VENTILATION | COVID-19 A In 2011, two further papers were published, collating performance data for mechanical, natural and hybrid ventilation systems and attempting to quantify the difference between displacement and mixing ventilation with regards to effectiveness at removing pathogens. Amir Aliabadi et al concluded that a vertical, upward-type displacement ventilation that introduces fresh, cool air near the bottom of the room is far superior to top-down mixing ventilation, as the buoyant force takes the warm and polluted air (possibly containing airborne pathogens) close to the ceiling and subsequently the exhaust for removal.8 Yonggao Yin et al managed to conduct comparative tests (again, in a hospital ward) and arrive at a numerical evaluation: 4ACHD > 6ACHM (displacement ventilation with 4ACH removed tracer gas and fine aerosols much more effectively than the mixing type ventilation with 6ACH).9 If any exhaust was located at low level, (ventilation flow against buoyancy) the pollutant concentration at breathing zone would be even worse than when using a mixing type of ventilation. The paper concluded that for the best result for pathogen removal, all exhausts must be located at high levels, preferably closer to the pollutant source with fresh air delivered low. Back to the present Several studies published in 2020 and 2021 arrived at similar conclusions. One focused specifically on the mixing ventilation in hospitals and established that even at 12ACH (equivalent to 120L.s-1 per person) the top-down mixing ventilation failed to remove virus pathogens from two-person wards.10 A team at the University of Cambridge found that mixing ventilation systems disperse airborne contaminants evenly throughout the space. These contaminants may include droplets and aerosols, potentially containing viruses. The conclusion was that displacement ventilation, which encourages vertical stratification and is designed to remove the polluted warm air near the ceiling, is the most effective at reducing the exposure risk.11 A University of Oregon study concluded that recirculating or mixing airflow has the potential for high spread of coronavirus-infected droplets within densely occupied spaces, even with just one person exhaling the virus droplets. Apart from the recirculation, transmission appears to be facilitated by the type and velocity of turbulent airflow designed to reach deep into the occupied space.12 Conclusion We need to take a closer look at how we ventilate buildings, especially with high occupancy. Thinking just in terms of ACH is way too simplistic. We spent the past few decades sealing office windows and fitting positive pressure, mixing ventilation into ceiling voids I believe we were going in the wrong direction. The question is how can we adapt existing buildings to a Covidconscious future? Not every building can be naturally ventilated although, surprisingly, many can and not every HVAC or every space can be easily converted to displacement ventilation (supplying C D Displacement ventilation needs considered design and architect and engineer to work together, which can be hard air slowly at low level and extracting at high level). Those hard-to-adapt spaces may need to transition to lower occupancy levels or to more individual setups (less open plan) accommodating more flexible working arrangements. This means that working from home might not always lead to a reduction in office space requirement and lower estate costs. CJ TOM LIPINSKI is founder and technical director at Ventive References: 1 Exaggerated risk of transmission of COVID-19 by fomites, The Lancet, bit.ly/CJSept21vent 2 How the world missed Covid-19s silent spread, The New York Times, bit.ly/CJSept21vent2 3 239 experts with one big claim: the coronavirus is airborne, The New York Times, bit.ly/CJSept21vent3 4 Roadmap to improve and ensure good indoor ventilation in the context of COVID-19, WHO, bit.ly/CJSept21vent4 5 SL Miller, WW Nazaroff, JL Jimenez, A Boerstra, G Buonanno, S J Dancer, J Kurnitski, LC Marr, L Morawska, C Noakes, 2020. Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event, International Journal of Indoor Environment and Health, 314-323. 6 Birnir, Bjrn, 2021. Ventilation and the SARS-CoV-2 Coronavirus Analysis of outbreaks in a restaurant and on a bus in China, and at a Call Center in South Korea, bit.ly/CJSept21vent5 7 Hua Qian, Yuguo Li, WH Seto, Patricia Ching, WH Ching, HQ Sun, 2010. Natural ventilation for reducing airborne infection in hospitals, Building and Environment Volume 45, Issue 3 559-565. 8 Aliabadi, Amir, A & Rogak, Steven & Bartlett, Karen & Green, Sheldon, 2011. Preventing Airborne Disease Transmission: Review of Methods for Ventilation Design in Health Care Facilities. Advances in preventive medicine 124064. 10.4061/2011/124064. 9 Y Yin, W Xu, JK Gupta, A Guity, P Marmion, A Manning, B Gulick, X Zhang & Q Chen, 2011. Experimental Study on Displacement and Mixing Ventilation Systems for a Patient Ward. HVAC&R Research 1175-1191. 10 Juan Ren, Yue Wang, Qibo Liu, Yu Liu, 2021. Numerical Study of Three Ventilation Strategies in a prefabricated COVID-19 inpatient ward. Building and Environment 188 (2021) 107467. 11 RK Bhagat, MSD Wykes, SB Dalziel, PF Linden, 2020. Effects of ventilation on the indoor spread of COVID-19. Journal of Fluid Mechanics Volume 903. 12 Leslie Dietz, Patrick F Horve, David A Coil, Mark Fretz, Jonathan A Eisen, Kevin Van Den Wymelenberg, 2020. 2019 Novel Coronavirus (Covid-19) Pandemic: Built Environment Considerations To Reduce Transmission. Applied and Environmental Science. 13 RK Bhagat, MSD Wykes, SB Dalziel, PF Linden, 2020. Effects of ventilation on the indoor spread of COVID-19. Journal of Fluid Mechanics Volume 903. 64 September 2021 www.cibsejournal.com CIBSE Sep 21 pp62-64 Covid ventilation.indd 64 27/08/2021 13:11