Reducing airborne infection risk of COVID-19 by locating air cleaners at proper positions indoor: Analysis with a simple model

A critical review of research methodologies for COVID-19 transmission in indoor built environment

The Coronavirus Disease 2019 (COVID-19) has caused massive losses for the global economy. Scholars have used different methods to study the transmission mode and influencing factors of the virus to find effective methods to provide people with a healthy built environment. However, these studies arrived at different or even contradictory conclusions. This review presents the main research methodologies utilized in this field, summarizes the main investigation methods, and critically discusses their related conclusions. Data statistical analysis, sample collection, simulation models, and replication transmission scenarios are the main research methods. The summarized conclusion for prevention from all reviewed papers are: adequate ventilation and proper location of return air vents, proper use of personal protective equipment, as well as the reasonable and strict enforcement of policies are the main methods for reducing the transmission. Recommendations including standardized databases, causation clarification, rigorous experiment design, improved simulation accuracy and verification are provided.

A CFD study on the effect of portable air cleaner placement on airborne infection control in a classroom

The utilization of portable air cleaners (PACs) is a recommended supplemental approach to help remove airborne pathogens and mitigate disease transmission in learning environments. To improve PAC effectiveness, science-based information is needed to optimize their implementation strategies such as the deployment location, height, and number of PACs. In this study, we developed a Computational Fluid Dynamics (CFD) model to assess how PACs perform in occupied classrooms equipped with displacement and mixing ventilation systems. The results show that PACs with a flow rate of 2.6 h-1 reduce the mean aerosol intake of all students by up to 66%. A key benefit of using PACs is to facilitate air mixing and movement in indoor environments with inadequate ventilation, thereby effectively reducing high aerosol concentrations near the infector. Furthermore, our results highlight the impact of PAC location on its performance. PACs achieve the best effectiveness when placed closed to the infector (within a distance <3 m). In the absence of knowing who is infected, deploying a PAC at the center of the room is recommended. Moreover, adjusting PAC flow discharge height to the breathing height of occupants (e.g., 0.9-1.2 m for seated people) can enhance their effectiveness in spaces with poor air mixing.

Effects of table based air curtains on respiratory aerosol exposure risk mitigation at face-to-face meeting setups

Face-to-face meetings on a conference table are a frequent form of communication. The short-range exposure risk of aerosol disease transmission is high in the scenario of susceptible facing the infectious person over the table. We propose a mitigation methodology using the air curtain to reduce direct exposure to virus-laden aerosols. A numerical model was validated with experimental data to simulate the dispersion of aerosols. A dynamic mesh was adopted to consider the head movement of a 3D thermal manikin model. Results show that nodding head increase the potential risk by 74 % compared to motionless. Subsequently, for a single air curtain, placing it in the middle of the table is more effective in preventing risks than on the sides. For double air curtains, increasing the distance between them has a greater risk reduction effect than a shorter distance. Increasing the air velocity or width is more effective than increasing the number of air curtains. A moderate velocity (1 m s-1) works well to reduce the risk of nasal breathing. A higher velocity (2 m s-1) is needed for the coughing scenario. For similar indoor environments, the air curtains on the table can offer active precautions without changing the current ventilation system.

Optimizing indoor air quality: CFD simulation and novel air cleaning methods for effective aerosol particle inhibition in public spaces

In contemporary building ventilation, displacement and mixing ventilation demand high air volumes for rapid virus elimination, resulting in elevated energy consumption. To minimize the spread of viruses and decrease energy consumption for ventilation, this study employed CFD to explore the efficacy of a downward uniform flow field in impeding the transmission of aerosol particles in a high-traffic public facility, like a supermarket. The findings indicate that the downward uniform flow field proves insufficient when individuals remain static for extended periods. A wind speed of 0.1 m/s or higher becomes essential to overpower the stationary thermal plume, which disrupts this flow field. In areas with human presence, however, this technique is found to be particularly efficient since mobile heat sources do not generate a fixed thermal plume. A 0.05 m/s downward uniform flow field can settle 90% of particles within just 22 s. This flow pattern contributes to the swift settling of aerosol particles and effectively diminishes their dispersion. Employing this flow pattern in public places with increased foot traffic, like supermarkets, can lower the risk of contracting novel coronavirus without augmenting energy consumption. In order to implement the flow field in a part of the domain, a new air purification device is proposed in this study. The device combined with shelves can optimize the flow field uniformity through the MLA (PSO-SVR) algorithm and alteration of the air distribution structure. The uniformity of the final flow field increased to 0.925. The combination of data-driven MLA with CFD showed good performance in predicting the flow field uniformity. These findings offer valuable insights and practical applications for the prevention and control of respiratory diseases, particularly in post-epidemic scenarios.

