important to realize that if such small particles were entrained in a patient’s respiratory airstream, they would be of the size most likely to elude the cilial and mucosal defenses of the upper respiratory tract and to deposit in the alveoli of the deep lung (Fig. 84-1). Since the early 1970s, investigators have enhanced the understanding of the respiratory fate of small particles as a function of their Stokes diameter.
TABLE 84-1 ASHRAE Filtration Standard 52.2 | |||||||||||||||||||||||||
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Quantitative information about particles is as reliable as the measuring instrumentation. By knowing the airborne spore concentration in a given air body and the tidal volume of the lung, one can estimate the probability of inhaling a certain quantity of pathogenic material.
Riley and Nardell (9) used the concept of infectious dose in the form of quanta to predict the probability of infection from the release of infectious particles. Using ventilation for infection control, one can achieve protection, to a degree, before reaching a point of diminishing returns (10), especially for agents such as Mycobacterium tuberculosis. Reliable assessment of biologic risks from airborne pathogens is difficult because of the variables that are intrinsic to living systems. Two Aspergillus spores or influenza virus particles may have widely differing potentials for causing infection, depending on such factors as viability of the spores or particles and the health status of the person inhaling them. To determine control strategies for such agents, it is first necessary to estimate what constitutes an infective dose and then to determine what sort of ventilation control system will reduce concentrations of the suspected pathogen to a safe or noninfective level (11). The movement of the air is an important aspect of ventilation efficacy because particles <5 µm behave similarly to a gas. We can test such aspects of ventilation to find the optimal factors, such as air velocity, for ensuring air cleansing. Marshall utilizes the AOA concept for demonstrating ventilation efficacy in BMT rooms. This analysis of the air in various locations in the room will demonstrate the wellventilated versus the poorly ventilated areas. The poorly ventilated areas have higher AOA readings, and hence, higher concentrations of infectious particles.
 FIGURE 84-1 Upper and lower respiratory tract (URT and LRT) deposition of idealized spherical particles as a function of diameter. (From Rhame FS, Mazzarella M, Streifel AJ, et al. Evaluation of commercial air filters for fungal spore removal efficiency. Third International Conference on Healthcare-Associated Infections. Atlanta, GA, 1990, with permission.) |
GENERAL VENTILATION PRINCIPLES
Although air is a gaseous mixture containing nitrogen, oxygen, carbon dioxide, and a number of trace elements, it behaves in accordance with the principles of fluid dynamics. In descriptions of ventilation systems, air is treated as though it were a liquid flowing through the system. Air moves in response to pressure. For liquids, the most common source of pressure is gravity. For gases, the most common source of pressure is temperature. The global system of air movement is powered by the rays of the sun. In a building HVAC system, pressure is provided by fans and blowers that push or pull air through the building. The most basic rule of airflow in a duct system is that air in must equal air out (12). For any two points in a closed duct, A1V1 = A2V2, where A1 is the cross-sectional area (measured in square feet) and V1 is the air velocity (in feet per minute). A1 V1 gives the airflow in cubic feet per minute (cfm). This equation indicates that if the ducts contract (reducing A), air speed, V, must increase proportionally to maintain the same cfm flow rate.
The basic rule of air pressure is TP = VP − SP, where TP is the total pressure in the system, VP is the velocity pressure, and SP is the static pressure. Velocity pressure is measured in the direction of airflow and is directly proportional to V, the speed of the moving air. Velocity pressure is always positive. Static pressure is the pressure a body of air exerts on its container, and it can be measured in all directions. Static pressure may be either positive or
negative. It is pressure that tends to either burst (positive pressure) or collapse (negative pressure) the duct. If a body of air increases in speed, the velocity pressure increases, whereas the static pressure drops.
negative. It is pressure that tends to either burst (positive pressure) or collapse (negative pressure) the duct. If a body of air increases in speed, the velocity pressure increases, whereas the static pressure drops.
TP, the total pressure, may be either positive or negative and is the sum of the static and velocity pressure. As a body of air moves through a duct system in response to pressure generated by a fan, the total pressure in the system decreases because of frictional losses between the moving air body and the walls of its container, the duct system. This concept is illustrated by a third equation, TP1 = TP2 − HL, which states that, for a body of air moving from point 1 to point 2, the total pressures at the two points differ by the frictional losses (HL) caused by the intervening run of duct.
