IHEEM DIGITAL WEEK – INFECTION CONTROL Types of UVC system


Looking at some existing UVC systems, the Professor first highlighted the ‘in duct’ type; its effectiveness depended on the number of lamps, their location, intensity, the airflow over them, and the susceptibility of the microorganism to UVC light. Her team has already modelled this approach, for example looking at the airflow over such lamps, and the dose that a single lamp received over time, and comparing this with experimental data from the Environmental Protection Agency in the US. She said: “Our model compared quite well with the EPA data, but from our CFD model we can see that different particles receive different doses. Some get way more than they need, and some considerably less, so the system is not very efficient. We can use such models to look at different lamp combinations and orientations, and, for instance, to highlight the differing impact of a single perpendicular lamp and a parallel one. You can see that the dose received from the parallel lamp is considerably higher. We can create very distinctive graphs for different lamp designs”


Looking at upper room UV, Professor Noakes showed slides of a system set up in a chamber room, with the air flow through the room, and the measurements of the UV field, and how it appears in a 2D plane. “You can see the field that it is producing,” she explained. “Although the UV is not visible, it gives out some visible blue light. We can model this, look at where in the room the flow goes, couple that with the UV device, and examine how the dose is distributed. This helps us to understand how effective these devices might be.”


Professor Noakes explained that her team had also looked at how they could model the UVC lamps in a ‘multi zone’ scenario, such as hospital, with wards being zones 1, 2, 3, and 4; circulation zones 5 and 6, and with ‘interaction’ between


•Multi-zone scenario, zonal flow model •Wells-Riley infection risk model


•UV disinfection assuming TB (coronavirus almost same)


•UV energy cost vs. ventilation


the two areas. She said: “In this modelling, we put an infector on Zone 1, and assumed an air change rate of three/hour, before asking what would happen if we increased this to six. We also looked at a scenario where, instead of increasing the air change rate, we put UV in zone 1 only, the patient zones only, or everywhere, and the results. We implemented this model with TB, but, by chance, the susceptibility of the coronavirus is almost the same.”


Cases and energy consumption calculated


The Professor explained that in the base case, both the number of new cases, and the energy consumption, were calculated. Doubling the ventilation rate halved case numbers, but doubled the energy consumption, but by installing the UV system – especially in the infected patient zone – the results were almost as good as with doubling the ventilation rate, but only at a marginally higher additional energy cost. Prof. Noakes added: “This modelling showed that even if we started putting the UVC everywhere, we still had a lower energy requirement than with the ventilation enhancements. I would never advise reducing your ventilation rate, but if you can’t achieve the rates you need, it’s worth considering if some of these UV germicidal devices are effective. They must, though, be sized right for the room.”


In summary, Prof. Noakes said: “We have strong evidence that the virus is transmitted when people are close together, and growing evidence that the aerosol transmission matters, particularly for ventilation. There remain many unknowns, however; for example, how much virus is in exhaled breath, which is critical. Nor do we know how this varies with different people, or with the disease. Equally, we don’t know how much virus you have to be exposed to get infected, and until then it will very difficult to


Scenario 1


2 3 4 5 6


ACH 3 6 3 3 3 3


UV


None None


Zone 1


Zones 1-4 Zones 5, 6 All zones


conduct quantitative risk assessments. We also don’t know the relative importance of the different transmission routes. We know that aerosols, droplets, and surfaces, all matter, but not how these all differ at different junctures in the disease, or in a hospital, compared with in the community. I suspect the transmission risks from very sick patients are far lower than for community cases.”


Precautions as we enter winter The Professor added: “We don’t yet fully understand the importance of the aerosols, particularly at longer distances. I do think there is an element of precautionary principle here as we go into winter – to think how we keep ventilation up. We also lack evidence for mitigation strategies; we’re currently throwing the book at it – cleaning and washing everything, wearing face masks, and using ventilation. While not certain of these measures’ relative importance, it’s looking like the facecoverings and ventilation matter, and we believe a lot of the cleaning might not be as necessary as we first imagined, although I would never recommend reducing your hospital cleaning.”


Professor Noakes conceded that the sector ‘lacks quote a lot of practical experience and expertise around deploying UVC technologies’. She said: “I think the professional engineering community really needs to consider how we build the information up here.” Finishing with ‘a few practical things to consider’, she said: “I would say, for now, assume aerosols matter, and identify where you have poorly ventilated multi- occupant spaces, reduce their use, and mitigate the transmission risk there. Equally, think about how to maintain ventilation throughout the winter; it may be better to air a room frequently than to use continuous ventilation, but also consider comfort, and that if you do change anything, you may create pressure changes.


M Zone 1 Zone 5 Zone 2 Zone 3 Zone 6 Zone 4


“Many people ask about desk fans; I would say that if they are within a patient space, and only providing comfort to that space, they are fine. However, I’d advise against positioning them where they might move air down a corridor or between zones. Really consider the risk in non-COVID areas too. The biggest risks are where you have asymptomatic transmission from staff or other patients, so don’t just consider COVID wards, but also corridors, waiting rooms, and small consulting and treatment rooms. Not so much your operating theatres, as they are very highly ventilated; it’s the small poorly ventilated spaces that pose the biggest risks.”


Modelling ward-level systems.


With that, Professor Noakes closed a very informative presentation, before taking questions.


hej November 2020 Health Estate Journal 39


©Dr Carl Noakes/University of Leeds


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