Evaluation of intervention measures in reducing the driver's exposure to respiratory particles in a taxi with infected passengers

In the fifth wave of the COVID-19 epidemic in Hong Kong in early 2022, the large number of infected persons caused a shortage of ambulances and transportation vehicles operated by the government. To solve the problem, taxi drivers were recruited to transport infected persons to hospitals in their taxis. However, many of the drivers were infected after they began to participate in the plan. To tackle this issue, the present study numerically evaluated the effectiveness of several intervention measures in reducing the infection risk for taxi drivers. First, experiments were conducted inside a car to validate the large-eddy simulation (LES)-Lagrangian model for simulation of particle transport in a car. The validated model was then applied to calculate the particle dispersion and deposition in a Hong Kong taxi with intervention measures that included opening windows, installing partitions, and using a far-UVC lamp. The results show that opening the windows can significantly reduce the driver's total exposure by 97.4 %. Installing partitions and using a far-UVC lamp can further reduce the infection risk of driver by 55.9 % and 32.1 %, respectively. The results of this study can be used to support the implementation of effective intervention measures to protect taxi drivers from infection.

Reducing airborne transmission of SARS-CoV-2 by an upper-room ultraviolet germicidal irradiation system in a hospital isolation environment

Upper-room ultraviolet germicidal irradiation (UVGI) technology can potentially inhibit the transmission of airborne disease pathogens. There is a lack of quantitative evaluation of the performance of the upper-room UVGI for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) airborne transmission under the combined effects of ventilation and UV irradiation. Therefore, this study aimed to explore the performance of the upper-room UVGI system for reducing SARS-CoV-2 virus transmission in a hospital isolation environment. Computational fluid dynamics and virological data on SARS-CoV-2 were integrated to obtain virus aerosol exposure in the hospital isolation environment containing buffer rooms, wards and bathrooms. The UV inactivation model was applied to investigate the effects of ventilation rate, irradiation flux and irradiation height on the upper-room UVGI performance. The results showed that increasing ventilation rate from 8 to 16 air changes per hour (ACH) without UVGI obtained 54.32% and 45.63% virus reduction in the wards and bathrooms, respectively. However, the upper-room UVGI could achieve 90.43% and 99.09% virus disinfection, respectively, with the ventilation rate of 8 ACH and the irradiation flux of 10 μW cm-2. Higher percentage of virus could be inactivated by the upper-room UVGI at a lower ventilation rate; the rate of improvement of UVGI elimination effect slowed down with the increase of irradiation flux. Increase irradiation height at lower ventilation rate was more effective in improving the UVGI performance than the increase in irradiation flux at smaller irradiation height. These results could provide theoretical support for the practical application of UVGI in hospital isolation environments.

The impact of high background particle concentration on the spatiotemporal distribution of Serratia marcescens bioaerosol

Airborne transmission is a well-established mode of dissemination for infectious diseases, particularly in closed environments. However, previous research has often overlooked the potential impact of background particle concentration on bioaerosol characteristics. We compared the spatial and temporal distributions of bioaerosols under two levels of background particle concentration: heavily polluted (150-250 μg/m3) and excellent (0-35 μg/m3) in a typical ward. Serratia marcescens bioaerosol was adopted as a bioaerosol tracer, and the bioaerosol concentrations were quantified using six-stage Andersen cascade impactors. The results showed a significant reduction (over at least 62.9%) in bioaerosol concentration under heavily polluted levels compared to excellent levels at all sampling points. The temporal analysis also revealed that the decay rate of bioaerosols was higher (at least 0.654 min-1) under heavily polluted levels compared to excellent levels. These findings suggest that background particles can facilitate bioaerosol removal, contradicting the assumption made in previous research that background particle has no effect on bioaerosol characteristics. Furthermore, we observed differences in the size distribution of bioaerosols between the two levels of background particle concentration. The average bioaerosols size under heavily polluted levels was found to be higher than that under excellent levels, and the average particle size under heavily polluted levels gradually increased with time. In conclusion, these results highlight the importance of considering background particle concentration in future research on bioaerosol characteristics.

