Upper-room Upper-room Ultraviolet Germicidal Irradiation Research Articles

Evaluation of upper-room ultraviolet germicidal irradiation system for air disinfection and energy conservation in multi-patient ward

High occupancy rates significantly increase the risk of airborne transmission of respiratory infectious diseases in multi-patient wards. While increasing air supply can effectively reduce airborne transmission, it also substantially raises energy consumption. Therefore, developing low-energy methods in multi-patient wards to suppress airborne transmission is essential. This study aims to evaluate the inactivation performance of upper-room ultraviolet germicidal irradiation (UVGI) against SARS-CoV-2 virus aerosols in a typical multi-patient ward using computational fluid dynamics at different ventilation rates, as well as its energy-saving effects. Upper-room UVGI reduced the virus aerosol concentrations, with a maximum decrease of up to 159 copies/m3 in the occupied zone (zone less than 2.1 m in height) observed at 8 ACH. However, increased ventilation rates limited the inactivation performance of upper-room UVGI. Two parameters of UVGI, irradiation height and irradiation flux, had been detailly evaluated. Elevating the irradiation height of upper-room UVGI could significantly enhance the inactivation performance. Increasing irradiation height from 0.3 to 0.6 m reduced the concentrations in the occupied zone by up to 11 copies/m3, while a further increase to 0.9 m decreased the concentrations by up to 55 copies/m3. However, increasing irradiation flux could only approximately linearly enhance the virus aerosol inactivation performance of upper-room UVGI. The energy consumption evaluation of upper-room UVGI shows that UVGI activation could conserve 100 kWh per year in one multi-patient ward, with a maximum saving of 333.7 kWh. These findings provide insights into the setups strategies and the energy-saving performance of upper-room UVGI systems in multi-patient wards.

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.

CFD modeling of room airflow effects on inactivation of aerosol SARS-CoV-2 by an upper-room ultraviolet germicidal irradiation (UVGI) system

Ultraviolet germicidal irradiation (UVGI) systems inactivate microorganisms indoors. Upper-room UVGI systems use wall- or ceiling-mounted fixtures to create an air disinfection zone above the occupied zone. The performance of upper-room UVGI systems varies with indoor airflow patterns induced by mechanical ventilation and thermal plumes from indoor heat sources. Little information is available on the effects of ventilation strategies on upper-room UVGI system performance for the control of viral aerosols in occupied spaces. This study simulated the effects of ventilation system characteristics in an office space on the ability of an upper-room UVGI system to inactivate viral aerosols with UV-C susceptibility representative of coronaviruses. UVGI reduced viral aerosol concentration by two orders of magnitude relative to the concentration without UVGI. Air change rates and air distribution strategy (mixing vs. displacement) had notable effects on the effectiveness of the UVGI system. For mixing ventilation, as the recirculation airflow rate increased from 0 to 5.3 h−1 for a room volume of 108 m3 with a fixed outdoor air change rate of 0.7 h−1, UVGI inactivation increased by 96.7%. Mixing ventilation with 100% outdoor air of 0.7 h−1 yielded airborne virus inactivation that was double that of displacement ventilation, due to enhanced air mixing.

Effect of ventilation strategy on performance of upper-room ultraviolet germicidal irradiation (UVGI) system in a learning environment

Upper-room ultraviolet germicidal irradiation (UVGI) system is recently in the limelight as a potentially effective method to mitigate the risk of airborne virus infection in indoor environments. However, few studies quantitatively evaluated the relationship between ventilation effectiveness and virus disinfection performance of a UVGI system. The objective of this study is to investigate the effects of ventilation strategy on detailed airflow distributions and UVGI disinfection performance in an occupied classroom. Three-dimensional computational fluid dynamics (CFD) simulations were performed for representative cooling, heating, and ventilation scenarios. The results show that when the ventilation rate is 1.1 h−1 (the minimum ventilation rate based on ASHRAE 62.1), enhancing indoor air circulation with the mixing fan notably improves the UVGI disinfection performance, especially for cooling with displacement ventilation and all-air-heating conditions. However, increasing indoor air mixing yields negligible effect on the disinfection performance for forced-convection cooling condition. The results also reveal that regardless of indoor thermal condition, disinfection effectiveness of a UVGI system increases as ventilation effectiveness is close to unity. Moreover, when the room average air speed is >0.1 m/s, upper-room UVGI system could yield about 90% disinfection effect for the aerosol size of 1 μm–10 μm. The findings of this study imply that upper-room UVGI systems in indoor environments (i.e., classrooms, hospitals) should be designed considering ventilation strategy and occupancy conditions, especially for occupied buildings with insufficient air mixing throughout the space.

