Alberta Health Services: COVID-19 Scientific Advisory Group Rapid Evidence Report, June 5, 2020 Transmission from HVAC Systems • 8 Buonanno et al. (2020) estimate the SARS-CoV-2 viral load emitted by a contagious subject on the basis of the viral load in the mouth, the type of respiratory activity (e.g. breathing, speaking), respiratory physiological parameters (e.g. inhalation rate), and activity level (e.g. resting, standing, light exercise). The authors conclude that the results obtained from the simulations highlight that a key role is played by proper ventilation in containment of the virus in indoor environments (Buonanno, Stabile, & Morawska, 2020). Morawska and Cao (2020) stated that based on the trend in the increase of infections, and understanding the basic science of viral infection spread, they strongly believe that the SARS-Cov-2 virus is likely to be spreading through the air and that it will take several months for this to be confirmed by science. The authors recommend all possible precautions against airborne transmission of SARS-CoV-2 virus in indoor scenarios be taken and that these precautions include increased ventilation rate, using natural ventilation, avoiding air recirculation, avoiding staying in another person’s direct air flow, and minimizing the number of people sharing the same environment based on Qian & Zeng (2018). Morawska & Cao (2020) also recommend personal protective equipment, in particular masks and respirators should be recommended, to be used in public places where density of people is high and ventilation potentially inadequate, as they can protect against infection (by infected individuals) and infecting others (L. Morawska & Cao, 2020). Other Viruses & HVAC Systems in Healthcare Settings Li et al. (2004) presented a detailed air distribution study of a hospital ward during a major nosocomial outbreak of SARS-CoV in Hong Kong in March 2003 (Y. Li, Huang, Yu, Wong, & Qian, 2004). Retrospective on-site inspections and measurements of the ventilation design and air distribution system, three months after the outbreak, showed that the flow rates in the supply diffusers and exhaust grilles were not balanced. Measurements performed using bio-aerosol generator (with diameters between 0.1 and 10um) placed in one of the beds next to the index patient’s bed (since it was occupied). At a height of 1.1m, concentration decreased as the virus laden bio-aerosols moved away from the index patient’s cubicle. The concentrations at the doorstep of patient’s toilet and store/clean room were relatively high – so risk if other patients in distant cubicles visited the toilets. Extraction fans in the store/cleaning room and in the patient’s toilet seems to have also contributed to spread of bio-aerosols from index patient’s cubicle to corridor and nurses station. Using their simulations and measurements, the predicted bio-aerosol concentration agreed with the spatial infection pattern which the authors indicated it suggested a probable airborne transmission route, in addition to the commonly accepted large droplet and close personal contact transmission (Y. Li et al., 2004). Yu et al. (2005) conducted a retrospective cohort study of the SARS-CoV outbreak, noted above, on a hospital ward at the Prince of Wales Hospital, China (Yu, Wong, Chiu, Lee, & Li, 2005). Information on roles of healthcare workers and the ventilation system (location and size of supply diffusers, exhaust grills, supply air temperature, and the air-flow rate through each supply diffuser, exhaust grille and exhaust fan) were collected. Dispersion of hypothetical virus-laden aerosols, originated from the index case patient’s bed, through the entire ward was Dbouk and Drikakis (2020) used computational multiphase fluid dynamics and heat transfer to investigate the transport, dispersion, and evaporation of saliva particles arising from a human cough (Dbouk & Drikakis, 2020). An ejection process of saliva droplets in air was applied to mimic the real event of a human cough. Their model took into account relative humidity, turbulent dispersion forces, droplet phase-change, evaporation, and breakup in addition to the droplet-droplet and droplet-air interactions. The authors further investigated the effect of wind speed on social distancing. For a mild human cough in air at 20 °C and 50% relative humidity, human saliva- disease-carrier droplets may travel up to unexpected considerable distances depending on the wind speed. When the wind speed was approximately zero, the saliva droplets did not travel 2 m, which is within the social distancing recommendations. However, at wind speeds varying from 4 km/h to 15 km/h, the saliva droplets can travel up to 6 m with a decrease in the concentration and liquid droplet size in the wind direction. The findings imply that considering the environmental conditions, the 2 m social distance may not be sufficient. Further research is required to quantify the influence of parameters such as the environment’s relative humidity and temperature among others. The authors further highlight that further research is required to assess the probability of viral transmission and that a holistic approach to address these questions is needed. This would require closer interactions between individuals in medicine, biology, engineering fluid physics and social sciences. 11.207
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