Subway stations are characterized by poor ventilation, high density of people, and close exposure, making them one of the high-risk places for the spread of infectious diseases. Based on the CFD (Computational Fluid Dynamic) method, this paper studied the effects of mask-wearing, personnel distance, ventilation volume, and air supply distance on the risk of cross-infection for people in queues in different risk areas of subway stations. The risk of cross-infection in the high-density queue in the subway station was evaluated and the reduction strategy was proposed. The results showed that in the process of close contact in the ticketing area without masks, the infected persons not only exposed the people in front of them to high risk but also increased the exposure risk of people on both sides of them. There is still a high risk of infection during close security checks, and it is recommended that security personnel wear masks. The prevention and control effect of comprehensive ventilation in subway stations is affected by air supply distance and ventilation volume. For areas close to the air supply outlet (< 0.5 m), increasing ventilation volume can significantly reduce the risk of infection. For the area far from the air outlet (>1 m), the effect of increasing the ventilation volume to reduce the risk is poor. The findings of this study are expected to provide a basis for the formulation of reasonable prevention and control measures, help relieve the pressure of epidemic prevention and control in subway stations, and save prevention and control costs.

Pressure differential deviations under static conditions and pressure convergence fluctuations under dynamic disturbances are widely reported problems with pressure differential control in pharmaceutical cleanrooms, yet their underlying mechanisms and key reasons remain insufficiently explored. This study performed a field survey and model-based simulations to identify the major influencing parameters and quantify their influence on pressure differentials. Twelve pharmaceutical cleanrooms with varying environmental control parameters were included in the field survey, all of which were served by a variable air volume (VAV) ventilation system. Large deviations between actual and design pressure differentials were found, ranging from 10% to 42.5%, and a total of 24 uncertain parameters and their respective uncertainty ranges were identified. Based on the field survey, a data-driven pressure differential response model was developed using MATLAB/Simulink platform. The model fully took into account the system dynamics and facilitated real-time monitoring and control of the pressure differential. Sobol-based sensitivity analysis was then conducted to identify key influencing parameters of pressure differential deviations. The simulated results revealed that static pressure differential deviations were predominantly influenced by pressure sensing accuracy, exhaust airflow accuracy, and duct impedance, while dynamic disturbances were mainly driven by room envelope airtightness and supply airflow accuracy. The interactions between connected zones were pronounced. Rooms with higher branch duct impedance experienced smaller pressure differential deviations due to natural buffering characteristics, while the parameter uncertainties in these rooms significantly affected pressure differential in other rooms. These findings offer practical guidance for the design and operation of precise pressure differential control in pharmaceutical cleanrooms.

The high-volume evacuation (HVE) is commonly employed as a primary source control measure for removing splatter emitted from mouth during dental treatments, but there is still a lack of comprehensive understanding of its efficiency. Based on our previous experiments on the emission characteristics during dental treatments, this study employed computational fluid dynamics (CFD) simulations to investigate the impact of emission parameters (droplet size, emission velocity, emission angle), HVE usage methods (distance between HVE and the droplet release source), and HVE suction flow rates on its removal efficiency. The effect of HVE on fallow time (FT) was also examined. Cumulative removal efficiency that accurately reflected the HVE effect was proposed as an evaluation index. It was found that emission velocity and distance between HVE and the source were key factors determining cumulative removal efficiency. When the distance was 4 cm, the cumulative removal efficiencies for low-velocity (0.8 m/s), medium-velocity (3.4 m/s), and high-velocity droplets (6.0 m/s) were approximately 97.9%, 73.6%, and 58.0%, respectively. For high-velocity droplets at 6.0 m/s, decreasing the distance from 4 cm to 2 cm and 1 cm increased the cumulative removal efficiency from 58.0% to 76.7% and 100%. This study was expected to enhance the understanding of HVE performance and provide information on its usage method. It also indicated the need for developing advanced control measures that could have high efficiency in removing both low-velocity and high-velocity droplets.

The present study develops a cost-effectiveness assessment model to analyze the performance of major operational parameters of central HVAC systems in terms of airborne transmission risk, energy consumption, and medical and social cost. A typical multi-zone building model with a central HVAC system is built numerically, and the effect of outdoor air (OA) ratio (from 30% to 100%) and filtration level (MERV 13, MERV 16, and HEPA) are assessed under the conditions of five climate zones in China. Compared with the baseline case with 30% OA and MERV 13 filtration, the airborne transmission risk in zones without infector is negligibly reduced with the increase in OA ratio and the upgrade of filtration level, owing to their slight modification on the equivalent ventilation rate of virus-free air. However, depending on climate zone, a 10% increase in OA ratio results in 12.5%–78.6% and 0.1%–8.6% increase in heating and cooling energy consumption, respectively, while an upgrade of filtration level to MERV 16 and HEPA results in an increase of 0.08%–0.2% and 1.4%–2.6%, respectively. Overall, when compared to the use of 100% OA ratio and HEPA filtration, the application of 30% or 40% OA ratio and MERV 13 filtration would save annually an energy and facility related cost of ＄29.4 billion in China, though giving an increase of approximately ＄0.1 billion on medical and social cost from the increased number of confirmed cases. This study provides basic method and information for the formulation of cost-effective operational strategies of HVAC systems coping with the airborne transmission, especially in resource-limited regions.

Numerous short-term exposure events in public spaces were reported during the COVID-19 pandemic, especially during the spread of Delta and Omicron. However, the currently used exposure risk assessment models and mitigation measures are mostly based on the assumption of steady-state and complete-mixing conditions. The present study investigates the dynamics of airborne transmission in short-term events when a steady state is not reached before the end of the events. Large-eddy simulation (LES) is performed to predict the airborne transmission in short-term events, and three representative physical distances between two occupants are examined. Both time-averaged and phase-averaged exposure indices are used to evaluate the exposure risk. The results present that the exposure index in the short-term events constantly varies over time, especially within the first 1/ACH (air changes per hour) hour of exposure between occupants in close proximity, posing high uncertainty to the spatial and temporal evolutions of the risk of cross-infection. The decoupling analysis of the direct and indirect airborne transmission routes indicates that the direct airborne transmission is the predominated route in short-term events. It suggests also that the general dilution ventilation has a relatively limited efficiency in mitigating the risk of direct airborne transmission, but determines largely the occurrence time of the indirect one. Given the randomness, discreteness, localization, and high-risk characteristics of direct airborne transmission, a localized method that has a direct interference on the respiratory flows would be better than dilution ventilation for short-term events, in terms of both efficiency and cost.