Performance of upper-room ultraviolet germicidal irradiation (UVGI) system in learning environments: Effects of ventilation rate, UV fluence rate, and UV radiating volume

• We study performance of upper-room ultraviolet germicidal irradiation (UVGI) systems. • Performance of upper-room UVGI system in a classroom varies with ventilation rate. • Non-uniform airflow pattern meaningfully influences the UVGI system performance. • UV radiating volume is more crucial for virus disinfection than UV fluence rate. • Doubling UV fluence rate from 25 to 50 μW∙cm-2 yields only moderate benefit. Previous studies show that upper-room ultraviolet germicidal irradiation (UVGI) systems can help contain infectious airborne viruses indoors. However, there has been a lack of research on the performance of an upper-room UVGI system in learning environments such as a school classroom. Since classrooms are more vulnerable to airborne transmission of diseases due to high occupancy for long hours, airborne infection characteristics are different from other occupied indoor environments (e.g., offices and residences). The objective of this study is to investigate UVGI system performance in a classroom considering detailed effects of ventilation rate, UV fluence rate, and UV radiating volume. Two analytical models, a one-zone and a two-zone material balance model, along with computational fluid dynamics (CFD) simulations, were employed to analyze viral aerosol concentrations under a representative range of classroom operating conditions. The CFD results show that increasing ventilation rate from 1.1 h –1 to 5 h –1 yields about 85% of airborne disinfection while doubling UV radiating volume results in a 60% disinfection. However, increasing UV fluence rate from 25 μW∙cm –2 to 50 μW∙cm –2 yields a moderate additional disinfection of 18%. Overall, the study results reveal that operating a UVGI system in an occupied classroom can markedly disinfect airborne viruses up to 96%, which is as effective as increasing ventilation rate more than five times. Furthermore, the results suggest that the one-zone and two-zone analytical models used in several previous studies could result in notably meaningful errors in analyzing viral aerosol concentrations, especially in occupied rooms with a highly non-uniform airflow distribution.

A Case Study of Upper-Room UVGI in Densely-Occupied Elementary Classrooms by Real-Time Fluorescent Bioaerosol Measurements.

Recently, the requirement to continuously collect bioaerosol samples using shorter response times has called for the use of real-time detection. The decreased cost of this technology makes it available for a wider application than military use, and makes it accessible to pharmaceutical and academic research. In this case study, real-time bioaerosol monitors (RBMs) were applied in elementary school classrooms—a densely occupied environment—along with upper-room ultraviolet germicidal irradiation (UVGI) devices. The classrooms were separated into a UVGI group and a non-UVGI control group. Fluorescent bioaerosol counts (FBCs) were monitored on 20 visiting days over a four-month period. The classroom with upper-room UVGI showed significantly lower concentrations of fine size (<3 μm) and total FBCs than the control classroom during 13 of the 20 visiting days. The results of the study indicate that the upper-room UVGI could be effective in reducing FBCs in the school environment, and RBMs may be applicable in reflecting the transient conditions of the classrooms due to the dynamic activity levels of the students and teachers.

A Simple Method for Evaluating the Performance of Louvered Fixtures Designed for Upper-Room Ultraviolet Germicidal Irradiation

