Low-tech modifications can reduce airborne COVID-19 in emergency hospitals, say researchers
The coronavirus pandemic is stressing bed space capacity in hospitals globally. Healthcare authorities are attempting to add thousands of additional bed spaces by temporarily adapting any available large open halls.
However, large air-conditioned halls tend to have top-down air-conditioning, which creates turbulent flows that can mix and spread droplets containing the virus very widely. At six full changes of the air in the hall in an hour, it may take over 20 minutes to dilute the concentration of smaller droplets produced in a cough to below a tenth of their original density. This, say the researchers, is plenty of time for droplets to travel beyond 20m, putting healthcare professionals in particular at risk as they move about through “a slowly refreshing miasma”.
Professor Andrew Woods of Cambridge’s BP Institute (BPI) and Professor Alan Short of the Department of Architecture have developed a series of practical solutions to reduce the concentration of airborne virus experienced by patients and healthcare workers in buildings converted into makeshift wards.
The designs, released today in a video report, involve relatively low-tech adaptations to ventilation systems and ward configuration, and are relevant for use in the UK and overseas. Because of the urgent need to share information relating to the pandemic, the preliminary report has been released ahead of a peer review publication process.
The team recommends subdividing the large floorplate of a typical hall into enclosed patient zones, a minimum of 16 metre square, with some 10-20 beds separated by solid partitions up to 3 metre height, and lighter clear polythene sheeting above taped up to prevent air leaking in or out. Air is drawn through the back of the patient bays by an outflow ventilation duct, removing dirty air from the space and providing a clean corridor space for healthcare workers.
“Effective ventilation is critically important in helping to suppress cross-infection, and nowhere more so than in an infectious diseases ward,” says Professor Alan Short from Cambridge’s Department of Architecture. “Patients coughing or being ventilated will project droplets, some containing the virus, as an aerosol. They are so small that they may take tens of minutes to fall to the floor as the droplet evaporates in still air.”
“Governments, healthcare decision-makers and construction workers are facing an extreme challenge in the urgent need to construct emergency hospitals,” says Professor Andy Woods, Director of the interdisciplinary BPI in Cambridge and co-author of the new video report. “Our work aims to highlight simple yet effective solutions that are relatively easy to install, implement, service and maintain.”
The team’s recommendations are based on physical laboratory experiments to test ventilation systems for two basic arrangements of beds: what is becoming a standard approach of placing hundreds of beds in an open hall with low level partitions, compared with arranging beds within enclosed patient bays so that, as far as possible, the exhaust air does not permeate the rest of the hall.
In the completely open version, ventilation air moves down to the ground and spreads out over the patient beds, leading to a highly mixed environment. When a patient coughs or releases aerosols, the flow pattern of the aerosols can extend across the space to other patient beds, even to patients across the corridor.
In the version subdivided into patient bays, the ventilation flow still comes down from the ceiling and moves into the patient bed-spaces and mixes, but a good proportion of this air is removed through exhaust ducts located behind the beds. When a patient produces aerosols within a bay, the aerosol concentration remains high in the bay and as air is drawn out through the exhaust duct this limits the aerosol transport into the main space.
“In a large hall, airflows mix up the airborne aerosols all too efficiently and disperse them through the space across patients and perhaps more significantly nurses and healthcare workers,” adds Woods. “A small measure, such as the installation of part-enclosed patient bays with exhaust ducts can help reduce this dispersion.”
“The strategies will work in many different climates,” adds Short. “We’ve developed viable low energy ventilation models for converted spaces in many other climate regions from Temperate Northwest to the Mediterranean, from Continental climates in China and central India to the Mid-West of North America, Canada and marine coastal climates globally.”
In particular, the Cambridge team is working with Professor L.S. Shashidhara, Dean of Research at Ashoka University, and advisor to the Indian government and architect CS Raghuram, to create viable conversions of marriage halls and sheds as emergency COVID-19 hospitals in India.
“Crucially, the measures we suggest are simple to implement as part of a rapid interior remodelling plan,” adds Short. “Our research shows that a small number of straightforward modifications would reduce risk in what is already a very risky environment.”
The video report is based on decades of research by Woods and Short on how particles like viruses are transported in mechanically ventilated spaces. Two key publications:
- Mingotti, N. and Woods, A.W. (2015) On the transport of heavy particles through an upward displacement-ventilated space. J. Fluid Mechanics 772: 478-507
- Short, C.A. (2017) The Recovery of Natural Environments in Architecture: Air, Comfort and Climate. Routledge, Abingdon, UK, [ISBN 978 1 138 65146 3]