These three rules provide the conceptual framework within which ventilation systems are designed. In a simple recirculating model, the fan creates sufficient positive pressure to force air through the supply ductwork and sufficient negative pressure to draw the air out of the rooms into the return ductwork and back to the fan, completing the circuit. The pressure generated by the fan must be sufficient to overcome the energy losses created by friction between the moving air and the duct system through which it travels. The ductwork blows air into the various rooms through supply openings. The air circulates in the room and then moves toward return openings that draw air back into the return duct system with negative pressure (suction). With consideration for “ceiling real estate,” the careful placement of supply and return/exhaust ducts in a room will help optimize the efficacy of particle removal.
The supply and return openings in the room illustrate an important difference between positive and negative pressure ventilation. An individual with healthy lungs can easily blow out a candle at arm’s length. The same healthy lungs could not generate enough negative pressure, or suction, to cause the flame to even flicker (13). The supply duct is comparable to blowing out the candle, whereas the exhaust is attempting to suck it out. We refer to the strong directional flow of positively pressured supply air as “throw,” whereas the negatively pressured exhaust duct has a “capture velocity” (Fig. 84-2). The control of such a ventilation system is facilitated by a sealed room. A tight seal on the room allows air to enter and escape only through the ducted openings, thus avoiding room surface air leakage problems. Such measures help to maintain consistent control of the ventilation (14,15). An additional advantage of a tight room seal is to control sound transmission. Important to hospital patient rights is the concept of the Health Insurance Portability and Accountability Act (HIPAA), which ensures patient privacy. Air is the medium for sound transmission, and the tight seal of a room enhances privacy as well as ensuring ventilation efficacy. To what standard of seal should we adhere? The standard for a weathertight house promotes a leakage rate per square foot of surface area in a house at 2.5 in2/ft2. This weathertight standard of leakage can apply to hospitals but should be expressed as the leakage volume of air at a specified pressure such as 0.1 cfm/ft2 of surface in a patient room (15). By establishing this leak rate, it assures that the “make-up” air for the exhaust will, in fact, come from the supply rather than “holes” in the room. Many leak holes can provide sufficient air volumes to negate the effect of the offset between exhaust and supply air volumes. This can create confusion, because low pressure differentials can result in fluctuating airflow directions: for example, a room flipping from negative to positive airflow in and out of the room. Such confusion has spurred exposure investigations concerned with the lack of consistent airflow monitoring on infectious patients with diseases such as tuberculosis or disseminating varicella zoster.
 FIGURE 84-2 Basic difference between flow and pressure openings. |
HOSPITAL VENTILATION SYSTEMS
In designing an HVAC system for any occupied building, one must properly size ducts and fans to provide the proper air pressures and duct velocities to meet the ventilation requirements of the entire building. Properly sized heating and cooling equipment and noise reduction enter into the total calculations, as does some sort of filtration or air-cleaning system. As air recirculates in a building, it builds up an increasing load of gaseous contaminants that are not readily removed by filtration. It is necessary to exhaust a certain percentage of this stale air and replace it with fresh outdoor air to ensure occupant health and comfort (16). A wide variety of systems have been used to meet these criteria. A few of the more common types with an eye toward the needs of the hospital environment are considered below.
Energy management is a formidable challenge for building management in the future. Hospitals have among the highest utility costs per square foot of any industry. Strategies for controlling ventilation costs in climates where heating and cooling are extreme have shifted to energy-saving concepts. Displacement ventilation is one such concept (17). Air is delivered from the bottom (low part of room) and, as the air is warmed, will rise to be extracted from the room. What we use today is contrary to physical forces: forcing air down when the natural tendency is for air to rise. Although energy savings are immense when allowing air to be discharged into a room at a higher temperature, the lack of space and/or design in most patient rooms does not allow for its large-scale implementation yet.
Central Air-Conditioning System
This system brings in fresh outdoor air and mixes it with recirculated air. This air mixture is filtered and conditioned for temperature and humidity according to institutional
requirements and then distributed to all building locations. This system is favored for its low cost and simplicity. In a large hospital, the major drawback of centrally conditioned air is the difficulty in adapting it to the specific requirements of local areas, which may have differing heating and cooling needs. This is a particular problem in cold climates in which rooms along the exterior shell require warmer air than rooms in the core. Large central supply ducts, which reduce noise by slowing airflow, require large amounts of space. Efforts to create local or zone conditions with additional equipment, such as extra heating and cooling coils or booster fans, rapidly increase costs and are often only partially effective.
requirements and then distributed to all building locations. This system is favored for its low cost and simplicity. In a large hospital, the major drawback of centrally conditioned air is the difficulty in adapting it to the specific requirements of local areas, which may have differing heating and cooling needs. This is a particular problem in cold climates in which rooms along the exterior shell require warmer air than rooms in the core. Large central supply ducts, which reduce noise by slowing airflow, require large amounts of space. Efforts to create local or zone conditions with additional equipment, such as extra heating and cooling coils or booster fans, rapidly increase costs and are often only partially effective.