ABSTRACTThe primary objective of the present work was to develop and validate a simple method for measuring the ultraviolet (UV) emission rate of louvered UV fixtures designed for upper-room ultraviolet germicidal irradiation (UVGI). A secondary objective was to compare the emission rates and energy usage of a few commercially available fixtures and explain the reasons for the differences observed. The ultraviolet emission rate of fixtures designed for upper-room UVGI is clearly an important metric for specifying the UV dosing requirements for a particular application—that is, determining how many UV fixtures should be installed in a given room. UV emission rate is also important for evaluating how efficiently the fixture utilizes the required electrical power input. The ratio of the UV emission rate to the electrical input is a useful parameter for ranking or improving fixtures. In this article, we describe “UV sensor traverse” and its validation. UV sensor traverse is a simple method for measuring UV emission rate by traversing the louvered face of a fixture with a UV sensor. Using this method, we show that a commercially available fixture with a cylindrical parabolic reflector with a tubular lamp has about 84% of the UV radiation exiting the fixture emitted from the back of the lamp, compared to only 21% for a fixture with a flat reflector and compact lamps. The energy-use efficiency of the former fixture is about five times greater than that of the latter fixture. In the fixture with the parabolic reflector, UV rays are redirected by the reflector so that they tend to be parallel to the louvers, allowing significantly more of the UV radiation emitted from the back of the lamp to exit the fixture than a fixture with a flat reflector, which simply alters the direction of the UV rays. To conserve energy and minimize the number of louvered UV fixtures required, well-designed parabolic reflectors are essential.

Localized air-conditioning with upper-room UVGI to reduce airborne bacteria cross-infection

Localized airflow can result in both temperature and pollutant concentration segregations with limited mixing between adjacent environmental zones. However, when the bacteria source is within the environmental zone, it becomes important to look for effective methods to disinfect the localized air and prevent cross-infection among occupants in the same zone. This work investigates the effectiveness of upper-room ultraviolet germicidal irradiation (UVGI) in reducing airborne bacteria transmission within a zone conditioned by localized ceiling-mounted airconditioning system that recirculates return air. A computational fluid dynamics (CFD) model was developed to simulate the transport and inactivation by UV of Serratia marcescens, an extremely UV-susceptible microorganism. The CFD–UV model was validated using measurements of airflow, temperature, and UV irradiance. Numerous CFD simulations were performed for a case study to determine the maximal fraction of return air that can be recirculated for energy conservation without violating the indoor air quality standards for CO2 and bacteria concentrations. Results revealed that a maximum of 23% return air can be tolerated when minimal UV output of 15 W is delivered to the space. The use of the optimal setting reduced the energy consumption of the system by 15% compared to 100% fresh air case without UVGI.

A validated numerical investigation of the ceiling fan's role in the upper-room UVGI efficacy

This investigation quantifies the upper-room ultraviolet germicidal irradiation (UVGI) efficacy in a room with a ceiling-mounted fan that blew air either upwards or downwards at three rotational speeds. The numerical modeling deployed a steady-state passive scalar (Eulerian) and particle tracking (Lagrangian) CFD with a rotating reference frame. Two wall-mounted fixtures horizontally collimated the irradiance field, which was measured with a flat sensor and imported into the numerical models. This study predicted the UVGI efficacy under an extreme range of microorganism susceptibilities to define relationships between the system performance and operational parameters. A mathematical expression with general validity for fraction remaining under perfect air-mixing conditions was analytically developed and used as a performance benchmark. The CFD predictions were validated by the experimental data, expressed as a fraction remaining at the room exhaust for two different microorganisms. Numerical predictions were in a good agreement with the experimental data. In general, the Lagrangian predictions agreed better with the measured data than the Eulerian predictions. Inclusion of a rotating fan significantly improved UVGI performance, but there was no benefit from increasing the fan speed beyond certain values. For this investigation where the microorganism source is located below the fan, the UVGI system performed most efficiently when the fan blew upward, with the optimal performance achieved for the moderate rotational speed of 107 rpm. The highest upper-room UVGI efficacy for this fan setup is due to the airflow and UV light fields enabling a delivery of the highest amount of UV irradiation to the microorganism.

New airborne pathogen transport model for upper-room UVGI spaces conditioned by chilled ceiling and mixed displacement ventilation: Enhancing air quality and energy performance

