Air exchange rates and advection-diffusion of CO2 and aerosols in a route bus for evaluation of infection risk

Wind velocity and dispersion/advection-diffusion of artificial droplets and droplet nuclei in a domed all-weather multi-purpose stadium

To evaluate the COVID-19 infection risk and the effectiveness of countermeasures at mass-gathering events, we measured the dispersion and advective diffusion of artificial droplets and artificial droplet nuclei at the Tokyo Dome, Japan (capacity 55,000 people). We also measured and evaluated the effectiveness of wearing masks and increasing the space between seating areas. If people were seated facing forward, artificial droplets did not reach the mouths of surrounding people, suggesting low risk of droplet transmission. For an artificially generated cough or sneeze, the volume of droplets deposited on the hair, back of the neck, and back of the human in front, and the backs of the seats in front, decreased by two to three orders of magnitude when a mask was worn, regardless of the type of mask. However, when the mask was worn with the nose out, the amount deposited on the back of the seat in front was reduced by only 17%. Even in seats with the highest particle concentration in the vicinity of the source, only 0.097%-0.24% of the generated droplet nuclei (1.0-3.0 μm) from the source were inhaled. Our results suggest that the infection risk at the Tokyo Dome via droplet and airborne transmission was low.

A COVID-19 cluster analysis in an office: Assessing the long-range aerosol and fomite transmissions with infection control measures

Simulated exposure to severe acute respiratory syndrome coronavirus 2 in the environment was demonstrated based on the actual coronavirus disease 2019 cluster occurrence in an office, with a projected risk considering the likely transmission pathways via aerosols and fomites. A total of 35/85 occupants were infected, with the attack rate in the first stage as 0.30. It was inferred that the aerosol transmission at long-range produced the cluster at virus concentration in the saliva of the infected cases on the basis of the simulation, more than 108 PFU mL-1. Additionally, all wearing masks effectiveness was estimated to be 61%-81% and 88%-95% reduction in risk for long-range aerosol transmission in the normal and fit state of the masks, respectively, and a 99.8% or above decline in risk of fomite transmission. The ventilation effectiveness for long-range aerosol transmission was also calculated to be 12%-29% and 36%-66% reductions with increases from one air change per hour (ACH) to two ACH and six ACH, respectively. Furthermore, the virus concentration reduction in the saliva to 1/3 corresponded to the risk reduction for long-range aerosol transmission by 60%-64% and 40%-51% with and without masks, respectively.

COVID-19 transmission and control in land public transport: A literature review

Land public transport is an important link within and between cities, and how to control the transmission of COVID-19 in land public transport is a critical issue in our daily lives. However, there are still many inconsistent opinions and views about the spread of SARS-CoV-2 in land public transport, which limits our ability to implement effective interventions. The purpose of this review is to overview the literature on transmission characteristics and routes of the epidemic in land public transport, as well as to investigate factors affecting its spread and provide feasible measures to mitigate the infection risk of passengers. We obtained 898 papers by searching the Web of Science, Pubmed, and WHO global COVID database by keywords, and finally selected 45 papers that can address the purpose of this review. Land public transport is a high outbreak area for COVID-19 due to characteristics like crowding, inadequate ventilation, long exposure time, and environmental closure. Different from surface touch transmission and drop spray transmission, aerosol inhalation transmission can occur not only in short distances but also in long distances. Insufficient ventilation is the most important factor influencing long-distance aerosol transmission. Other transmission factors (e.g., interpersonal distance, relative orientation, and ambient conditions) should be noticed as well, which have been summarized in this paper. To address various influencing factors, it is essential to suggest practical and efficient preventive measures. Among these, increased ventilation, particularly the fresh air (i.e., natural ventilation), has proven to effectively reduce indoor infection risk. Many preventive measures are also effective, such as enlarging social distance, avoiding face-to-face orientation, setting up physical partitions, disinfection, avoiding talking, and so on. As research on the epidemic has intensified, people have broken down many perceived barriers, but more comprehensive studies on monitoring systems and prevention measures in land public transport are still needed.

Numerical performance of CO2 accumulation and droplet dispersion from a cough inside a hospital lift under different ventilation strategies

The impact of mechanical ventilation on airborne diseases is not completely known. The recent pandemic of COVID-19 clearly showed that additional investigations are necessary. The use of computational tools is an advantage that needs to be included in the study of designing safe places. The current study focused on a hospital lift where two subjects were included: a healthy passenger and an infected one. The elevator was modelled with a fan placed on the middle of the ceiling and racks for supplying air at the bottom of the lateral wall. Three ventilation strategies were evaluated: a without ventilation case, an upwards-blowing exhausting fan case and a downwards-blowing fan case. Five seconds after the elevator journey began, the infected person coughed. For the risk assessment, the CO2 concentration, droplet removal performance and dispersion were examined and compared among the three cases. The results revealed some discrepancies in the selection of an optimal ventilation strategy. Depending on the evaluated parameter, downward-ventilation fan or no ventilation strategy could be the most appropriate approach.

