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Airborne Bacteria Inactivation in a Hospital Ward by Ultraviolet Irradiation

S. Sadrizadeh1,2,3†

, S. Stensson4, S. Marashian

5, I. Walker

2 and S. Holmberg

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KTH Royal Institute of Technology, Fluid and Climate Technology, Stockholm 10044, Sweden Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA Center for the Built Environment, University of California - Berkeley, California 94720, USA RISE Research Institutes of Sweden, Borås 50115, Sweden Shiraz University, School of Mechanical Engineering, Shiraz, Iran

†Corresponding email: SSadrizadeh@lbl.gov, Sasan@berkeley.edu

SUMMARY

This study considers numerical modelling based on the Computational Fluid Dynamics technique for a hospital ward with an upper-room ultraviolet germicidal irradiation fixture. A two-bed hospital ward equipped with a ceiling-level low velocity ventilation diffuser was considered. The airflow field was considered steady state and ultraviolet distribution was treated as a scalar flux. Different particle sizes were simulated representing the pathogenic contaminants released from patients’ breathing zones. The results confirm the effectiveness of ultraviolet germicidal irradiation. However, an optimization study should be performed to enhance the disinfection efficiency of the system. Ultraviolet germicidal irradiation is an effective technique for airborne bacteria inactivation which potentially can be used to prevent the spread of certain infectious diseases.

INTRODUCTION

Cross-infection is the transmission of a pathogen from one person to another due to a poor barrier protection (Cruickshank, 1942). The most common are nosocomial cross infections, usually referred as hospital-acquired infections (HAIs). Although cross-infection routines are normally adopted for all patients in hospitals and health care units, such infections are still common and persist among medical wards and general surgical units. HAIs can place a substantial clinical and financial burden on the healthcare system, patients, and their families (Sadrizadeh, 2016; Sadrizadeh & Holmberg, 2014a). Such hospital-related infections add an estimated minimum of $4.5 billion annually to the cost of health care systems and kill more people in the US than AIDS, breast cancer and auto accidents combined (Centers for Disease Control (CDC), 1992). Hospital wards are generally considered one of the most important pathogen sources due to the respiratory activity of a potentially infectious patient. The airborne droplets exhaled from patient's mouth normally contain infectious agents (H Qian et al., 2006), they can reach neighbouring patients, and can

travel even farther in the presence of ventilation systems. The size distribution and amount of such droplets in exhaled breath may vary extensively (Papineni & Rosenthal, 1997). Transmission of pathogenic microorganisms can be effectively curtailed in hospitals and health care facilities by means of an efficient ventilation system (Sadrizadeh & Holmberg, 2014a; Sadrizadeh, Holmberg, & Tammelin, 2014), proper personnel clothing (Sadrizadeh & Holmberg, 2014b; Sadrizadeh, Holmberg, & Nielsen, 2016) and reducing foot traffic and activities in critical areas (Sadrizadeh, Tammelin, Ekolind, & Holmberg, 2014). It is generally agreed that integration of an effective ventilation system and air-pressure control in hospitals is essential to minimize unplanned airflow penetration through building envelopes and interior spaces (Fontana & Quintino, 2014; Lidwell, 1977). In addition to the above mentioned family of solutions, several research studies have also indicated the benefits from the implementation of ultraviolet germicidal irradiation (UVGI) in hospital environments (Sung & Kato, 2010; Yang, Lai, & Wu, 2016). This technology has a long history of being used for the disinfection of air streams, primarily in environments with higher risk of airborne pathogen transmission such as healthcare facilities, hospital wards and operating rooms. The installation of UVGI units diminished as extensive usage of antibiotics began in the late 1950s and beyond (Brickner, 2003). However, application of this technology has again received increasing attention due to the global threat posed by antibiotic resistance. It is proven that UVGI induces irreparable damage to bacterial DNA, and interrupts their replication ability, thus rendering them non-infectious microorganisms (Kowalski, 2009). Upper-room (upper-air) and in-duct systems are the most common form of UVGI for air disinfection in an enclosed environment. UVGI units can be installed in the upper-room area of a ward in order to interrupt the transmission of airborne infectious diseases and prevent cross contamination among the hospitalized patients. In this study, the application of an upper-room UVGI system on the distribution of airborne particles in a

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hospital ward is examined numerically via computational fluid dynamics (CFD). We aimed to investigate the effect of a UVGI system on the overall particle concentration within the ward.

METHODS

Figure-1 shows a two-bed hospital ward which has been used in our previous works (Sadrizadeh & Holmberg, 2015; Sadrizadeh & Nielsen, 2016). The room dimensions are 4.2 m 3.6 m 2.5 m. Two beds with a parallel gap distance of 1.4 m, each with dimensions of 2.0 m 0.8 m 0.8 m (H), were placed in the ward centre.

