Turn Up the Lights, Leave them On and Shine them All Around-Numerical Simulations Point the Way to more Efficient Use of Far-UVC Lights for the Inactivation of Airborne Coronavirus

Abstract

It has been demonstrated in laboratory environments that ultraviolet-C (UVC) light is effective at inactivating airborne viruses. However, due to multiple parameters, it cannot be assumed that the air inside a room will be efficiently disinfected by commercial germicidal ultraviolet (GUV) systems. This research utilizes numerical simulations of airflow, viral spread, inactivation by UVC and removal by mechanical ventilation in a typical classroom. The viral load in the classroom is compared for conventional upper-room GUV and the emerging "Far-UVC." In our simulated environment, GUV is shown to be effective in both well and poorly ventilated rooms, with greatest benefit in the latter. At current exposure limits, 18 commercial Far-UVC systems were as effective at reducing viral load as a single upper-room GUV. Improvements in Far-UVC irradiation distribution and recently proposed increases to exposure limits would dramatically increase the efficacy of Far-UVC devices. Modifications to current Far-UVC devices, which would improve their real-world efficacy, could be implemented now without requiring legislative change. The prospect of increased safety limits coupled with our suggested technological modifications could usher in a new era of safe and rapid whole room air disinfection in occupied indoor spaces.

Figure 1

Room 230 in Physics & Astronomy at the University of St Andrews. Three of the four air inlets can be seen on the right near the ceiling opposite the three windows on the left side of this classroom.

Figure 2

Steady‐state velocity fields displayed in units of m s⁻¹ from OpenFOAM simulations with 1.4ACH and 6.8ACH. The images show representative slices of the velocity fields in vertical and horizontal planes. The four air inlets are on the upper right hand sides and the three windows on the lower left sides.

Figure 3

A one pixel‐wide slice in the $x‐z$ (left) and $x‐y$ (right) planes showing the fluence rate patterns of a typical commercial Far‐UVC device, an isotropic pattern, and a simulated upper‐room device. The $x‐y$ fluence rate map is taken at a height of 2.2 m. The Far‐UVC devices are shown with a scaling to provide a fluence rate appropriate for the current TLV at a height of 2 m. Notice that the upper‐room device provides a much higher fluence rate than the Far‐UVC devices in the area above head height, leading to rapid virus inactivation in this region. Low‐intensity light from the upper‐room device that is scattered from the walls and ceiling can be seen in the lower room regions.

Figure 4

Three‐dimensional representation of the fluence rates within the room for different Far‐UVC lighting scenarios. Left four panels show fluence rates arising from the illumination pattern of typical commercial Far‐UVC devices, right four panels for isotropic lights

Figure 5

Relative concentrations as a function of time after a one‐time release of particles within the room. Left panels are for simulations with 1.4ACH and right panels for 6.8ACH. The uppermost panels assume the Far‐UVC lights operate continuously at the current TLV by delivering a maximum fluence rate of 0.8 μW cm⁻² at a height of 2 m. The middle and bottom panels increase these TLV fluence rates by a factor of twenty and one hundred, respectively. The different linestyles are displayed in the caption and explained in the text.

Figure 6

As for Fig. 5, but for ‘continuous release’ where 7080 particles are released from a height of 0.5 m (as described in the Section ‘Particle dissemination’) every second of the simulation

Figure 7

As for Fig. 6, but now comparing ventilation with eighteen commercial devices operating either in continuous mode or on duty cycles with on/off times as indicated.

Figure 8

Example of particle trajectories for different irradiation and intensity patterns for Far‐UVC devices. Unless otherwise stated, the airflow is for the simulation with 6.8ACH. In each of the figures, the air inlets are on the right and the particle trajectories can be seen exiting the three open windows on the left. The trajectories display the inactivation as a function of position from where a particle is emitted either for an hour of elapsed time that the simulation represents or until it exits the simulation via a window. Note the bottom right figure that shows the rapid inactivation achievable for a single isotropic light that delivers twenty times the current TLV at a height of 2 m

Figure 9

Inactivation simulations for different $k_V$ values representing viruses that are more or less susceptible to UVC. Upper panels for one‐time release of particles and lower panels for continuous release