High Ventilation Rates Research Articles

Human exposure to air contaminants under the far-UVC system operation in an office: effects of lamp position and ventilation condition

The far-UVC (222 nm) system has emerged as a solution for controlling airborne transmission, yet its effect on indoor air quality, particularly concerning positioning, remains understudied. In this study, we examined the impact of far-UVC lamp position on the disinfection and secondary contaminant formation in a small office. We employed a three-dimensional computational fluid dynamics (CFD) model to integrate UV intensity fields formed by different lamp positions (ceiling-mounted, wall-mounted, and stand-alone types) along with the air quality model. Our findings reveal that the ceiling-mounted type reduces human exposure to airborne pathogens by up to 80% compared to scenarios without far-UVC. For all the lamp positions, O3 concentration in the breathing zone increases by 4–6 ppb after one hour of operation. However, it should be noted that a high concentration zone (> 25 ppb) forms near the lamp when it is turned on. Moreover, ventilation plays a crucial role in determining human exposure to airborne pathogens and secondary contaminants. Increasing the ventilation rate from 0.7 h−1 to 4 h−1 reduces airborne pathogen and secondary contaminant concentrations by up to 90%. However, caution is warranted as higher ventilation rates can lead to elevated O3 indoors, especially under conditions of high outdoor O3 concentrations.

Ventilation effectiveness and incomplete mixing in air distribution design for airborne transmission

How ventilation should be arranged to be effective at reasonable air change rates is one key question as ventilation criteria and standard airborne disease transmission models are based on the well-mixed assumption, but air distribution patterns lead to non-uniform spatial concentrations. In this study a new method for ventilation effectiveness application in ventilation design for airborne transmission was developed and tested with tracer gas measurements in 22 rooms. Contrary to existing ventilation effectiveness values measured with distributed source, the developed method uses a couple of point source locations corresponding to an infector to quantify infection risk for each occupant. Novelty of the method is new ventilation effectiveness indicator that makes it possible to describe the effect of spatial variation of concentration and risk with single parameter. Quanta were used as input data to calculate the ventilation rate supplied by air distribution system corresponding to a specified risk level, but the differences between studied cases do not significantly depend on the quanta values. Application of the method to measured rooms showed that simple ventilation effectiveness calculation from average concentration at the breathing height, not requiring quanta data, provided lower ventilation effectiveness and higher ventilation rate in all cases. In many cases the difference in required ventilation rates was only a few percent, but in some large spaces it exceeded 10% with maximum of 39% in large open plan office with high concentration differences. Measured ventilation effectiveness values ranging from 0.5 to 1.4 indicate a substantial improvement potential in many cases.

Experimental and numerical study on upper-room Far-UVC system under different ventilation schemes to disinfect airborne microorganisms in indoor environments

Many deadly infectious diseases are transmitted through airborne routes in indoor environments. Far UV upper-room system combined with proper ventilation is regarded as an effective approach to disinfect airborne microorganisms. Despite the current advances in the far UV upper-room system, various types of data are still lacking to guide the efficient design and the safe operation. This study combines field air sampling, laboratory experiments and computational fluid dynamics simulations to advance the knowledge on the potential of far UV radiation for disinfection and its safety design. Our findings reveal that seven out of the eight measured room spaces are categorised as slightly polluted (1201–2000 CFU m−3) or polluted (>2000 CFU m−3). Proteobacteria and Firmicutes constitute 65.2 ± 10.6 % and 21.2 ± 5.68 % of the bacterial composition in congregate settings, respectively. Ultraviolet radiation proves effective in deactivating pathogenic microorganisms, characterized by a decay rate constant ranging from 0.0446 m2 J−1 to 0.464 m2 J−1, depending on the wavelength and the type of pathogenic microorganisms. The time response of Ultraviolet radiation is up to 12 min to achieve 92.1 % sterilisation. Despite the reduction in the sterilisation ratio of Ultraviolet radiation, maintaining a high ventilation rate is highly recommended to eliminate genetic material from pathogenic microorganisms. When the excimer lamp is installed higher than 1.9 m above the floor level, the radiation intensities at both standing and sitting height become lower than the safety threshold value of 0.166 W m−2. A combination of UV radiation at 222 nm and ventilation is effective for neutralising both gram-positive and -negative bacteria and can contribute to stop the spread of infectious diseases in congregate settings.