The maximum allowable return air ratio in chilled ceiling (CC) and mixed displacement ventilation (DV) system for good air quality is regulated by acceptable levels of CO2 concentration not to exceed 700ppm and airborne bacterial count to satisfy World Health Organization (WHO) requirement for bacterial count not to exceed 500CFU/m3. Since the CC/DV system relies on buoyancy effects for driving the contaminated air upwards, infectious particles will recirculate in the upper zone allowing effective utilization of upper-room ultraviolet germicidal irradiation (UVGI) to clean return air. The aim of this work is to develop a new airborne bacteria transport plume-multi-layer zonal model at low computational cost to predict bacteria concentration distribution in mixed CC/DV conditioned room without and with upper-room UVGI installed. The results of the simplified model were compared with layer-averaged concentration predictions of a detailed and experimentally-validated 3-D computational fluid dynamics (CFD) model.The comparison showed good agreement between bacteria transport model results and CFD predictions of room air bacteria concentration with maximum error of ±10.4CFU/m3 in exhaust air. The simplified model captured the vertical bacteria concentration distribution in room air as well as the locking effect of highest concentration happening at the stratification level.The developed bacteria transport model was used in a case study to determine the return air mixing ratio that minimizes energy consumption and maintains acceptable IAQ with and without UVGI. Results showed that the use of upper-room UVGI resulted in 35% in energy saving, whereas the use of in-duct UVGI achieved no more than 12% energy saving, both compared to 100% fresh air case.

Influence of ceiling fan's speed and direction on efficacy of upperroom, ultraviolet germicidal irradiation: Experimental

Increasing a ceiling fan's speed from its lowest setting of 61 rpm, which resulted in 0.77 m3/s of airflow, to its highest setting of 176 rpm, which resulted in 2.5 m3/s of airflow, or having the fan blow either upward or downward had no statistically significant effect on the efficacy of upper-room ultraviolet germicidal irradiation (UVGI). This outcome suggests that air circulation due to the ceiling fan was sufficient and that any additional increase would not improve efficacy. Numerous experimental studies on upper-room UVGI in which fans were used to provide air mixing have been published. However, none have quantified the air movement produced by these fans or described their tests in sufficient detail to allow results to be compared to predictions using computational fluid dynamics (CFD). The present work provides the required information. In addition to the usual boundary conditions needed for CFD, we made experimental measurements of UV susceptibility of the microorganisms used in the upper-room UVGI tests. We measured UV susceptibilities for Mycobacterium parafortuitum and Bacillus atrophaeus spores to be 0.074 and 0.018 m2/J, respectively. In a previous publication, we reported the spatial distribution of fluence rate, which is also needed for predicting efficacy from CFD. In a companion paper referred to as Part II, upper-room UVGI efficacy was predicted by both Eulerian and Lagrangian CFD and compared to the experimental results from the present study.

Influence of Bioaerosol Source Location and Ceiling Fan Direction on Eggcrate Upper-room Ultraviolet Germicidal Irradiation.

BackgroundEggcrate upper-room ultraviolet germicidal irradiation (UVGI), an engineering control method for reducing the airborne transmission of infectious diseases, was recently developed as an alternative to conventional upper-room UVGI using conventional louvered fixtures. A UV screen, which is composed of open-cell eggcrate panels supported in a frame designed for a conventional suspended ceiling, was used to minimize UV radiation in the lower room. A ceiling fan, which was blowing upward directly above the microbiological source, provided vertical air exchange between the upper and lower room. This system has been shown to be significantly more effective than conventional upper-room UVGI.Study DesignIn the present study, the microbiological source location and the airflow direction due to the ceiling fan were varied in order to evaluate their impact on germicidal efficacy.ResultsThe test results clearly showed that placing an aerosol source directly underneath an upward blowing ceiling fan produces the maximum efficacy.ConclusionsThe likely explanation for this outcome is that the fan sucks the microorganisms emitted by the source into the UV beam before being mixed with the air in the room. This is somewhat analogous to local exhaust ventilation in which the contaminant is removed prior to being mixed with the air in the room. Thus, when possible, the ceiling fan should be blowing upward and directly above the source. However, for experimental testing, the source location should be varied in order to access the range of germicidal efficacies that can be expected.

Eggcrate UV: a whole ceiling upper-room ultraviolet germicidal irradiation system for air disinfection in occupied rooms

A novel whole ceiling upper-room ultraviolet germicidal irradiation (UVGI) system [eggcrate ultraviolet (UV)] has been developed that incorporates open-cell 'eggcrate'-suspended ceiling panels and bare UV lamps with a ceiling fan. Upper-room UVGI is more effective for air disinfection than mechanical ventilation at much lower installation and operating costs. Conventional upper-room UVGI fixtures employ multiple tightly spaced horizontal louvers to confine UV to the upper-room. These louvered fixtures protect occupants in the lower-room from UV-induced eye and skin irritation, but at a major cost to fixture efficiency. Using a lamp and ballast from a conventional upper-room UVGI fixture in the eggcrate UV system, the germicidal efficacy was markedly improved even though the UV radiation emitted by the lamp was unchanged. This fundamental change in the application of upper-room UVGI air disinfection should permit wider, more effective application of UVGI globally to reduce the spread of airborne infection.