Dual Duct System
This system has a central system that separately produces two air streams, one hot and the other cold, which are then parallel-ducted throughout the building. Each room is provided with a mixing box in which the two air streams are blended. This allows individual thermostats and volume controls for each room. Although more expensive and difficult to install, this system can provide a number of microclimates without much add-on equipment. The principle drawbacks are the degree of care required in installing the system and the sound baffling required to reduce the noise created by faster airflows within the smaller ductwork. Other variations in the air-handling system may be unique to a regional climate condition that design engineers have considered in the ventilation specifications. This may be a factor for consideration when humidifying or dehumidifying the air.
The control of water in the air-handling system is paramount for controlling potential allergens and pathogens associated with the growth of microorganisms on fibrous insulation (18,19). There can be considerable air-handling system variation when designing for the climate. All designs, however, require careful maintenance and operational considerations for infection control. For example, a local fan coil system has often been used in hospital areas requiring supplemental cooling. Such climate control is often provided with local systems that recirculate ambient air and provide dehumidification and cooling. Such systems, although engineered for temperature control, do not accommodate airpurification control. The fan coil drain pans, if not properly maintained, become reservoirs for local fungal contamination (20). Air conditioners may also be reservoirs for fungal growth or accumulation (21,22). Such systems should be discouraged for areas in which immunocompromised patients are hospitalized. Recent outbreak investigations have demonstrated prolific mold growth on cold ducted systems, either on filters or associated with mixing boxes.
Filtration
Hospital HVAC systems are often required to perform additional tasks related to the prevention of healthcareassociated infections. By appropriate use of air-filtration technology, a hospital air-handling system can deliver air that is virtually particle-free to areas where such a level of protection is needed. The problem presented by such a rigorous filtration system is the energy cost involved. Most filters scrub the air by trapping particles in dense pleated media using impaction and interception for particles >1 µm and diffusion or electrostatic attraction for particles <1 µm. Dense filters impede the flow of air and cause a loss of system pressure. To maintain effective air velocities in the ductwork, a more powerful fan must be installed to overcome this pressure drop across the filter (23).
Filters are rated by their percentage of efficiency. A number of different test methods are used to rate air filters (24,25). Most common are the dioctyl phthalate (DOP) and dust spot tests. The DOP test challenges an air filter with an aerosol 0.3 µm in diameter. A light-scattering instrument downstream measures the penetration of the filter by these particles. A filter that can arrest 99.97% of the DOP particles is referred to as a high-efficiency particulate air (HEPA) filter. This method actually counts particles as a measurement of efficiency.
The dust spot test is used to rate less rigorous filters. This test uses atmospheric air or a defined dust as the challenge. Air upstream and downstream from the tested filter is drawn through filter paper. The samples are then compared for opacity using a photometer. Although not quantitative in evaluating particle reduction, this test measures the ability of a filter to reduce the dirt load of an air stream. Kuehn (26) and Rhame et al. (27) have shown that dust spot methods can measure high-efficiency removal of particles.