Experimental study on the effect of longitudinal ventilation on spill fire characteristics in sloped tunnels

A series of spill fire tests were carried out in a reduced-scale tunnel (6 m × 0.6 m × 0.45 m), considering different discharge rates (60–140 ml/min), ventilation rates (0∼1.26 m/s), and tunnel slopes (3 %, 5 %, 7 %). Some crucial characteristic parameters during the quasi-steady combustion stage of tunnel spill fires were analyzed, including the diffusion shape of the liquid fuel, mass burning rate per unit area, flame morphology and plume temperature, and the critical ventilation velocity. The results show that as the tunnel slope increases, the diffusion shape of the liquid fuel transitions from circular to an ellipse shape and then to rectangular or linear shape. Moreover, the mass burning rate per unit area decreases with increasing ventilation rates at low ventilation (V < 0.5 m/s), while it remains almost constant at high ventilation rates (V ≥ 0.5 m/s). This could be attributed to variations in flame tilt behavior under different ventilation conditions. Predictive models for the flame tilt angle and the vertical temperature distribution of tunnel spill fires under the effect of ventilation were further established and verified. Furthermore, the critical ventilation velocity of tunnel spill fires remains essentially unchanged as the discharge rate (or fire source power) increases. This phenomenon was explained by analyzing the resistance of the spill fire plume and the buoyancy generated by the smoke temperature. This work could serve as a critical reference for assessing disaster risks and executing emergency rescue operations during tunnel spill fires.

Investigation of the effects of indoor air stability on the airborne transmission of droplets from violent respiratory events

Indoor air stability is a condition under which the indoor temperature gradients can affect the trajectories of constant breathing flows. However, it remains unclear how indoor air stability affects the airborne contaminants from violent respiratory events such as coughing. Here, we report a study of the dispersion of airborne droplets from coughing under different indoor air stability conditions. The distributions of droplets from coughing processes under stable and unstable conditions were compared. The results revealed that the stable conditions confined the coughed droplets to the breathing zone of the droplet host, whereas the unstable conditions intensified the dispersion of droplets and reduced their local concentration. The dimensionless vertical distance of the droplet cloud under the stable condition was 4 times longer than that of the unstable condition at t = 15 s. In addition, we observed that high ventilation rate caused an intense mixing of the indoor air, thus weakening the effect of indoor air stability on the propagation of droplet cloud. Personal exposure of the stable condition was 6.5 times larger than that of the unstable condition at a ventilation rate of 2.1 ACH, while it decreased to lower than half of that of the unstable condition at 7.1 ACH and 12.3 ACH. Personal exposure to the indoor environment should be assessed by simultaneously considering the indoor air stability conditions and ventilation rates. This study has provided insights into the transmission of cough droplets in indoor environments and has practical significance for preventing the spread of contagious diseases.

Tritium management in ITER test blanket systems port cell for maintenance operations

Four Test Blanket Systems (TBS) will be tested in the International Thermonuclear Experimental Reactor equatorial ports #16 and #18 to verify tritium breeding and heat extraction technology. A significant quantity of tritium would be produced in TBM, and partly released into the port cell from the pipework of TBS or other high-temperature components due to its strong mobility and high permeation. The port cell should be accessible during equipment maintenance and human intervention. This work built a multi-dimensional geometric model to characterize HTO transport in the port cell, absorption/desorption, and diffusion in walls and discussed the effect of paint thickness, ventilation rate, source term, and epoxy properties on detritiation efficiency. The results suggest that a 0.1–0.16 mm paint with the lowest HTO solubility is optimal from the compromise between quick cleanup and tritiated waste decommission. A higher ventilation rate could accelerate detritiation while minimizing the radioactive source by a tritium-resisting layer is the most direct method. The optimized design options for reducing the time required to reach 1 DAC in 12 h still need further discussion because of the delayed HTO source from epoxy paint and dead zone of the flow field.

Dynamic and stationary monitoring of air pollutant exposures and dose during marathons

