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! # % & % ( ) +,. / / &,7 1!! 8,,,,7!! +, 5! 7 9 Engineering control of respiratory infection and low energy design of healthcare facilities YuguoLi 1, JulianTang 2,3, CatherineNoakes
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! # % & % ( ) +,. / / &,7 1!! 8,,,,7!! +, 5! 7 9 Engineering control of respiratory infection and low energy design of healthcare facilities YuguoLi 1, JulianTang 2,3, CatherineNoakes 4,*, MichaelJHodgson 5, 1. Department ofmechanicalengineering, UniversityofHongKong, Pokfulam, HongKong 2. LeicesterRoyal Infirmary, Leicester, UnitedKingdom 3. AlbertaProvincialLaboratoryforPublicHealth, Edmonton,Alberta, Canada 4.PathogenControlEngineeringInstitute, SchoolofCivilEngineering,Woodhouse Lane, Universityof Leeds, Leeds LS2 9JT, United Kingdom 5. Office ofoccupationalmedicine,occupational Safetyand HealthAdministration,Departmentof Labor, USA Corresponding author: Tel: , Fax: ABSTRACT Indoor microorganism and infection have become an emerging direction in indoor air quality research science. Airborne droplet nuclei can serve as carriers of respiratory infectious diseases. The study of expiratory droplets and their exposure control has received particular attention since the 2003 SARS epidemics and the 2009 influenza pandemics. Little is known about how effective the commonly-used indoor environment control strategies are for infection control. Significant questions also exist on what are the ventilation requirements for airborne infection control. There is a broad range of relevant important issues including the exposure risk, and effective control methods in various indoor settings such as hospitals, homes, schools and offices. What is known is that the minimum required ventilation rate for infection control in hospitals can be much higher than the general health and comfort requirement in homes and offices. This has resulted in significant energy efficiency issues in healthcare facilities. This review considers the current knowledge on airborne transmission of infection and the potential implications of a move to low energy design, particularly in hospitals, on the risk of infection. The review outlines active research and development on reducing hospital energy use while improving infection control and discusses the potential for conducting clinical trials to gain the necessary evidence to support changes in hospital ventilation design. 2 CURRENT PRACTICE AND GUIDELINES IN INFECTION CONTROL About 7% of total annual worldwide deaths, i.e. 4 million people, are due to viral respiratory infections (WHO, 2004). Among them, influenza is a serious threat; influenza epidemics occur worldwide each year and the 2009 avian flu and H7N9 outbreaks have heightened awareness of the risk posed by new strains of the virus. With many serious respiratory diseases associated with transmission in high occupancy indoor environments, the design of the environment and particularly the ventilation is understandably a key feature of infection control in healthcare settings worldwide. However with a global move to reduce the energy consumption of all buildings through measures such as improving airtightness to reduce uncontrolled infiltration, ventilation of healthcare buildings has come under scrutiny and questions are being raised as to whether such high ventilation rates are indeed necessary. It is imperative to consider how a change in healthcare building design policies may impact on infection transmission. To discuss the implication of low energy design in healthcare facilities on infection control, it is useful to review first the current knowledge, practice and guidelines in infection control. Microorganisms are believed to be transmitted from person to person or between animals and people through a number of mechanisms including physical contact, large droplets, or through fine aerosols. Infection control guidelines worldwide for all approaches are typically based on a three-stage hierarchy(cdc 2005): Administrative control; Environmental control; Personal protective equipment. In the case of transmission by aerosols and droplets, identification and separation of infectious patients remains the first line of defence(cdc 2005), however environmental control approaches and application of personal protective equipment have a particularly important role to play depending on the disease and its transmission. The distinction between airborne and droplet transmission is based on assumed behavior of different sized particles in air. The designation of fine particulates as those with a diameter 5 µm, those 10 µm as large particles or droplets, has long been the basis of fundamental infection control strategies; as developed by the Association of Professionals for Infection control (APIC 2013), Centers for Disease Control (CDC) and World Health Organization(WHO 2007). Simplistically, droplet precautions recommend wearing surgical masks within 6 feet (2m) of patients with an infection. The basis for this approach is the theory of