Effective hospital filtration systems that have been evaluated for air cleaning demonstrate the removal of particles at the 90% efficiency level for the removal of particles >0.5µm in diameter. Often filter system failure is associated with defective filter housing rather than filter media failure. Outdoor air is initially filtered through 20% to 40% efficient media, mixed with recirculating air, and sent through a 90% dust spot-efficient filter. These 90% filters have been demonstrated to provide nearly 100% efficiency in removing particles 1 to 5 µm in diameter with a lower pressure drop than when the 99.97% HEPA filters are used. Modern filters designed with larger surface areas can provide high-filter efficiency while maintaining relatively low pressure drops compared with previous versions of the HEPA filter. Distributing such clean air throughout the system provides an additional layer of safety to all occupants at risk for airborne pathogens. Then, where required, rooms or zones can be HEPA-filtered for a higher degree of protection. Modern filtration technology is creating minimal pressure drop filters featuring enhanced fiber electrostatic qualities and increased surface area of the filter. Although reduced resistance pressure while maintaining high-filter efficiency is beneficial for cost savings, careful consideration for proven long-term efficiency is necessary to prevent problems such as filter failure due to a shielded charge on synthetic filter fibers (Raynor et al. [28]). High-efficiency filter innovation certainly helps provide sufficient air volume to assist in maintaining essential air quality parameters in hospitals (which often become deficient in air-volume delivery and exhaust as the building ages). Such systems reduce risks created by opening and shutting doors and from transporting vulnerable patients for procedures that cannot be performed in specially protected areas. Filtration and room air exchanges continue to dominate priorities for air quality (29,30) in prevention of aspergillosis. However, the combination of appropriate ventilation parameters (filtration, air exchanges, and especially pressure management) helps to ensure control of the many sources of opportunistic filamentous fungal infections plaguing the immunocompromised host (31).
AREAS REQUIRING SPECIAL VENTILATION
Certain areas in the hospital have special ventilation systems as described in the HVAC handbook (2) and Facilities Guidelines Institute’s Guidelines for Design and Construction of Health Care Facilities (3), and now the ASHRAE Standard 170: Ventilation for Health Care Facilities (1). Air systems have been designed to meet these specific needs, most commonly, operating rooms, positive-pressure protective environments, negative-pressure isolation units, and local air control flow life islands (Table 84-2). Each of these situations has specific ventilation requirements related to the prevention of healthcare-associated infection or occupational exposure to airborne infectious diseases or medicated aerosols or gases. All operate on the underlying principle that clean air should move from less contaminated to more contaminated areas (clean to dirty airflow). To more clearly illustrate the principles involved, a specific patient, pathogen, or procedure is discussed here for each type of situation.
Protective Environments
Operating Room Surgery is by nature a process requiring invasive procedures that expose host tissues to the outside environment, creating the potential for exposure to external agents, such as bacteria and fungi. Therefore, in the operating room, the surgical site and instrument table should be considered the cleanest area, and infection control efforts should be directed toward providing protection through appropriate ventilation control.
Surgical site infection is a well-documented surgical complication (32). Procedural practices including aseptic technique and prophylactic antibiotics provide the first line of defense, but it has been shown that removing bacteria and fungi from operating room air helps to minimize infection (33,34). Microorganisms shed by humans are the most common airborne agents in a correctly designed operating room with appropriate air filtration (35). Large volumes of air filtered through high-efficiency filters should be provided from panels in the operating room ceiling over the surgical site. The downward force of air from the ceiling supply diffuser provides a focused ventilated area around the surgical site that is constantly washed by a high-volume flow of clean air. Such airflow moves particles away from the operating table toward the air returns at the margins of the room. It is important that this directional airflow of filtered air is delivered in such a manner that infectious particles shed by the operating team are swept away toward the return ducts and not trapped and recirculated within the vicinity of the procedure. The more objects there are that interrupt the airflow pattern, the greater the turbulence will be. Special clean room laminar flow ventilation with HEPA filtration has been used in orthopedic cases to prevent the consequences of surgical site infections. A vertical flow system designed to provide a downward flow of air over the surgical site actually increases the air exchanges in the cleanest zone (36,37). Air delivery from a horizontal direction does not provide an extra benefit, because personnel and equipment in the way of the directed airflow cause turbulence and potential trajectory of problematic particles toward the surgical site. Vertical flow is preferred over horizontal airflow for space management and infection control considerations. Memarzadeh and Manning (38) performed computational fluid dynamic studies that reinforced the empirical findings of Lidwell (37) that a vertical flow with velocity from 30 to 35 linear feet per minute (lfpm) (0.15-0.18 m/s) could be achieved at the surgical site. If air supply can provide a laminar flow regimen albeit at a lower velocity than the official definition of laminar flow of 90 lfpm (0.45 m/s), control of the shed particles over the surgical site is realistic. In addition, AOA evaluations help establish (independent of test and balance report) the degree of ventilation efficacy. This can be conducted to check air transit times of any common gas, such as carbon dioxide, from which we can ascertain the time that it takes for a particle (<5 µm) to move from the source to the extractor vents in the respective rooms. Readings of 3 to 5 ft/s indicate satisfactory air movement (8).
TABLE 84-2 Summary of Special-Ventilation Hospital Areas | ||||||||||||||||||||||||||||||||||||
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