Marathon running significantly increases breathing volumes and, consequently, air pollution inhalation doses. This is of special concern for elite athletes who ventilate at very high rates. However, race organizers and sport governing bodies have little guidance to support events scheduling to protect runners. A key limitation is the lack of hyper-local, high temporal resolution air quality data representative of exposure along the racecourse. This work aimed to understand the air pollution exposures and dose inhaled by athletes, by means of a dynamic monitoring methodology designed for road races. Air quality monitors were deployed during three marathons, monitoring nitrogen dioxide (NO2), ozone (O3), particulate matter (PMx), air temperature, and relative humidity. One fixed monitor was installed at the Start/Finish line and one mobile monitor followed the women elite runner pack. The data from the fixed monitors, deployed prior the race, described daily air pollution trends. Mobile monitors in combination with heatmap analysis facilitated the hyper-local characterization of athletes' exposures and helped identify local hotspots (e.g., areas prone to PM resuspension) which should be preferably bypassed. The estimation of inhaled doses disaggregated by gender and ventilation showed that doses inhaled by last finishers may be equal or higher than those inhaled by first finishers for O3 and PMx, due to longer exposures as well as the increase of these pollutants over time (e.g., 58.2 ± 9.6 and 72.1 ± 23.7 μg of PM2.5 for first and last man during Rome marathon). Similarly, men received significantly higher doses than women due to their higher ventilation rate, with differences of 31–114 μg for NO2, 79–232 μg for O3, and 6–41 μg for PMx. Finally, the aggregated data obtained during the 4 week- period prior the marathon can support better race scheduling by the organizers and provide actionable information to mitigate air pollution impacts on athletes' health and performance.

A new tool for energy conservation in operating room: The role of machine learning models in enhancing airflow control

Maintaining high ventilation rates in operating rooms (ORs) is critical to ensure the safety of healthcare professionals and patients. Yet, the significant energy consumption associated with ventilation systems is a growing global concern. While traditional approaches have predominantly focused on optimizing ventilation design, limited attention has been given to operational control strategies. This study aims to control ceiling air supply speed by exposure aerosol concentrations exhaled by the patient. It achieves this by integrating empirical data with machine learning (ML) models, effectively balancing the need for maintaining optimal ventilation conditions and ensuring a safe aerosol exposure concentration. Experimental results from a full-scale OR setup, manipulating ceiling air supply speed, respiratory flow rate, and surgical bed height, generated 81 datasets. These datasets served as the basis for assessing the performance of 11 ML models. Notably, the extra trees (ET) and multi-layer perceptron (MLP) models outperformed the others, exhibiting impressive coefficient of determination (R2) values of 0.954 and 0.904, respectively. Moreover, the models were further enhanced by assessing their robustness with Gaussian noise and interval prediction. This study presents a novel approach that holds significant relevance for the practical control of ventilation systems, particularly with regard to health and energy efficiency considerations.

Optimization of the Air-Conditioning Energy Performance and Daylight Performance of AResidential Building According to the Bioclimatic Design Principles: An Application to the Moroccan Mediterranean

The bioclimatic architecture concept refers to an approach that takes into account the various characteristics of a building environment to make it more comfortable for its occupants. This work aims to improve the passive design parameters of an air-conditioned residential building located in the north of Morocco, in accordance with bioclimatic principles. The bioclimatic chart diagram is used to select the passive design measures that are the most appropriate for the north Morocco climate characteristics. Then, a set of design parameters are selected for more delimitation in the optimization study. The optimization problem is multi-objective and aims to find the design solution that simultaneously includes the best air-conditioning energy performance and daylight performance. The obtained results showed that the multi-objective optimum design solution is characterized by massive walls and roof, exterior insulation, double window glazing type, and a high summer ventilation rate. Also, a small glazing area with a large sun-shading covering is needed for the east facade. Ultimately, the building performance analysis revealed that the optimum bioclimatic design solution fully meets the requirements establishedby the Moroccan Building Thermal Regulation (MBTR), leading to an energy performance improvement of about 52%.

The effects of ventilation conditions on mitigating airborne virus transmission

The effects of ventilation strategies on mitigating airborne virus transmission in a generic indoor space representative of a lobby area or information desk found in a hotel, company, or cruise ship are presented. Multiphase computational fluid dynamics simulations are employed in conjunction with evaporation modeling. Four different ventilation flow rates are examined based on the most updated post-COVID-19 pandemic standards from health organizations and recent findings from research studies. Three air changes per hour provide the best option for minimizing droplet spreading at reasonable energy efficiency. Thus, a higher ventilation rate is not the best solution to avoid spreading airborne diseases. Simultaneous coughing of all occupants revealed that contagious droplets could be trapped in regions of low airflow and on furniture, significantly prolonging their evaporation time. Multiphase flow simulations can help define ventilation standards to reduce droplet spreading and mitigate virus transmission while maintaining adequate ventilation with lower energy consumption. The present work significantly impacts how heat, air-conditioning, and ventilation systems are designed and implemented.

