ABSTRACT
Escalating demands of assessing airborne disease infection risks had been awakened from ongoing pandemics. An inhalation index linked to biomedical characteristics of pathogens (e.g. TCID 50 for coronavirus delta variant) was proposed to quantify human uptake dose. A modified Wells-Riley risk-assessment framework was then developed with enhanced capability of integrating biological and spatiotemporal features of infectious pathogens into assessment. The instantaneous transport characteristics of pathogens were traced by Eulerian-Lagrangian method. Droplets released via speaking and coughing in a conference room with three ventilation strategies were studied to assess occupants' infection risks using this framework. Outcomes revealed that speaking droplets could travel with less distance (0.5 m) than coughing droplets (1 m) due to the frequent interaction between speaking flow and thermal plume. Quantified analysis of inhalation index revealed a higher inhalation possibility of droplets with nuclei size smaller than 5 µ m , and this cut-off size was found sensitive to ventilation. With only 60-second exposure, occupants in the near-field of host started to have considerable infection risks (approximately 20%). This risk was found minimising over distance exponentially. This modified framework demonstrated the systematic analysis of airborne transmission, from quantifying particle inhalation possibility, targeting specific disease's TCID 50 , to ultimate evaluation of infection risks.
ABSTRACT
Social distance will remain the key measure to contain COVID-19 before the global widespread vaccination coverage expected in 2024. Containing the virus outbreak in the office is prioritised to relieve socio-economic burdens caused by COVID-19 and potential pandemics in the future. However, "what is the transmissible distance of SARS-CoV-2" and "what are the appropriate ventilation rates in the office" have been under debate. Without quantitative evaluation of the infection risk, some studies challenged the current social distance policies of 1-2 m adopted by most countries and suggested that longer social distance rule is required as the maximum transmission distance of cough ejected droplets could reach 3-10 m. With the emergence of virus variants such as the Delta variant, the applicability of previous social distance rules are also in doubt. To address the above problem, this study conducted transient Computational Fluid Dynamics (CFD) simulations to evaluate the infection risks under calm and wind scenarios. The calculated Social Distance Index (SDI) indicates that lower humidity leads to a higher infection risk due to weaker evaporation. The infection risk in office was found more sensitive to social distance than ventilation rate. In standard ventilation conditions, social distance of 1.7 m-1.8 m is sufficient distances to reach low probability of infection (PI) target in a calm scenario when coughing is the dominant transmission route. However in the wind scenario (0.25 m/s indoor wind), distance of 2.8 m is required to contain the wild virus type and 3 m is insufficient to contain the spread of the Delta variant. The numerical methods developed in this study provide a framework to evaluate the COVID-19 infection risk in indoor environment. The predicted PI will be beneficial for governments and regulators to make appropriate social-distance and ventilation rules in the office.
ABSTRACT
Inhaled viral droplets may immediately be expelled and cause an escalating re-transmission. Differences in the deposition location of inhaled viral droplets may have a direct impact on the probability of virus expelling. This study develops a numerical model to estimate the region-specific deposition fractions for inhalable droplets (1-50 µ m) in respiratory airways. The results identified a higher deposition fraction in the upper airways than the lower airways. Particularly for droplets larger than 10 µ m, the relatively high deposition fraction in the oral/laryngeal combined region warns of its easy transmission through casual talking/coughing. Moreover, considering droplet sizes' effect on virus loading capacity, we built a correlation model to quantify the potential of virus expelling hazards, which suggests an amplified cascade effect on virus transmission on top of the existing transmission mechanism. It therefore highlights the importance of considering the instant expelling possibilities from inhaled droplets, and also implies potentials in restricting a rapid secondary transmission by measures that can lower down droplet deposition in the upper airways.
ABSTRACT
Urgent demands of assessing respiratory disease transmission in airliner cabins had awakened from the COVID-19 pandemics. This study numerically investigated the cough flow and its time-dependent jet-effects on the transport characteristics of respiratory-induced contaminants in passengers' local environments. Transient simulations were conducted in a three-row Boeing 737 cabin section, while respiratory contaminants (2 µm-1000 µm) were released by different passengers with and without coughing and were tracked by the Lagrangian approach. Outcomes revealed significant influences of cough-jets on passengers' local airflow field by breaking up the ascending passenger thermal plumes and inducing several local airflow recirculation in the front of passengers. Cough flow could be locked in the local environments (i.e. near and intermediate fields) of passengers. Results from comparative studies also revealed significant increases of residence times (up to 50%) and extended travel distances of contaminants up to 200 µm after considering cough flow, whereas contaminants travel displacements still remained similar. This was indicating more severe contaminate suspensions in passengers' local environments. The cough-jets was found having long and effective impacts on contaminants transport up to 4 s, which was 8 times longer than the duration of cough and contaminants release process (0.5 s). Also, comparing to the ventilated flow, cough flow had considerable impacts to a much wider size range of contaminants (up to 200 µm) due to its strong jet-effects.