Upper-room Ultraviolet Germicidal Irradiation Research Articles

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.

Contaminant removal effectiveness evaluation of ventilation, UR-UVGI, and air cleaner using CFD in negative pressure isolation ward

A negative pressure isolation ward (NPIW) is designed to create negative pressure to prevent the spread of airborne viruses. Therefore, it is necessary to increase the contaminant removal effect in the NPIW. To reduce indoor air contaminants, upper room-ultraviolet germicidal irradiation (UR-UVGI) and air cleaner are being used. Unlike the UR-UVGI, the air cleaner generates airflow. Therefore, the contaminant removal effect may differ according to the ventilation methods. This study is to compare ventilation, UR-UVGI, and air cleaner contaminant removal effects using CFD and Contaminant Removal Effectiveness (CRE) in the NPIW. As a result, the CRE of the ceiling and wall exhaust methods were different, and the CRE of the URUVGI and air cleaner cases were also different according to the two ventilation methods. Therefore, when applying UR-UVGI and air cleaner, it is necessary to consider the ventilation method.

Pulsating jet ventilation add-ons performance for reducing the contaminant spread in classrooms: Portable air cleaners vs. upper room UVGI

This paper examines the extent to which “add-on” cleaning devices in a classroom ventilated with a pulsating jet ventilation (PJV) system are able to reduce the contaminant spread risks between students when one of them is infected. Two types of “add-on” devices are investigated: portable air cleaners (PACs) and upper room ultraviolet germicidal irradiation (UR-UVGI) lamps. The cleaning effectiveness of each system is evaluated by comparing the contaminant concentration reduction ratio (CR) within the breathing layer in each group of students and the intake fraction (iF) with their corresponding values of standalone PJV operation under the same conditions. Detailed 3D computational fluid dynamic (CFD) models were developed and validated against published experimental data. A parametric analysis was performed by varying the PJV supply flowrate and its period of intermittency to establish the PJV operating conditions that maximize the cleaning and protection effectiveness of each “add-on” device at acceptable thermal comfort and CO2 levels.It was found that for maximum cleaning effectiveness, the PJV system is recommended to operate with an UR-UVGI system at supply flowrate between 250 l/s and 350 l/s with a range of intermittency periods between 4 min and 5 min. However, for a maximum protection against cross-contamination, the PJV system should operate with PAC devices at 400 m3/h for PJV flowrate ranging between 250 l/s and 350 l/s irrespective of the intermittency period with iFmax values between 1.47×10−4 and 0.67×10−4. The addition of “add-ons” resulted in a maximum of 7% increase in the overall energy consumption.

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.

Exploring the Distribution of Ultraviolet Radiation Intensity of Parabolic Reflector at Different Overall Heights

The global pandemic of coronavirus disease 2019 (COVID-19) has posed a serious threat to human society on both perspectives of health and economy. Ultraviolet (UV) has been verified capable to damage the DNA and RNA of various microorganisms, including the new coronavirus. The parabolic reflector, a key component of the upper-room ultraviolet germicidal irradiation (UVGI) device, plays a vital role in collating and restraining ultraviolet light. In order to deeply understand the influence of the reflector in the device on the spatial distribution and numerical value of the ultraviolet radiation intensity, we measured and simulated the efficacy of the reflector equipped with a single-tube UV lamp. At a distance of 50cm from the reflector, a virtual plane with a side length of 40cm×40cm parallel to the exit port was introduced. At room temperature of 20 °C, measured and simulated reflectors were carried out at three heights A, B, and C, which were 25 cm, 20 cm, and 15 cm. We have concluded that as the height of the reflector increases, the irradiance value in the space also increases; then decreasing the height of the reflector is more conducive to the uniform distribution of the irradiance in the space. In conclusion, exploring the appropriate reflector height to obtain the balance of UV intensity and distribution is meaningful for improving the efficiency of the upper layer device

Spatial analysis of the impact of UVGI technology in occupied rooms using ray-tracing simulation.

The use of Ultraviolet Germicidal Irradiation (UVGI) devices in the upper zones of occupied buildings has gained increased attention as one of the most effective mitigation technologies for the transmission of COVID-19. To ensure safe and effective use of upper-room UVGI, it is necessary to devise a simulation technique that enables engineers, designers, and users to explore the impact of different design and operational parameters. We have developed a simulation technique for calculating UV-C fluence rate within the volume of the upper zone and planar irradiance in the lower occupied zone. Our method is based on established ray-tracing light simulation methods adapted to the UV-C wavelength range. We have included a case study of a typical hospital patient room. In it, we explored the impact of several design parameters: ceiling height, device location, room configuration, proportions, and surface materials. We present a spatially mapped parametric study of the UV-C irradiance distribution in three dimensions. We found that the ceiling height and mounting height of the UVGI fixtures combined can cause the largest variation (up to 22%) in upper zone fluence rate. One of the most important findings of this study is that it is crucial to consider interreflections in the room. This is because surface reflectance is the design parameter with the largest impact on the occupant exposure in the lower zone: Applying materials with high reflectance ratio in the upper portion of the room has the highest negative impact (over 700% variation) on increasing hot spots that may receive over 6mJ/cm2 UV dose in the lower occupied zone.