The effects of desk-partition layout on cough droplet dynamics relating to seasonal influenza

The use of physical dividers as control measures for short-range viral transmission in indoor settings has gained increasing attention. However, the understanding regarding their correct usage under different seating arrangements is incomplete. In this study, we focused on assessing the effectiveness of physical dividers in impeding the transient transmission of cough droplets in three representative layouts using the large-eddy simulation technique with the Eulerian–Lagrangian model. We computationally also investigated the effects of ventilation on viral droplet transmission under three representative desk-divider layouts. The ventilation approach was tested using two different ventilation rates (1.0 and 1.8 m/s). A comparative analysis of the ventilation flow fields, topologies, and particle directions has been studied. The findings indicate that the sitting arrangements, ventilation rates, and partition layouts play a significant role in designing effective control measures for indoor infection under the office setup considered. The protected occupied zone ventilation (POV) worked best in situations of low ventilation (1.0 m/s) for cross-partition in protecting the three healthy persons. In addition, the POV performed well for low (1.0 m/s) and high (1.8 m/s) ventilation rates for face-to-face layout. The side-by-side configuration performed poorly for the POV considered in protecting the healthy person seated directly opposite the infected person. The numerical predictions may be used to validate other experimental studies and educate office workers and engineers on indoor airborne infection control.

Performance improvement of a biotechnology vaccine cleanroom for contamination control

Cleanrooms play an important role in various industries, such as pharmaceuticals, specifically for vaccine cleanrooms by strictly regulating contaminants, temperature, humidity, and pressure to ensure product reliability, particularly in critical areas. This study investigates a biotechnology vaccine cleanroom using a comprehensive field measurement test and numerical simulation analysis. Field measurements were conducted during the at-rest and operational occupancy states. As a result, it was discovered that the operational condition has a higher concentration than the at-rest condition. To mitigate this issue, the numerical simulation improvement strategy employed in this study aimed to reduce the concentration under operational conditions. The strategy proposed in this simulation is determining the ventilation rate and arranging the outlet air grilles. To assess and evaluate the performance of the cleanroom, we utilized ventilation and removal efficiency metrics. These indices served as important metrics for evaluating the cleanroom's efficiency in controlling contaminants. In at-rest conditions, ventilation and removal efficiency were 90.0 % and 86.2 %, respectively. However, under operational conditions, these metrics dropped to 81.3 % and 80.1 %, respectively, in the baseline case. After the proposed improvement design, the ventilation and removal efficiency in the operational condition could improve by up to 6.8 % and 5.2 %, respectively. A proper arrangement of outlet air grilles could control the decrease in concentration. In addition, a higher ventilation rate also could decrease the concentration. However, it caused high energy consumption. Therefore, the ventilation rate must be designed optimally to compromise contamination control and energy saving.

Impacts of energy saving measures on IEQ, task performance and COVID-19 contagion risk in public buildings: analysis of a case-study in Bozen-Bolzano, Italy

The recent 2022 energy crisis has pushed several Italian public administrations to adopt solutions to reduce energy demand for space heating in public buildings. Reduction of HVAC system operation time, decrease of heating setpoint and changes to ventilation schedules are some examples of typically implemented measures. However, some of them are in direct competition with those adopted so far to limit the risk of COVID-19 contagion, such as higher ventilation rates. Furthermore, limited operation time and lower setpoints can lead to suboptimal thermal comfort conditions for workers in public buildings, with subsequent decrease of their task performances. In this framework, to assess the global efficacy of the most recent energy saving policies in public buildings, this research focuses not only on the quantification of energy savings but also on the analysis of increased COVID-19 contagion risk, as well as performance loss due to suboptimal building operation conditions. To investigate the impact of new building operation policies for energy savings, an intermediate storey of a public office building in Bolzano, Italy, was selected as a case-study.

Impact of street canyon morphology on heat and fluid flow: An experimental water tunnel study using simultaneous PIV-LIF technique

Urban areas are known for their complex atmospheric environments, with the building morphology having a significant impact on local climate patterns, air quality, and overall urban microclimate. Understanding the heat transport and fluid flow in complex urban environments is crucial for improving urban climate resilience, which remains an open frontier in the field of urban studies. To gain a more profound insight into the physical processes occurring in urban areas, particularly within street canyons, we conducted an experimental investigation in a large-scale water tunnel. This study involved the simultaneous examination of heat and flow fields, carried out at high spatial and temporal resolutions, utilizing Laser-induced Fluorescence (LIF) for heat analysis and Particle Image Velocimetry (PIV) for flow analysis. Our results of heat and flow in different street canyons indicate that the flow is significantly influenced by a combination of factors, including canyon configuration, the presence of buoyant force, and the magnitude of the approaching flow. The ventilation rate and heat flux from the street canyon, which are key factors shaping the urban microclimate, are found dominated significantly by the street canyon morphology. For instance, changing the aspect ratio of a street canyon results in a significant change of air ventilation rate, ranging from as low as 0.02 to as high as 1.5 under the same flow conditions. Additionally, canyons with high air ventilation rates exhibit significant heat flux removal at the canyon roof level, which is accurately described by the local Richardson number.