Multi-scale risk assessment and mitigations comparison for COVID-19 in urban public transport: A combined field measurement and modeling approach

The outbreak of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has caused an unparalleled disruption to daily life. Given that COVID-19 primarily spreads in densely populated indoor areas, urban public transport (UPT) systems pose significant risks. This study presents an analysis of the air change rate in buses, subways, and high speed trains based on measured CO2 concentrations and passenger behaviors. The resulting values were used as inputs for an infection risk assessment model, which was used to quantitatively evaluate the effects of various factors, including ventilation rates, respiratory activities, and viral variants, on the infection risk. The findings demonstrate that ventilation has a negligible impact on reducing average risks (less than 10.0%) for short-range scales, but can result in a reduction of average risks by 32.1%-57.4% for room scales. When all passengers wear masks, the average risk reduction ranges from 4.5-folds to 7.5-folds. Based on our analysis, the average total reproduction numbers (R) of subways are 1.4-folds higher than buses, and 2-folds higher than high speed trains. Additionally, it is important to note that the Omicron variant may result in a much higher R value, estimated to be approximately 4.9-folds higher than the Delta variant. To reduce disease transmission, it is important to keep the R value below 1. Thus, two indices have been proposed: time-scale based exposure thresholds and spatial-scale based upper limit warnings. Mask wearing provides the greatest protection against infection in the face of long exposure duration to the omicron epidemic.

Spatial distributions of airborne transmission risk on commuter buses: Numerical case study using computational fluid and particle dynamics with computer-simulated persons

Commuter buses have a high passenger density relative to the interior cabin volume, and it is difficult to maintain a physical/social distance in terms of airborne transmission control. Therefore, it is important to quantitatively investigate the impact of ventilation and air-conditioning in the cabin on the airborne transmission risk for passengers. In this study, comprehensive coupled numerical simulations using computational fluid and particle dynamics (CFPD) and computer-simulated persons (CSPs) were performed to investigate the heterogeneous spatial distribution of the airborne transmission risk in a commuter bus environment under two types of layouts of the ventilation system and two types of passenger densities. Through a series of particle transmission analysis and infection risk assessment in this study, it was revealed that the layout of the supply inlet/exhaust outlet openings of a heating, ventilation, and air-conditioning (HVAC) system has a significant impact on the particle dispersion characteristics inside the bus cabin, and higher infection risks were observed near the single exhaust outlet in the case of higher passenger density. The integrated analysis of CFPD and CSPs in a commuter bus cabin revealed that the airborne transmission risk formed significant heterogeneous spatial distributions, and the changes in air-conditioning conditions had a certain impact on the risk.

Evaluation of shields and ventilation as a countermeasure to protect bus drivers from infection

We evaluated the deposition of droplets and droplet nuclei-generated by simulated coughing and talking from three points in a bus-on the driver's face and on surfaces around the driver (e.g., the steering wheel), based on whether countermeasures were taken, and assuming that an infected passenger was talking to the driver. When a shield, such as acrylic boards or polyvinyl chloride (PVC) sheets, was used as the countermeasure, the deposition of artificial droplets (>4 μm), emitted from beside or behind the driver, on his eyes, mouth, and cheeks reduced by two to three orders of magnitude or more. Deposition on the surfaces around the driver was also reduced following the use of shields. For artificial droplet nuclei (1.3 μm of polystyrene latex (PSL)) emitted from atomizers beside the driver, the operation of the ventilation fan (VF) and air conditioner (AC), and defroster (DEF) greatly reduced the driver's exposure, while the use of the shield did not. The infection risk of the driver was estimated through exposure to the virus via transfer to the mucosa via hands or surface-to-finger, direct adhesion on the mucosa, and direct inhalation of droplets and droplet nuclei. This is under the assumption that the droplets and droplet nuclei measured in this study are 40% the diameter of those after immediately leaving the mouth of the infected person and are constant regardless of particle size. When using the shield, total infection risk via droplet, airborne, and contact transmission was decreased by 75.0-99.8%. When the shield was not installed, the infection risk decreased by 9.74-48.7% with the operation of the VF, AC, and/or DEF.