settling times by particle size; large droplets will rapidly deposit on surfaces in the vicinity of the infectious source. Traditionally, patients at risk for transmitting disease through large particles have not been placed in respiratory isolation. On the other hand, patients with infections believed to be transmitted by fine aerosols through the airborne route have traditionally required far more intense protection strategies, including early identification, isolation into an airborne infection isolation room with specialist ventilation, and the use of a fit tested respiratorbystaffin such aroom. This distinction remains the mainstay of infection control, and ASHRAE has recently developed a position paper outlining overall engineering concerns and summarizing implications for engineers of many of the infectious diseases transmitted by airborne route and by large droplets (ASHRAE, 2013). ASHRAE fundamentally relies on cognizant authorities in the development of its formal positions; the importance here being the implications for control strategies. Only in healthcare have control strategies explicitly considered infections in ventilation requirements, with ahierarchy of measures recommended by guidance. Local exhaust ventilation can facilitate source control in high risk procedures, pressurization minimizes transport of airborne microorganisms and other pollutants between spaces, a high ventilation rate dilutes and removes airborne microorganisms and well defined ventilation patterns can minimize transfer within a space. Air cleaning through filtration or other means can provide local control and enable recirculation of air. Guidance on these approaches is set out in a range of standards and guidance including ANSI/ASHRAE/ASHE Standard170 (2013), FGI Guidelines for Design and Construction of Hospital and Outpatient Facilities (2014) and various CDC guidance including those on TB(2005) in the US and Health Technical Memorandum (Department of Health, 2007) in the UK. For example all these standards stipulate that patient and ward areas should have higher than average ventilation rates to minimize opportunist transmission from undiagnosed sources, while high risk areas such as operating rooms and isolation rooms have ventilation rates in excess of 10 ACH. Other buildings such as schools, public access buildings, and offices have mostly ignored any consideration of infectious hazards, although even here there is increasing awareness of the relationship between the building and health, and filtration, dedicated exhaust to the outside and ultraviolet germicidal irradiation(uvgi) have been explored in some studies. Nevertheless, in practice, this distinction between fine and large particles as separate modes of transmission is becoming ever harder to maintain. Ten years ago, a review of the topic in a premier journal in medicine suggested that although there were clearly agents that relied on only one or only the other mode of transmission, i.e., large or small particles, there was a third group, organisms that were facultative users of both (Roy and Milton 2004). Shortly thereafter, a CDC review suggested that a common agent, influenza A, might fit that description and that the critical issue was then not whether airborne, droplet, or both but in what proportion transmission occurred (Tellier 2006). The 4 issue has smoldered for many years in other common viral illnesses, including even the common cold, where despite widespread beliefs to the contrary, experimental arrays show airborne transmission (Dick et al 1987). Recently, Nielsen and Li have modeled particle movement over short distances and suggest that airborne transmission of infectious diseases over short range might in fact follow the same pattern as large droplet transmission (Liu et al 2010, Li 2011) Theproblem has cometo ahead withtheincreasein international travel andthethreat ofpandemicmigration,no less important because the degree and importance of airborne transmission of influenza and the risks posed by new strains remains unknown. There is also concern over other infections, particularly virulent or emerging diseases. It is long established that smallpox, a now eradicated disease, was transmitted through the airborne route (Milton 2012). In fact, careful review of older literature has raised questions about even hemorrhagic fever cases resulting from airborne transmission (Carey et al 1972; Roels et al 1995), a phenomenon reinforcing the current biosafety laboratory precaution recommendations. The 2003 SARs outbreak demonstrated only too well the challenge of dealing with a global outbreak of a new disease and the importance the environment can play in transmission (Li et al 2005). Preventing airborne transmission in pandemic settings and ensuring infrastructure is sufficiently robust to deal with current and new threats requires far more intense approaches and represents a growing challenge for engineers. EXPOSURE TO EXPIRATORY DROPLETS AND RESPIRATORY INFECTION Although the relative importance of large versus fine particle