Virus spreading in cruiser cabin

This paper concerns virus droplet simulations in a typical cruiser's cabin. Effects of ventilation rates and positions of the coughing person were investigated. The study also emphasizes the importance of including evaporation models to simulate the process accurately. A higher ventilation rate is not always the best strategy to avoid the spread of airborne diseases, as saliva droplets can spread further at high ventilation rates. Regardless of the ventilation strategy, they evaporate faster than the room's air renewal. One should aim for minimum droplet spreading inside the cabin and different ventilation strategies for occupied cabins. The authors propose using ventilation systems at medium flow rates of around 120 m3/h or three air changes per hour when a cabin is occupied. This value is also close to the recommended value of 108 m3/h from the latest standard by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. The suggested value minimizes droplet spreading while maintaining good ventilation, comfort, and energy consumption.

Simulation and analysis of wind and thermal environments in an office with window shutters

By designing an office with window shutters and setting different window sizes, the indoor wind and thermal environments were investigated at different heights. Opening windows improved the thermal conditions in the office, especially in high temperature rooms. Despite generation of high ventilation rates at many locations, the ventilation rates were good within the main areas. It appeared critical to supply air through opening of the windows. A novel simulation method was explored. Meanwhile, an appropriate numerical simulation scheme was chosen to evaluate the effects of various office structures on thermal comfort, which can thus provide an important reference for the office building design.

Systemic Anticancer Therapy and Thromboembolic Outcomes in Hospitalized Patients With Cancer and COVID-19

Systematic data on the association between anticancer therapies and thromboembolic events (TEEs) in patients with COVID-19 are lacking. To assess the association between anticancer therapy exposure within 3 months prior to COVID-19 and TEEs following COVID-19 diagnosis in patients with cancer. This registry-based retrospective cohort study included patients who were hospitalized and had active cancer and laboratory-confirmed SARS-CoV-2 infection. Data were accrued from March 2020 to December 2021 and analyzed from December 2021 to October 2022. Treatments of interest (TOIs) (endocrine therapy, vascular endothelial growth factor inhibitors/tyrosine kinase inhibitors [VEGFis/TKIs], immunomodulators [IMiDs], immune checkpoint inhibitors [ICIs], chemotherapy) vs reference (no systemic therapy) in 3 months prior to COVID-19. Main outcomes were (1) venous thromboembolism (VTE) and (2) arterial thromboembolism (ATE). Secondary outcome was severity of COVID-19 (rates of intensive care unit admission, mechanical ventilation, 30-day all-cause mortality following TEEs in TOI vs reference group) at 30-day follow-up. Of 4988 hospitalized patients with cancer (median [IQR] age, 69 [59-78] years; 2608 [52%] male), 1869 had received 1 or more TOIs. Incidence of VTE was higher in all TOI groups: endocrine therapy, 7%; VEGFis/TKIs, 10%; IMiDs, 8%; ICIs, 12%; and chemotherapy, 10%, compared with patients not receiving systemic therapies (6%). In multivariable log-binomial regression analyses, relative risk of VTE (adjusted risk ratio [aRR], 1.33; 95% CI, 1.04-1.69) but not ATE (aRR, 0.81; 95% CI, 0.56-1.16) was significantly higher in those exposed to all TOIs pooled together vs those with no exposure. Among individual drugs, ICIs were significantly associated with VTE (aRR, 1.45; 95% CI, 1.01-2.07). Also noted were significant associations between VTE and active and progressing cancer (aRR, 1.43; 95% CI, 1.01-2.03), history of VTE (aRR, 3.10; 95% CI, 2.38-4.04), and high-risk site of cancer (aRR, 1.42; 95% CI, 1.14-1.75). Black patients had a higher risk of TEEs (aRR, 1.24; 95% CI, 1.03-1.50) than White patients. Patients with TEEs had high intensive care unit admission (46%) and mechanical ventilation (31%) rates. Relative risk of death in patients with TEEs was higher in those exposed to TOIs vs not (aRR, 1.12; 95% CI, 0.91-1.38) and was significantly associated with poor performance status (aRR, 1.77; 95% CI, 1.30-2.40) and active/progressing cancer (aRR, 1.55; 95% CI, 1.13-2.13). In this cohort study, relative risk of developing VTE was high among patients receiving TOIs and varied by the type of therapy, underlying risk factors, and demographics, such as race and ethnicity. These findings highlight the need for close monitoring and perhaps personalized thromboprophylaxis to prevent morbidity and mortality associated with COVID-19-related thromboembolism in patients with cancer.