A review on indoor airborne transmission of COVID-19– modelling and mitigation approaches

In the past few years, significant efforts have been made to investigate the transmission of COVID-19. This paper provides a review of the COVID-19 airborne transmission modeling and mitigation strategies. The simulation models here are classified into airborne transmission infectious risk models and numerical approaches for spatiotemporal airborne transmissions. Mathematical descriptions and assumptions on which these models have been based are discussed. Input data used in previous simulation studies to assess the dispersion of COVID-19 are extracted and reported. Moreover, measurements performed to study the COVID-19 airborne transmission within indoor environments are introduced to support validations for anticipated future modeling studies. Transmission mitigation strategies recommended in recent studies have been classified to include modifying occupancy and ventilation operations, using filters and air purifiers, installing ultraviolet (UV) air disinfection systems, and personal protection compliance, such as wearing masks and social distancing. The application of mitigation strategies to various building types, such as educational, office, public, residential, and hospital, is reviewed. Recommendations for future works are also discussed based on the current apparent knowledge gaps covering both modeling and mitigation approaches. Our findings show that different transmission mitigation measures were recommended for various indoor environments; however, there is no conclusive work reporting their combined effects on the level of mitigation that may be achieved. Moreover, further studies should be conducted to understand better the balance between approaches to mitigating the viral transmissions in buildings and building energy consumption.

A CFD-based framework to assess airborne infection risk in buildings

The COVID-19 pandemic has prompted huge efforts to further the scientific knowledge of indoor ventilation and its relationship to airborne infection risk. Exhaled infectious aerosols are spread and inhaled as a result of room airflow characteristics. Many calculation methods and assertions on risk assume 'well-mixed' flow conditions. However, ventilation in buildings is complex and often not showing well-mixed conditions. Ventilation guidance is typically based on the provision of generic minimum ventilation flow rates for a given space, irrespective of the effectiveness in the delivery of the supply air. Furthermore, the airflow might be heavily affected by the season, the HVAC ventilation, or the opening of windows, which would potentially generate draughts and non-uniform conditions. As a result, fresh air concentration would be variable depending upon a susceptible receptor's position in a room and, therefore, associated airborne infection risk. A computational fluid dynamics (CFD) and dynamic thermal modelling (DTM) framework is proposed to assess the influence of internal airflow characteristics on airborne infection risk. A simple metric is proposed, the hourly airborne infection rate (HAI) which can easily help designers to stress-test the ventilation within a building under several conditions. A case study is presented, and the results clearly demonstrate the importance of understanding detailed indoor airflow characteristics and associated concentration patterns in order to provide detailed design guidance, e.g. occupancy, supply air diffusers and furniture layouts, to reduce airborne infection risk.

Transmission characteristics of infectious pathogen-laden aerosols in a negative-pressure operating room

The infectious pathogen-laden aerosols generated by infected patients have a significant impact on the safety of surgical staff in highly clean negative-pressure operating rooms. Understanding the transmission characteristics of infectious pathogen-laden aerosols is therefore essential. Therefore, in this study, we conducted experiments in a full-size negative-pressure operating room, and the Phi-X174 phage was used as a bioaerosol release source to investigate the migration and deposition of bioaerosols. The high-concentration spatial regions and high-concentration deposition surfaces of the bioaerosols in the operating room were determined. The results showed that there was a high concentration of bioaerosols in the vortex region below the medical lamp for extended periods of time. Three surgical staff members close to the patient's surgical site had high bioaerosol concentrations at their facial sampling points, with the highest concentration reaching 16,553 PFU/m³ . At the end of bioaerosol generation, 99.9% of the bioaerosols were discharged within 10 mins. The bioaerosol deposition rates per unit area were high at 1.48%/m², 0.46%/m² and 1.79%/m² for the central control panel, storage cabinet, and door surface, respectively. This research can be used as a scientific reference for controlling bioaerosols and determining key disinfection parts in a negative-pressure operating room.

Estimating the restraint of SARS-CoV-2 spread using a conventional medical air-cleaning device: Based on an experiment in a typical dental clinical setting

OBJECTIVES

Droplets or aerosols loaded with SARS-CoV-2 can be released during breathing, coughing, or sneezing from COVID-19-infected persons. To investigate whether the most commonly applied air-cleaning device in dental clinics, the oral spray suction machine (OSSM), can provide protection to healthcare providers working in clinics against exposure to bioaerosols during dental treatment.