transmission drives protection strategies in usual and pandemic settings, little research has explored the content of those droplets and the implications for transmission. The source represents the primary factor in transmission, although others modify the transmission process. These factors may be grouped as follows: The infector and source (expiratory droplet number and size, respiratory activity, virus concentration, social contact), The environment(air/surface temperature, moisture, contamination, air flow pattern, ventilation rate, usage ward, consulting room, waiting room, isolation room, operating theatre etc.), The virus (survival, site of infection), The susceptible(immunity, age, social contact) etc. While controlling the source is desirable, it is challenging particularly in health care facilities (Menzies 2000). Approaches such as masking infectious patients while they are moved within a hospital building(cdc, 2009) and applying local exhaust ventilation in certain respiratory procedures are advocated. However in the wider hospital environment controlling transmission focuses on the transmission route, and the ventilation and building design becomes a major factor. With the growing threat of pandemics, since the early 2000s, from SARS, H5N1, and H1N1, engineering control strategies become not only relevant but of growing importance in defining the environment factor in the transmission process. Understanding the mechanism of exposure is important to develop and understand an effective control strategy. For example, ventilation intervention decreases airborne transmission by extracting, directing the flow of airborne infectious agents away from susceptible persons and/or by diluting and removing airborne infectious agents from room air. If airborne transmission represents more of a hazard than previously considered, general dilution ventilation and approaches such as air cleaning represent more important aspects of ventilation for public health than previously considered. It also follows that a reduction in ventilation rates, as may be desired in low energy healthcare buildings, potentially poses an increasing hazard. However this is only part of the picture. Ventilation impacts particles that remain airborne, but the influence on droplet transmission or deposited particles and the subsequent exposure of susceptible persons to infectious agents through contact exposure is unclear. There is some evidence that air cleaning approaches reduce surface contamination but the impact on infection risk and the effectiveness of this as an intervention are largely unknown. The uncertainty of the airborne route for many respiratory diseases remains a crucial bottleneck of using ventilation as a community intervention measure. In the case of influenza, the US$ 8 million project Evaluating Modes of Influenza Transmission (EMIT) is one of the most recent studies that may offer the first evidential result soon (EMIT 2013). However such studies are complex and challenging, and in most cases evidence has to be derived from studies exploring specific aspects of the transmission process. More than 10 studies explore the size distribution of expiratory droplets dueto coughing,sneezing, speaking and talking(e.g. Morawska et al 2009; Chao et al 2009; Xie et al2009). Thedata byduguid (1946)has beenwidelyused inrisk assessment modelsintheliterature(e.g. Nicas et al 2005; Yang and Marr 2011). Substantial uncertainty and differences exist in these measurements, possibly due to individual inhomogeneity, instrumentation, and factors such as study control. Moreover, these studies generally characterize 6 healthy volunteers, so while they yield valuable information on potential for human dispersion of particles, infection risks can onlybeinferredbyextrapolation.survival of viruses andbacteria has beenstudied as afunction ofrhand temperature in air (Harper 1961; Yang and Marr 2011) and on surfaces (Nicas and Jones 2009; Weber and Stilianakis 2008) yet it can be hardtolinkthisdatato infection riskinhumans. More recently researchers have explored the implications of specific viral content at various particle sizes. In a small study of patients presenting with influenza-like-illness, i.e., influenza A or B virus confirmed by rapid test, Fabian et al (2008) identified viral particles in the coughed secretions of three(60%) of the five patients infected with influenza A virus and one(14%) of the seven infected with influenza B virus. Exhaled influenza virus RNA generation rates ranged from 3.2 to20influenza virus RNAparticles perminute. Over 87% of particles exhaledwereunder1micronin diameter, with unambiguous airborne potential. Bishoff et al (2013) recently studied patients with influenza-like illness to measure viral particles at m (1 foot), m (3 feet), and m(6 feet) from the patient's head. Twenty-six patients (43%) released influenza virus into room air, with 5 (19%) emitting up to 32 times more virus than others. Emitters surpassed the airborne 50% human infectious dose of influenza virus at all sample locations. The