Indoor Environment Control Strategies: Air Flow and Distribution
It is known that infectious aerosols travel through the air on prevailing air currents after generation from infected humans. Air flow dynamics can influence infectious aerosol exposure. Air flow influences the aerosol control efficacy of building-level ventilation, filtration, and germicidal UV systems. It also influences outdoor exposure to potentially infectious aerosols.
Air flow patterns were determined to predict the spatial distribution of SARS cases in a high-rise apartment complex, from the source throughout the apartment complex, reaching multiple buildings [1, 2]. A plume of virus generated from the stack appeared to rise up through a building and into the occupied spaces, and prevailing wind currents could have moved viral aerosols across the courtyard to buildings downwind. A similar outbreak within a high-rise apartment building has been described for SARS-CoV-2 .
Interest in air flow as an infection control strategy has often focused on hospitals where infectious aerosol exposure can be elevated and where ill patients may have limited movement. Air flow considerations are commonly included in the design of hospital isolation wards and isolation rooms. Negative pressure isolation aims to move potentially contaminated room air through an exhaust and reduce or eliminate exposure to individuals in the adjacent rooms or corridors. This benefits infection control, given evidence of airborne infection spread in hospitals. A SARS superspreading event within a hospital ward was consistent with computational fluid dynamics showing the dispersion of air flow and predicted relative aerosol viral concentrations from an infected person’s cubical throughout the ward [4, 5]. Similarly, a large MERS outbreak in a hospital was linked with air flow between patient rooms . Multi-zone modelling of a university dormitory wing showed that two students who presented with acute respiratory infection were in the only two rooms in the wing to be connected by an air flow from the room of the student with the initial symptom presentation . Documentation of infectious MERS in the air outside of a makeshift MERS isolation unit suggested a failure of the air flow control , underscoring the importance of careful air flow system implementation in high risk settings.
A series of airflow characterization and modelling studies have been done in the context of hospital settings. Experiments to characterize air flow dynamics related to a variety of ventilation systems were carried out by Qian and colleagues . Although downward ventilation, displacement ventilation, mixing ventilation, and personalized ventilation devices influenced air flow in a room with a single patient, they found that exhausting air at the top of the room represented a relatively effective means of removing aerosols, attributed to the upward air movement from thermal plume. But although displacement ventilation can help remove contaminants efficiently in some settings, it could lead to increased exposure of healthcare workers standing over sick patients . Provision of laminar flow air around operating room patients has been explored to reduce risk of transmission to or from infected and/or immunologically vulnerable patients.
In some cases, abundant air flow into an indoor environment may provide enough air exchange to rapidly dilute contaminated air between the infectious source and a susceptible person nearby. This can reduce critical exposure without consideration of air flow pattern, unless a susceptible person is directly downwind of the infectious case. Natural ventilation through window opening and cross ventilation resulted in increased air exchange in Peruvian hospitals, achieving up to 17 air changes per hour (ACH) in a consulting room and 66 ACH in a waiting room . A study of natural ventilation in an UK hospital showed the ability for cross-ventilation by window opening to reach air exchange up to 27 ACH with strong outside wind and uniform aerosol exposure across an open plan ward . Mechanically supplied air can supplement low, natural, air flow delivery to assure an abundance of air exchange, with good mixing, which reduces exposure to infectious aerosols potentially below the infectious dose. Air mixing with abundant ventilation and air flow to efficiently exhaust or clean contaminated air represent helpful layers of protection against infection transmission.
Portable air cleaners deployed in occupied grade-school classrooms led to measurable decreases of airborne particles and clean air delivery equivalent over 5 ACH . Tracer gas spatial distribution and particulate matter (PM) detection in various zones within the classroom suggested a well-mixed environment. Good air mixing and uniform particle distribution may be expected with some air turbulence in the room generated by people moving around, breathing, thermal plumes, natural infiltration and natural outdoor airflow from open windows or doors, any mechanical ventilation or fans, and the air movement from deployed air cleaners. However, there are some scenarios where thermal locking of air could reduce the level of cleaned or outdoor air that reaches the breathing zone of the occupied space. This may occur if warm air from an HVAC system enters at or near the ceiling of the room and flows across the upper room space toward an exhaust without mixing with the breathing zone. The addition of air currents from portable air cleaners between the upper and lower zones of an indoor space could interrupt thermal locking, resulting in increased mixing and greater clean air delivery to the breathing zone.
The placement of portable air cleaners could influence the overall level of contamination in a well-mixed space and could also provide some level of source control to reduce transfer of infectious aerosols between people interactive at closer range. A study of an office environment showed that air cleaner placement close to an aerosol source resulted in an elevated cleaning efficiency . On the other hand, placement under a desk away from the aerosol source and with less mixing potential resulted in the lowest cleaning efficiency. This study helps illustrate why distancing between potentially infected and susceptible people helps prevent transmission. Before particles can mix well with the ambient room air, they are highly concentrated at and around the source. Ideally exhaled air from an infected person could get mostly pulled through the air cleaner before moving throughout the room. The strength of the flow generated by the pull of an air cleaner can help determine the extent to which source aerosols get filtered before mixing into the room.
If an infectious person is occupying a space and shedding pathogens into inhalable aerosols, contamination of room air is inevitable, unless a well-fitting mask with high filtration efficiency is used to trap infectious aerosols at the source. Assuming some infectious aerosols get released, they will begin to mix with room air rapidly. Quick entrainment of infectious aerosols by directed air flows leading toward cleaners or exhaust could reduce exposure to others occupying the indoor space. An example of directed airflow around an infectious person is a ventilated headboard (such as that developed by NIOSH) that exhausts patient generated aerosols reducing exposure to others nearby .
Although the physics of air mixing is a dynamic process that responds to numerous environmental conditions, one question of interest is, “at what distance between people does highly concentrated exhaled infectious aerosol dissipate to well-mixed levels within the room?” The assumption of a well-mixed space may underestimate exposure at close range, while overestimating exposure at longer range. Any air flow dynamics that can reduce the exposure at closer distances to the source, may be especially useful for preventing transmission when maintaining longer distance is not feasible. Air flow dynamics could be further investigated for its potential to reduce risk between public transportation passengers, diners in restaurants or dining halls, and in hospitals. Planes and trains can offer relatively high ventilation, >20 ACH, but air flow patterns may influence the risk between riders in relatively close proximity (e.g., within the same row, or within nearby rows).
Airflow considerations can also influence the effectiveness of upper room GUV. A study of GUV inactivation of aerosolized bacteria (regarded as a representative model for enveloped viruses such as SARS-CoV-2 or influenza), demonstrated a doubling of GUV inactivation effectiveness with the addition of a ceiling fan to draw air up into the GUV irradiance zone from the occupied zone below . The effectiveness of GUV could increase in lower versus higher ventilated spaces, perhaps due to increased GUV exposure to larger volumes of room air that exit the room at a slower rate .
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