The impact of climate change on cognitive performance of children in English school stock: A simulation study

Children in England spend around 30% of their time in schools to gain knowledge and skills. Climate change could impact schools' thermal environments and children's learning performance by impairing their cognitive ability. This study presents an evaluation approach to investigating and quantifying climate change's impact on the cognitive performance of children across English school stocks. The study also evaluates the potential of possible strategies for mitigating the impacts of climate change. The results show that future climates are projected to increase cognitive performance loss of children in school archetypes representative of school stocks, with variations based on regional climate characteristics. Increasing ventilation rates proves to be an effective means of reducing cognitive performance loss, while its effectiveness diminishes as outdoor temperatures rise in the future. Thus, the introduction of air conditioning becomes a potentially more beneficial strategy, despite the associated increase in cooling energy demand. Moreover, higher ventilation rates in air-conditioned classrooms can further improve children's cognitive performance. The use of cognitive performance loss as a Key Performance Indicator (KPI) allows for better communication and understanding of climate change risks faced by schools among building and non-building experts. The proposed evaluation approach remains adjustable and can be continuously updated and enhanced as new insights from psychological research emerge.

Numerical study of the effects of ventilation rates on supercavity dynamics and acoustic characteristics

This paper presents a numerical simulation investigation of the hydrodynamic and acoustic characteristics of ventilated supercavities that operate at various ventilation rates. The large eddy simulation (LES) and Ffowcs Williams–Hawkings (FW-H) methods are used to solve the unsteady flow field and noise. The results show that the ventilation rate plays an important role in cavity closure forms and can improve hydrodynamic performance. There is a transition from re-entrant flow closure to a stable double-vortex tube closure form at the supercavity tail as the ventilation rate increases. Meanwhile, under the condition of a high ventilation rate, the re-entrant flow region in the cavity gradually shrinks and hairpin vortices appear, the body length of the cavity increases,and the unsteadiness at the supercavity tail is weakened. However, the turbulence intensity and integral scale both increase with the ventilation rate. Moreover, it is found that the noise is closely related to supercavity closure form. Ventilated supercavities with closure forms of double-vortex tubes have better noise performance. Interestingly, as the ventilation rate increases, the total sound pressure levels are improved. The wavenumber-frequency spectra of supercavity turbulent pulsating pressures under studied ventilation rates are similar, but there are some differences in some local areas.

Ventilation strategy for simultaneous management of indoor particulate matter and airborne transmission risks – A case study for urban schools in South Korea

During the COVID-19 pandemic, schools in South Korea augmented ventilation rates by opening windows to avoid airborne transmission risks. However, this also elevated indoor PM2.5 concentrations, creating a trade-off problem between the two risks.This study quantitatively analyzed the impact of ventilation rate on indoor PM2.5 concentrations and COVID-19 transmission risk in a typical classroom. The results showed that indoor PM2.5 concentrations asymptotically approached outdoor ones with the augmentation of ventilation. The impact of ventilation augmentation on indoor concentration decreased with the ventilation rate. The time evolution of air transmission risk was obtained from the balance equation of quanta generated by infected people to estimate the infection probability and effective reproduction number. The transmission risk was a complex function of the ventilation rate, quanta generation rate per infected person, and mask filtration efficiency.The combination of information from the analyses of the two risks provides feasible solutions to the problem of resolving the two conflicting risks simultaneously. For example, high ventilation rates are allowable in mild climates with low outdoor PM concentrations. They can ease mask filtration efficiencies and elevate allowable activity levels while wearing high-efficiency masks and limiting students' activity levels must be enforced under cold climates with high outdoor PM concentrations.This study provides important insights into the trade-off between ventilation rate and COVID-19 transmission risk in classrooms. In addition, the results can be used to develop strategies to mitigate both risks and ensure the safety of students and staff.