Minimizing the exposure of airborne pathogens by upper-room ultraviolet germicidal irradiation: an experimental and numerical study.

There has been increasing interest in the use of upper-room ultraviolet germicidal irradiation (UVGI) because of its proven effectiveness in disinfecting airborne pathogens. An improved drift flux mathematical model is developed for optimizing the design of indoor upper-room UVGI systems by predicting the distribution and inactivation of bioaerosols in a ventilation room equipped with a UVGI system. The model takes into account several bacteria removal mechanisms such as convection, turbulent diffusion, deposition and UV inactivation. Before applying the model, the natural die-off rate and susceptibility constants of bioaerosols were measured experimentally. Two bacteria aerosols, Escherichia coli and Serratia marcescens, were tested for this purpose. It was found out that the general decay trend of the bioaerosol concentration predicted by the numerical model agrees well with the experimental measurements. The modelling results agree better with experimental observations for the case when the UVGI inactivation mechanism dominates at the upper-room region than for the case without UVGI. The numerical results also illustrate that the spatial distribution of airborne bacteria was influenced by both air-flow pattern and irradiance distribution. In addition to predicting the local variation of concentration, the model assesses the overall performance of an upper-room UVGI system. This model has great potential for optimizing the design of indoor an upper-room UVGI systems.

Safety of Upper-Room Ultraviolet Germicidal Air Disinfection for Room Occupants: Results from the Tuberculosis Ultraviolet Shelter Study

We evaluated the safety of room occupants in the Tuberculosis Ultraviolet Shelter Study (TUSS), a double-blind, placebo-controlled field trial of upper-room ultraviolet germicidal irradiation (UVGI) at 14 homeless shelters in six U.S. cities from 1997 to 2004. Data collection involved administering questionnaires regarding eye and skin irritation to a total of 3,611 staff and homeless study subjects. Among these subjects, there were 223 reports of eye or skin symptoms. During the active UV period, 95 questionnaires (6%) noted such symptoms, and during the placebo period, 92 questionnaires (6%) did so. In the 36 remaining cases, either the UV period when symptoms took place was unknown or the symptoms spanned both periods. There was no statistically significant difference in the number of reports of symptoms between the active and placebo periods. One definite instance of UV-related keratoconjunctivitis occurred, resulting from a placement of a bunk bed in a dormitory where a single bed had been used when the UV fixtures were first installed. These findings demonstrate that careful application of upper-room UVGI can be achieved without an apparent increase in the incidence of the most common side effects of accidental UV overexposure.

Fundamental Factors Affecting Upper-Room Ultraviolet Germicidal Irradiation—Part II. Predicting Effectiveness

Compared with increasing outdoor air ventilation rate, upper-room ultraviolet germicidal irradiation (UVGI) is an attractive technology for lowering the indoor concentration of airborne microorganisms and thereby reducing the risk of airborne transmission of disease. With relatively modest vertical air circulation, most of the air in a room can be irradiated in a relatively brief time period without noise or significant power consumption. The hypothesis tested in this study is that the efficacy of upper-room UVGI to inactivate or kill airborne infectious microorganisms can be determined from an index of UVGI effectiveness, a dimensionless parameter designed to characterize adequacy of vertical air circulation, amount of UVGI provided, and their interaction. This index of effectiveness, which is determined independently of microbiological testing, was found to correlate well with experimental measurements made in a room-size chamber. A comparison of two other dimensionless parameters—the irradiation number and mixing number, from which effectiveness index is calculated—provides insight into whether the quantity of UV provided to the upper room or the intensity of the vertical air circulation is the controlling factor for effective application of upper-room UVGI. The irradiation number is calculated from the UV power output of the fixture(s), a parameter that is fixture specific and much easier to measure than mean fluence rate. An equation was also developed that relates UV fixture power output to mean fluence rate in either the irradiated zone or the entire room. In addition, reductions in viable microorganism concentration due to UVGI predicted from a two-box model are compared with experimental measurements.