Fig.1: Hospital ward (a) and its CAD model in CFD (b)

Incoming air was introduced to the room from a ceiling-level rectangular supply inlet with an air exchange rate of 6 ACH and temperature of 20 °C. Two UV lamps are located on the two walls of the room. The model for the UV dose distribution was developed by considering how the dose varied with the airflow in the ward. The UV irradiance field was defined as a passive scalar field. Two patients with a heat load of 58 W/m

2, representing

an adult person at rest (ISO 7730:2005), were considered, each with a respiratory breathing rate of 6.0 L/min airflow rate and a frequency of 10 min

-1 (Hua

Qian, Li, Nielsen, & Hyldgaard, 2008). A source of airborne microorganisms between 0.5–12 μm (Tang, Li, Eames, Chan, & Ridgway, 2006) was considered to be discharged together with exhalation air from the patient’s mouth. Figure-2 represents a normal-breathing pattern at rest which was implemented in this study.

Figure 2: Normal breathing pattern at rest

In this study, airborne droplet motion and airflow modelling based on computational fluid dynamics (CFD) is determined by iteratively solving the fundamental conservation equations for mass, momentum, and energy. The airflow was simulated using the RSM turbulence model and particle trajectories were tracked by a Lagrangian-based discrete random walk model. A scape boundary

condition for the discrete phase was adapted on the UVGI plane to simulate the particle-free airflow streams in the upper room section. The discrete ordinates (DO) radiation model is used to incorporate the radiation part of heat transfer. A detailed description of the simulation approach, including the validation, was previously given by the authors (Sadrizadeh et al., 2016; Sadrizadeh, Holmberg, et al., 2014) and thus it is not repeated here. The CFD calculations were carried out using commercial finite volume code Fluent 18.0. A mesh sensitivity analysis was performed, using three different grid densities, to evaluate the effects of grid sizes on the results. Besides the grid independence test required verifying the computations, the transient analysis was subjected to a time-step sensitivity analysis to obtain time step independent airflow distributions.

RESULTS AND DISCUSSION

Figure-3 shows air velocity contour map in a vertical plane through the air supply diffuser.

Fig.3: Air velocity contour map in a vertical plane through the air supply diffuser

The air from the supply opening flows along the ceiling toward the opposite wall, is forced down and flows out through two exhaust openings. It is clear that the airflow is complex with overall and local recirculation. Exhaled air (including airborne particles) from the patient on the right-hand side flows toward the other patient and is recirculated back again to the same location.

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Fig.4: Contour plots of predicted microorganism distribution in the breathing plane

Figure-4 shows predicted microorganism distribution at the breathing plane in the absence of a UVGI system. The particle concentration is normalized by the steady-state nominal particle concentration at the patient’s mouth. Results show that the concentration of airborne pathogens was lower close to the inlet. Almost 20 % of exhaled microorganisms can reach from one patient to the other, which may result in cross-infection.

Fig.5: volume concentration of airborne microorganisms within the ward room

Figure-5 shows the volumetric droplet concentration both before and after UVGI operation. To reduce computational cost and time in the problem-solving procedure, only 100 minutes of a normal ward situation was simulated while the UVGI system was off. However, more working time of the ward needs to be simulated in order to reach steady state in regards to particles. It is worth noting that the droplet’s concentration curve did not exhibit a steady exponential curve since the airflow field within the ward was not fully mixed. After 100 minutes of initial time, the UVGI started to operate and eliminate the airborne droplets from the ward environment which resulted in a drastic reduction of the airborne particle concentration.

According to the simulation results, it seems that any airborne microorganism that enters the upper-room part is efficiently inactivated, however not all microorganisms are actually transported to the irradiated zone. Results showed that less than 3 % of exhaled microorganisms from one patient can reach the other one.

CONCLUSIONS

A numerical study based on computational fluid dynamics was carried out to exhibit the importance of considering the room ventilation system in conjunction with a UVGI system. Results show that an upper-air UVGI installation is an effective means of reducing concentrations of airborne pathogens that cause healthcare-associated infections. UVGI would be a valuable complement to the main ward room ventilation system. This system uses a short wavelength light to deactivate microorganisms by destroys the nucleic acid in the bacteria and disrupts their DNA. Although the numerical simulation conducted here is for a single wardroom with a very simple ventilation system, it is likely that similar behaviour would be seen with different room designs.

REFERENCES

Brickner, P. W. (2003). The Application of Ultraviolet

Germicidal Irradiation to Control Transmission of

Airborne Disease: Bioterrorism Countermeasure.