CFD optimization of return air ratio and use of upper room UVGI in combined HVAC and heat recovery system

Several strategies and techniques have been recently developed to decrease energy consumption in heating, ventilation, and air-conditioning (HVAC) systems. One method is to recirculate a fraction of return air and make use of the remaining part (exhaust) to preheat fresh air in order to reduce the heating load. However, limitations must be imposed to the return fraction to maintain acceptable indoor air quality (IAQ).The aim of this paper is to develop a computational fluid dynamics (CFD) model to predict the dispersion of CO2 and airborne bacteria in all-air HVAC systems with aforementioned heat recovery system for known return air ratios as well as the disinfection of indoor air by upper room Ultraviolet Germicidal Irradiation (UVGI).The developed model can be put to practical use for estimating the maximum allowable return that minimizes energy cost of the integrated system while maintaining acceptable IAQ. It can also be used to predict the minimum required UV output to eliminate any excess in bacteria count in the breathing zone without the need for additional fresh air intake at the supply.

Evaluation of UR-UVGI System for Sterilization Effect on Microorganism Contamination in Negative Pressure Isolation Ward

A negative pressure isolation ward prevents the outflow of airborne microorganisms from inside the ward, minimizing the spread of airborne contamination causing respiratory infection. In response to recent outbreaks of severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), Korea has increased the number of these facilities. However, airborne contaminants that flow into the ward from adjacent areas may cause secondary harm to patients. In this study, the sterilization effect of upper-room ultraviolet germicidal irradiation (UR-UVGI) on microorganisms generated within the negative pressure isolation ward and those flowing inward from adjacent areas was evaluated through field experiments and computational fluid dynamics (CFD) analysis, to assess the potential of this approach as a supplementary measure to control such microorganisms. The sterilization effect was found to be not high because of high-level ventilation. CFD analysis under various conditions shows that the sterilization effect for indoor-generated microorganisms varies with the level of UV radiation, the source locations of the indoor-generated microorganisms, air supplies and exhausts, the UVGI system, and the airflow formed under the specified conditions. Our results show that when the UVGI system is installed in the upper part of the ward entrance, contaminated air from adjacent area is strongly sterilized.

A pilot study on the disinfection efficacy of localized UV on the flushing-generated spread of pathogens

The process of toilet-flushing can generate flushing-associated water droplets which can potentially expose humans to pathogen-laden aerosols. Very little is known about such aerosol dissemination or the means for minimizing exposure to these aerosols. This study has evaluated the efficacy of ultraviolet waveband C (UV-C) for disinfection of flushing-generated pathogen-laden aerosols through tests with localized disinfection systems for airborne and surface contaminations. Three types of bacteria were chosen for investigation: Staphylococcus epidermidis, Escherichia coli, and Salmonella typhimurium. Tests were conducted with UV-C tubes of 5 W and 10 W. High levels of disinfection efficacies were observed, ranging from 76% to 97% for bacteria-laden aerosols at sources of emission, and efficiencies of 53% to 79% for surface samples in localized systems. The results from the localized systems were further compared with those obtained with an upper-room ultraviolet germicidal irradiation (UVGI) system. As it is important to note, the UV-C doses and ozone emissions for the localized systems were found well below the limits recommended in current guidelines. This research has shown that the disinfection of flushing-generated pathogen-laden aerosols in proximity to the source of emission was more effective than at the more distant sites where aerosols may be dispersed to the environment.

Experimental and numerical study of the performance of upper-room ultraviolet germicidal irradiation with the effective Z-value of airborne bacteria

ABSTRACTUpper-room ultraviolet germicidal irradiation (UR-UVGI) is recommended for the defense against airborne pathogens in poorly ventilated rooms in public buildings or high risk environments, such as hospitals. Currently, there are few studies on the performance of UR-UVGI with the effective bacteria susceptibility constant (eZ-value) obtained from UR-UVGI experiment. In this study, the Eulerian model for the inactivation of UVGI was improved to consider the difference between exposure time and computational time, and was validated by our previous experiments. The method was applied to study numerically the performance of the UR-UVGI for 3, 6, and 10 air changes per hour (ACH) with the application of the eZ-value of airborne bacteria. Moreover, the eZ-values of commonly found bacteria, including Serratia marcescens, Staphylococcus epidermidis, Pseudomonas alcaligenes, and Micrococcus luteus, were obtained from a series of experiments in a full-scale environmental chamber equipped with a UR-UVGI fixtur...

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.