Use of CFD Modelling to Optimise the Design of Upper-room UVGI Disinfection Systems for Ventilated Rooms

The installation of upper-room ultraviolet germicidal irradiation (UVGI) devices in ventilated rooms has the potential to reduce transmission of infections by an airborne route. However, the performance of such devices is dependant on several factors including the location of the lamp and the ventilation airflow in the room. This study uses a CFD model to evaluate the performance of UVGI devices by considering the cumulative UV-C dose received by the bulk room air in a ventilated room. By evaluating the UV dose rather than the resulting micro-organism inactivation the methodology can be used to optimise UVGI systems at the design stage, particularly when the source location of bioaerosol contaminants is not known. The study investigates the relationships between the lamp location, lamp power, ventilation system and room heating in a small, ventilated room. The results show that with ventilation air supplied at low level and extracted at high level the UVGI system performs better than with the air supplied at high level and extracted close to the floor. In addition the results show the presence of a heater in the room is unlikely to have a detrimental effect on performance and may promote mixing to increase the extent of disinfection.

05/02857 Transport properties of non-spherical nanoparticles studied by Brownian dynamics: theory and numerical simulations

This investigation quantifies the upper-room ultraviolet germicidal irradiation (UVGI) efficacy in a room with a ceiling-mounted fan that blew air either upwards or downwards at three rotational speeds. The numerical modeling deployed a steady-state passive scalar (Eulerian) and particle tracking (Lagrangian) CFD with a rotating reference frame. Two wall-mounted fixtures horizontally collimated the irradiance field, which was measured with a flat sensor and imported into the numerical models. This study predicted the UVGI efficacy under an extreme range of microorganism susceptibilities to define relationships between the system performance and operational parameters. A mathematical expression with general validity for fraction remaining under perfect air-mixing conditions was analytically developed and used as a performance benchmark. The CFD predictions were validated by the experimental data, expressed as a fraction remaining at the room exhaust for two different microorganisms. Numerical predictions were in a good agreement with the experimental data. In general, the Lagrangian predictions agreed better with the measured data than the Eulerian predictions. Inclusion of a rotating fan significantly improved UVGI performance, but there was no benefit from increasing the fan speed beyond certain values. For this investigation where the microorganism source is located below the fan, the UVGI system performed most efficiently when the fan blew upward, with the optimal performance achieved for the moderate rotational speed of 107 rpm. The highest upper-room UVGI efficacy for this fan setup is due to the airflow and UV light fields enabling a delivery of the highest amount of UV irradiation to the microorganism.

95/00812 Carbon dioxide capture from power stations

The maximum allowable return air ratio in chilled ceiling (CC) and mixed displacement ventilation (DV) system for good air quality is regulated by acceptable levels of CO2 concentration not to exceed 700 ppm and airborne bacterial count to satisfy World Health Organization (WHO) requirement for bacterial count not to exceed 500 CFU/m3. Since the CC/DV system relies on buoyancy effects for driving the contaminated air upwards, infectious particles will recirculate in the upper zone allowing effective utilization of upper-room ultraviolet germicidal irradiation (UVGI) to clean return air. The aim of this work is to develop a new airborne bacteria transport plume-multi-layer zonal model at low computational cost to predict bacteria concentration distribution in mixed CC/DV conditioned room without and with upper-room UVGI installed. The results of the simplified model were compared with layer-averaged concentration predictions of a detailed and experimentally-validated 3-D computational fluid dynamics (CFD) model.The comparison showed good agreement between bacteria transport model results and CFD predictions of room air bacteria concentration with maximum error of ±10.4 CFU/m3 in exhaust air. The simplified model captured the vertical bacteria concentration distribution in room air as well as the locking effect of highest concentration happening at the stratification level.The developed bacteria transport model was used in a case study to determine the return air mixing ratio that minimizes energy consumption and maintains acceptable IAQ with and without UVGI. Results showed that the use of upper-room UVGI resulted in 35% in energy saving, whereas the use of in-duct UVGI achieved no more than 12% energy saving, both compared to 100% fresh air case.