Effective Air Hygiene System Design for Airborne Contagion Exposure Control in Health Care Facilities - Experimental and Numerical Studies

針對醫療建築中基於空氣傳播污染物接觸控制的有效潔淨空調系統設計 - 實驗和數值研究

Student thesis: Doctoral Thesis

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Awarding Institution
Supervisors/Advisors
Award date2 Nov 2018

Abstract

The air we breathe is incompletely free of airborne contagion as no biologically clean air exists. In most cases, the microbes in the general environment are insufficient to cause a response (infection or disease) in people, due to the protections by their immune systems. As a result, the goal of air hygiene system design is to provide an acceptable air quality as against pure air. Thus, air hygiene systems remain an essential component of maintaining adequate thermal comfort and indoor air quality (IAQ). Nevertheless, it consumes over 60% of the total building energy. Unlike other types of non-residential buildings, health care facilities are expected to serve as a healing centre for the patients while providing a healthy and comfortable environment for the caregivers and the visitors. Contrastingly, the healthcare environments are often found in cases of nosocomial infections. Hence, in addition to regulating temperature, humidity and airflow gradients, the ventilation systems in health care facilities are tasked with dilution and removal of fugitive gases and airborne pathogens. Infection transmission involves a complex interaction among the infectious agent(s), susceptible host(s) and the environment. It remains imperative to minimise the quantity of infectious agent(s) at the breathing zone of the susceptible occupants with effective air hygiene systems. Regardless, finding the right balance between the emitted pathogens, environmental conditions and exposure control in the healthcare facilities remains a challenge.

Exposure risk to airborne contagion is dependent on emission source strength and exposure limits. Thus, human exposure risk assessment requires proper quantification of the critical components: source strength, environmental parameters and target concentrations. Nonetheless, determining the virulence of infectious agents remain herculean task. As a result, existing studies are highly prescriptive-based due to lack of reliable target concentrations for many airborne contagions. Consequentially, the conventional approach is to use surrogates in the form of tracer gas, smoke, non-biological particles, and benign microbes. Although these surrogates can give a general idea on risk characterisation of different intervention and conditions, they are limited in that little semblance exists between the surrogates and the biological agents. This study bridges the gap by employing the benchmark dose (BMD) approach to develop quanta infective concentration (QiC) and reference concentration (RfC) for airborne contagion. The method is applied to natural person-to-person influenza epidemiology cases. At benchmark references of 63.2% and 10%, the QiC and RfC were computed as 68.6 pfu/m3 and 0.0796 pfu/m3 respectively. Applying QiC as emission source strength and RfC as target concentration, the study investigated the performance of air hygiene system in a bay-designed multibed patient ward mockup. Series of experiments were conducted using Taguchi Robust design methods. Findings from the study revealed that the air hygiene system design based on ASHRAE S170 baseline appears incapable of maintaining the air hygiene levels at the RfC. Results also indicate that outdoor air change rate and respiratory personal protective equipment (PPE) has significant effects on exposure than any other factors. However, while the outdoor air change rate has the highest influence on the exposure concentration, respiratory PPE affects the system robustness most. The study shows that it is possible to maintain exposure to airborne contagion in a patient ward at a reference concentration with adequate consideration for noise factors under effective air hygiene system conditions.

The results of capability analysis suggest that off-target conditions may lead to higher exposure and/or infection risk on one hand and resource (energy consumption, environmental impacts, etc.) inefficiency on the other. Also, the findings indicate that if we go with the notion of selecting design parameters that provide the least exposure concentration, without considering the variability in the exposure, a preferred choice may lead to "incapable air hygiene process". Such preference indicates that over 50 per cent non-conformance exposure would exist in the patient ward as against less than one per cent under the robust air hygiene system. These findings suggest the need for caution in the healthcare facility performance assessment techniques, which consider only process position (mean exposure concentration) and neglecting the process dispersion (variance) effects. Further, the study found that in terms of robustness, stratum air distribution system outperforms the mixing system. With an increment of over nine decibels in robustness, stratum air distribution has the potential to improve indoor air quality and/or reduce the cost of air quality by up to 150 per cent over the conventional system. Additionally, the combination of robust design method with capability analysis serves as a potential alternative to multi-objective optimisation of air quality and cost/energy metrics. The presented hybrid approach involves one objective of on-target exposure concentration, and thus remains a promising option to improve air quality and cost efficiency. A better understanding of the ideal function of air hygiene systems is needed to maximise the benefits. It is hoped that this study will motivate some further insights into the building system’s design in general and air hygiene system in particular for preventing exposure risk to airborne contagion in hospital environments.