primarily small influenza virus particles (diameter, 4.7 µm) showed decreasing concentrations with increasing distance from the patient's head (P .05). The authors questioned the current paradigm of localized droplet transmission during non-aerosol-generating procedures NIOSH researchers (Noti 2012) studied influenza generation in a simulated patient room and recovered particles in all aerosol fractions (5.0% in 4 m aerodynamic diameter, 75.5% in 1-4 m, and 19.5% in 1 m; n = 5). They demonstrated that tightly sealed masks (glue, not so practical in living workers) to the face blocked entry of 94.5% of total virus and 94.8%ofinfectious virus (n =3).Atightlysealedrespiratorblocked 99.8% oftotal virus and 99.6% of infectious virus (n = 3). Apoorlyfitted respirator blocked 64.5% oftotal virus and66.5%ofinfectious virus(n =3). A mask documented to be loosely fitting by a PortaCount fit tester, to simulate how masks are worn by healthcare workers, blocked entryof 68.5%of totalvirus and 56.6%ofinfectious virus (n = 2). Milton(2013) studied the effect of placing surgical masks on patients to capture influenza virus in large droplet spray. Fine particles contained 8.8 (95% CI 4.1 to 19) fold more viral copies than did coarse particles, supporting Bischoff et al (2013). Surgical masks reduced viral copynumbers in thefinefraction by2.8 fold (95%CI1.5 to 5.2) andin the coarsefractionby25 fold (95%CI 3.5 to 180). Overall, masks produced a3.4 fold (95% CI 1.8 to 6.3)reductionin viral aerosol shedding. Finally, Lindsley et al (2010) showed that such airborne particles, of influenza A, influenza B, and RSV, migrated from an urgent care clinic throughout a health care facility. These results supported the possibility that influenza and RSV can be transmitted by the airborne route and suggest that further investigation of the potential of these particles to transmit infection is important. The bottom line from these recent particle size and content studies is that fine particles contribute substantially to the airborne particle load especially in the facultative airborne viruses; however the contribution to the burden of disease remains unclear. No arrays of studies are under way to develop predictive tests on which aspect is more important and therefore which transmission prevention strategy should be primary. To assess the impact and magnitude of such risks, engineers will need to engage and work with a multi-disciplinary team of physicians and hospital infection control teams (HICTs) to make such an assessment. REDUCING HEALTHCARE BUILDING ENERGY USE WHILE MAINTAINING OR IMPROVING EFFECTIVE AIRBORNE INFECTION CONTROL Given the current climate, it is clear that low-energy considerations will eventually need to be applied to hospitals. So, on what basis should there be concern about the risks of increases in airborne hospital-acquired infections(hais), if we change design and introduce new technology to save energy? Is there any risk at all? Although not systematically investigated in a controlled manner, there are a large number of reviews and case reports that demonstrate that an effective ventilation system, working at a minimum number of ACHs is required for both thermal comfort and good airbornehygienelevels in hospitals (Li et al 2007; Eames et al 2009; Aliabadi et al 2011). Howeverin ahospital environment it is not simplytheventilationalonethat adds to theenergycost. As well as higher ventilation rates, infection control generally leads to a higher usage of related environmental control strategies such as use of HEPA filters which add pressure losses and higher fan power, greater water quality control (for legionella), 8 more control of moisture level due to the impact of moisture content on survival of pathogens etc(ashrae, 2013). The question is, is it possible to develop low-energy design without compromising infection control? To answer this question, we need to examine different low energy design strategies. It may be useful to categorize low energy design strategies into three categories. Category A - those with direct impact on infection control (such as reducing ventilation rate, air disinfection, use of natural ventilation; see WHO 2009) Category B- those with indirect impact on infection control (such as use of displacement ventilation, use of separate dehumidification control system, use of the liquid desiccant dehumidification systems etc.) Category C- those with no impact on infection control (such as good housekeeping measures like turning off lights when not in use, using renewable or CHP energy sources or installing energy efficient control systems such as variable speed drives). Within all categories, good design and maintenance of any technology is the first step in ensuring energy efficiency. For example a well constructed isolation room with a good level of airtightness, well designed airflow pattern and pressurization and properly commissioned ventilation and control system is likely to require less energy to operate th
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