METHOD

In this study, we measured and characterized the temporal and spatial variations in bioaerosol concentration and deposition with and without the use of the OSSM using an experimental design in a dental clinic setting. Serratia marcescens (a bacterium) and ΦX174 phage (a virus) were used as tracers. The air sampling points were sampled using an Anderson six-stage sampler, and the surface-deposition sampling points were sampled using the natural sedimentation method. The Computational Fluid Dynamics method was adopted to simulate and visualize the effect of the OSSM on the concentration spatial distribution.

RESULTS

During dental treatment, the peak exposure concentration increased by up to 2-3 orders of magnitude (PFU/m³) for healthcare workers. Meanwhile, OSSM could lower the mean bioaerosol exposure concentration from 58.84 PFU/m³ to 4.10 PFU/m³ for a healthcare worker, thereby inhibiting droplet and airborne transmission. In terms of deposition, OSSM significantly reduced the bioaerosol surface concentration from 28.1 PFU/m³ to 2.5 PFU/m³ for a surface, effectively preventing fomite transmission.

CONCLUSION

The use of OSSM showed the potential to restraint the spread of bioaerosols in clinical settings. Our study demonstrates that OSSM use in dental clinics can reduce the exposure concentrations of bioaerosols for healthcare workers during dental treatment and is beneficial for minimizing the risk of infectious diseases such as COVID-19.

New dose-response model and SARS-CoV-2 quanta emission rates for calculating the long-range airborne infection risk

Predictive models for airborne infection risk have been extensively used during the pandemic, but there is yet still no consensus on a common approach, which may create misinterpretation of results among public health experts and engineers designing building ventilation. In this study we applied the latest data on viral load, aerosol droplet sizes and removal mechanisms to improve the Wells Riley model by introducing the following novelties i) a new model to calculate the total volume of respiratory fluid exhaled per unit time ii) developing a novel viral dose-based generation rate model for dehydrated droplets after expiration iii) deriving a novel quanta-RNA relationship for various strains of SARS-CoV-2 iv) proposing a method to account for the incomplete mixing conditions. These new approaches considerably changed previous estimates and allowed to determine more accurate average quanta emission rates including omicron variant. These quanta values for the original strain of 0.13 and 3.8 quanta/h for breathing and speaking and the virus variant multipliers may be used for simple hand calculations of probability of infection or with developed model operating with six size ranges of aerosol droplets to calculate the effect of ventilation and other removal mechanisms. The model developed is made available as an open-source tool.

Finding the proper position of supply and return registers of air condition system in a conference hall in term of COVID-19 virus spread

The outbreak of the COVID-19 has affected all aspects of people's lives around the world. As air transmits the viruses, air-conditioning systems in buildings, surrounded environments, and public transport have a significant role in restricting the transmission of airborne pathogens. In this paper, a computational fluid dynamic (CFD) model is deployed to simulate the dispersion of the COVID-19 virus due to the coughing of a patient in a conference hall, and the effect of displacement of supply and return registers of the air conditioning system is investigated. A validated Eulerian-Lagrangian CFD model is used to simulate the airflow in the conference hall. The particles created by coughing are droplets of the patient's saliva that contain the virus. Three cases with different positions of supply and return registers have been compared. The simulation results show that case1 has the best performance; since after 80 s in case 1 that the inlet registers are in the longitudinal wall, the whole particles are removed from space. However, in other cases, some particles are still in space.

Development of a Novel Tabletop Device With Suction and Sanitization of Droplets against COVID-19

Background Coronavirus disease 2019 and other viruses are transmissible by aerosols and droplets from infected persons. This study aimed to develop a portable device that can trap droplets and deactivate viruses, and verify whether the device in an enclosed room can suction droplets and sanitize them using a filter and an ultraviolet-C (UVC) light-emitting diode. Materials and methods The portable device was evaluated by placing it 50 cm away from the droplet initiation point. A particle image velocimetry laser dispersed into a sheet form was used to visualize the droplets splashed on the irradiated sagittal plane and captured using a charge-coupled device camera at 60 frames per second. The images were overlaid and calculated to determine the percentage of the droplets beyond the portable device. Droplets with a particle size larger than 50 µm that dispersed and were deposited more than 100 cm away were measured using a water-sensitive paper. The effect of UVC sanitization on viruses captured by a high-efficiency particulate air (HEPA) filter was determined using a plaque assay. Results The percentage of droplets was 13.4% and 1.1% with the portable device OFF and ON, respectively, indicating a 91.8% reduction. The deposited droplets were 86 pixels and 26 pixels with the portable device OFF and ON, respectively, indicating a 68.7% reduction. The UVC deactivated more than 99% of the viruses on the HEPA filter surface in 5 minutes. Conclusions Our novel portable device can suck and fall the dispersed droplets, and an active virus was not observed on the exhaust side.