Occupant UV exposure measurements for upper-room ultraviolet germicidal irradiation

The threshold limit value (TLV) guideline for ultraviolet (UV) radiation specifies that irradiance measurements to ensure occupant safety be taken over an angle of 80° at the sensor. The purpose of this study was to evaluate the effect of an 80° field of view (FOV) tube on lower room UV-C irradiation measurements.Measurements were made in an experimental chamber at a height of 1.73m with and without an FOV tube. The FOV tube reduced the lower room irradiance readings by 18–34%, a statistically significant reduction compared to the bare sensor.An 80° FOV tube should be used for lower room irradiance measurements to comply with the TLV guideline. The resulting lower readings would allow more UV-C radiation in the upper room without compromising occupant safety. More UV-C radiation in the upper room could increase efficacy of UVGI systems for reducing transmission of airborne infectious diseases.In addition, recommendations are made to standardize lower room irradiance measurement techniques.

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.

Upper room UVGI effectiveness with dispersed pathogens at different droplet sizes in spaces conditioned by chilled ceiling and mixed displacement ventilation system

This paper investigates by modeling and experimentation the dispersion of bacteria and performance of Upper Room Ultraviolet Germicidal Irradiation (UVGI) in spaces conditioned by combined chilled ceiling (CC) and mixed displacement ventilation (DV) system. The effects of gravitational settling and deposition were taken into account in mathematical modeling to reasonably simulate the transport of pathogen via expiratory droplets of all size ranges. The model was experimentally validated with and without the use of UVGI for different ventilation rates. Experiments were conducted in a CC/DV test room with a constant generation of Serratia marcescens, an extremely UV-susceptible microorganism to measure the airborne bacteria concentration in the room air and quantify the bacteria deposition on horizontal surfaces. The experimental bacteria colony forming units (CFU) count was used to validate a CFD model developed to predict the deposition of pathogen carriers. The mathematical model was then substantiated using the CFD predictions of pathogen concentration in the space for large carrying particles. The validated mathematical model was then used to study the effect of pathogen-carrying particle sizes on the microbiological indoor air quality. Results showed that the droplet size affects the value of maximal bacteria concentration and the height at which it occurs. Moreover, the UV disinfection rate achieved in the upper zone decreases from 88% to 78% when the size of pathogen-carrying particles increases from 2.5 μm to 20 μm meaning that greater percentage of bacteria are removed by deposition when bacteria is carried by large particles.

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.

Modeling infection risk and energy use of upper-room Ultraviolet Germicidal Irradiation systems in multi-room environments

The effectiveness of ultraviolet irradiation at inactivating airborne pathogens is well proven, and the technology is also commonly promoted as an energy-efficient way of reducing infection risk in comparison to increasing ventilation. However, determining how and where to apply upper-room Ultraviolet Germicidal Irradiation devices for the greatest benefit is still poorly understood. This article links multi-zone infection risk models with energy calculations to assess the potential impact of a Ultraviolet Germicidal Irradiation installation across a series of inter-connected spaces, such as a hospital ward. A first-order decay model of ultraviolet inactivation is coupled with a room air model to simulate patient room and whole-ward level disinfection under different mixing and ultraviolet field conditions. Steady-state computation of quanta-concentrations is applied to the Wells–Riley equation to predict likely infection rates. Simulation of a hypothetical ward demonstrates the relative influence of different design factors for susceptible patients co-located with an infectious source or in nearby rooms. In each case, energy requirements are calculated and compared to achieving the same level of infection risk through improved ventilation. Ultraviolet devices are seen to be most effective where they are located close to the infectious source; however, when the location of the infectious source is not known, locating devices in patient rooms is likely to be more effective than installing them in connecting corridor or communal zones. Results show an ultraviolet system may be an energy-efficient solution to controlling airborne infection, particularly in semi-open hospital environments, and considering the whole ward rather than just a single room at the design stage is likely to lead to a more robust solution.

Student absenteeism and the comparisons of two sampling procedures for culturable bioaerosol measurement in classrooms with and without upper room ultraviolet germicidal irradiation devices

Upper room ultraviolet germicidal irradiance (UVGI) has been shown to reduce the concentration of bioaerosols in controlled chambers. However, there is a lack of experimental results on the reduction of bioaerosol concentrations by UVGI devices in actual uncontrolled buildings. This study was carried out in an American elementary school in the Midwest. Two sampling procedures were carried out in six selected classrooms with similar dimensions that were separated into two groups: (1) UVGI exposure group and (2) non-UVGI control group. Two-stage Tisch culturable impactors were utilized to collect airborne culturable bacteria and fungi. Monthly samples were collected during unoccupied period in sampling Procedure A and during close-to-occupied periods in sampling Procedure B. Student absenteeism data were collected. Nonparametric statistical methods were applied. Neither analysis of microorganisms nor student absenteeism showed a significant difference between the UVGI exposure and non-UVGI control groups in Procedure A. Analysis of the airborne culturable fine and total bacteria levels (1–8 µm) was significantly lower in the exposure classroom than those of the control classroom using Procedure B ( P values &lt; 0.05). The result indicates that collecting airborne bacteria close to occupied time could be more effective in evaluating the performance of upper room UVGI. In this case study, upper room UVGI can reduce culturable bioaerosols in a crowed environment like classrooms.