Public Health Reports, 118(2), 99–114.

Centers for Disease Control (CDC). (1992). Public health

focus: surveillance, prevention, and control of

nosocomial infections. MMWR. Morbidity and Mortality

Weekly Report, 41(42), 783–7.

Cruickshank, R. (1942). Cross-infection in hospital wards.

Public Health, 56, 17–19.

Fontana, L., & Quintino, A. (2014). Experimental analysis of

the transport of airborne contaminants between

adjacent rooms at different pressure due to the door

opening. Building and Environment, 81, 81–91.

ISO 7730:2005 - Ergonomics of the thermal environment --

Analytical determination and interpretation of thermal

comfort using calculation of the PMV and PPD indices

and local thermal comfort criteria. (2005) (Vol. 2005).

Kowalski, W. (2009). Ultraviolet germicidal irradiation

handbook: UVGI for air and surface disinfection.

Ultraviolet Germicidal Irradiation Handbook: UVGI for

Air and Surface Disinfection.

Lidwell, O. M. (1977). Air exchange through doorways. The

effect of temperature difference, turbulence and

ventilation flow. J. Hyg.(Cambridge), 79, 141–154.

Papineni, R. S., & Rosenthal, F. S. (1997). The size

distribution of droplets in the exhaled breath of healthy

human subjects. Journal of Aerosol Medicine : The

Official Journal of the International Society for Aerosols

0 30 60 90 120 1500

1000

2000

3000

4000

5000

6000

time [min]

Dro

ple

ts/m

3

UVGI off

UVGI on

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in Medicine, 10(2), 105–116.

Qian, H., Li, Y., Nielsen, P. V., & Hyldgaard, C. E. (2008).

Dispersion of exhalation pollutants in a two-bed

hospital ward with a downward ventilation system.

Building and Environment, 43(3), 344–354.

Qian, H., Li, Y., Nielsen, P. V, Hyldgaard, C. E., Wong, T.

W., & Chwang, a T. Y. (2006). Dispersion of exhaled

droplet nuclei in a two-bed hospital ward with three

different ventilation systems. Indoor Air, 16(2), 111–28.

Sadrizadeh, S. (2016). Design of Hospital Operating Room

Ventilation using Computational Fluid Dynamics (PhD

thesis). KTH Royal Institute of Technology.

Sadrizadeh, S., & Holmberg, S. (2014a). Effect of a Mobile

Ultra-clean Laminar Airflow unit on particle distribution

in an operating room. Particuology, 18, 170–178.

Sadrizadeh, S., & Holmberg, S. (2014b). Surgical clothing

systems in laminar airflow operating room: a numerical

assessment. Journal of Infection and Public Health,

7(6), 508–516.

Sadrizadeh, S., & Holmberg, S. (2015). Cross-infection in a

hospital wardroom with individual return openings. In In

Proceedings of Healthy Buildings 2015 Europe. May

18-20th 2015, Eindhoven–The Netherlands.

Sadrizadeh, S., Holmberg, S., & Nielsen, P. V. (2016). Three

distinct surgical clothing systems in a turbulent mixing

operating room equipped with mobile ultraclean

laminar airflow screen: A numerical evaluation.

Science and Technology for the Built Environment,

22(3), 337–345.

Sadrizadeh, S., Holmberg, S., & Tammelin, A. (2014). A

comparison of vertical and horizontal laminar

ventilation systems in an operating room: A numerical

study. Building and Environment, 82, 517–525.

Sadrizadeh, S., & Nielsen, P. V. (2016). Modelling of

coughed droplets in a hospital ward. In In Proceedings

of 12th REHVA world congress, CLIMA Conference.

May 22-25, 2016; Aalborg - Denmark.

Sadrizadeh, S., Tammelin, A., Ekolind, P., & Holmberg, S.

(2014, April). Influence of staff number and internal

constellation on surgical site infection in an operating

room. Particuology, pp. 42–51. Chinese Society of

Particuology.

Sung, M., & Kato, S. (2010). Method to evaluate UV dose of

upper-room UVGI system using the concept of

ventilation efficiency. Building and Environment, 45(7),

1626–1631.

Tang, J. W., Li, Y., Eames, I., Chan, P. K. S., & Ridgway, G.

L. (2006). Factors involved in the aerosol transmission

of infection and control of ventilation in healthcare

premises. The Journal of Hospital Infection, 64(2),

100–14.

Yang, Y., Lai, A. C. K., & Wu, C. (2016). Study on the

disinfection efficiency of multiple upper-room ultraviolet

germicidal fixtures system on airborne

microorganisms. Building and Environment, 103, 99–

110.

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