Association between the infection probability of COVID-19 and ventilation rates: An update for SARS-CoV-2 variants

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the current coronavirus disease 2019 (COVID-19) pandemic, is evolving. Thus, the risk of airborne transmission in confined spaces may be higher, and corresponding precautions should be re-appraised. Here, we obtained the quantum generation rate (q) value of three SARS-CoV-2 variants (Alpha, Delta, and Omicron) for the Wells-Riley equation with a reproductive number-based fitted approach and estimated the association between the infection probability and ventilation rates. The q value was 89-165 h^-1 for Alpha variant, 312-935 h^-1 for Delta variant, and 725-2,345 h^-1 for Omicron variant. The ventilation rates increased to ensure an infection probability of less than 1%, and were 8,000-14,000 m³ h^-1, 26,000-80,000 m³ h^-1, and 64,000-250,000 m³ h^-1 per infector for the Alpha, Delta, and Omicron variants, respectively. If the infector and susceptible person wore N95 masks, the required ventilation rates decreased to about 1/100 of the values required without masks, which can be achieved in most typical scenarios. An air purifier was ineffective for reducing transmission when used in scenarios without masks. Preventing prolonged exposure time in confined spaces remains critical in reducing the risk of airborne transmission for highly contagious SARS-CoV-2 variants.

Evaluation of SARS-CoV-2 transmission in COVID-19 isolation wards: On-site sampling and numerical analysis

Although airborne transmission has been considered as a possible route for the spread of SARS-CoV-2, the role that aerosols play in SARS-CoV-2 transmission is still controversial. This study evaluated the airborne transmission of SARS-CoV-2 in COVID-19 isolation wards at Prince of Wales Hospital in Hong Kong by both on-site sampling and numerical analysis. A total of 838 air samples and 1176 surface samples were collected, and SARS-CoV-2 RNA was detected using the RT-PCR method. Testing revealed that 2.3% of the air samples and 9.3% of the surface samples were positive, indicating that the isolation wards were contaminated with the virus. The dispersion and deposition of exhaled particles in the wards were calculated by computational fluid dynamics (CFD) simulations. The calculated accumulated number of particles collected at the air sampling points was closely correlated with the SARS-CoV-2 positive rates from the field sampling, which confirmed the possibility of airborne transmission. Furthermore, three potential intervention strategies, i.e., the use of curtains, ceiling-mounted air cleaners, and periodic ventilation, were numerically investigated to explore effective control measures in isolation wards. According to the results, the use of ceiling-mounted air cleaners is effective in reducing the airborne transmission of SARS-CoV-2 in such wards.

Temporal and spatial far-ultraviolet disinfection of exhaled bioaerosols in a mechanically ventilated space

Far-UVC with a peak wavelength of 222 nm can potentially be used to inactivate exhaled bioaerosols in an efficient and safe manner. Therefore, this study aimed to experimentally explore the effectiveness of a 222 nm far-UVC light for inactivating bioaerosols, represented by E. coli, exhaled from a manikin in a chamber with mechanical ventilation. The spatial irradiance distribution from the far-UVC light was measured. The susceptibility constant (z-value) for E. coli under the far-UVC light was experimentally obtained. The temporal and spatial concentrations of the bioaerosols exhaled from the manikin were measured under three typical ventilation rates (4, 10, and 36 ACH). According to the results, when the far-UVC light was turned on, the bioaerosol concentrations were lower than those without the far-UVC light under all three ventilation rates, suggesting that far-UVC light can effectively disinfect E. coli under mechanical ventilation. However, the disinfection efficiency of the far-UVC light decreased as the ventilation rate increased, which indicated that the far-UVC light played a more important role in bioaerosol removal under a lower ventilation rate. In general, the results supported the feasibility of using 222 nm far-UVC light for disinfection of exhaled bioaerosols in mechanically ventilated spaces to reduce infection risks.