AIRBORNE INFECTION CONTROL


Online Yorkshire Branch of the Month event participants (l-r) Cath Noakes, Chris Davies, and Pete Sellars.


the ventilation in use, the size, temperature, and humidity of the space, and the sampling methodology. To test these is challenging; indeed it’s recognised that there is both a lack of test facilities able to undertake this work, and of test standards. It’s extremely difficult to compare data from different manufacturers, and, as a manufacturer, to get testing undertaken to a particular quality.”


Showing further slides, Prof. Noakes emphasised ‘the importance of ventilation rates’, in terms of their effect on introducing an air cleaning device into a room with different flow rates. She said: “If your room is already ventilated at 10 air changes/hour, it’s unlikely putting in an air cleaning unit will make much difference.”


Real-world performance It was also very difficult to show how effectively such devices worked ‘in reality’. She pointed to a slide: “This is a chamber test where we compared experimental data with CFD studies, with a device mounted on the wall, ceiling, or very close to the source. We got the best results with it on the wall, or close to the source, but in the ceiling, it appeared to make matters worse, although of course it can’t in reality. In fact what had happened was that the device changed the flow pattern in the room, and then, when you sampled at a particular location, it appeared to give you a higher concentration, all demonstrating how challenging it is to actually measure and model these things under real-world circumstances.”


Different types of device Turning to the different device types available, the Professor said she considered filtration-type devices as among the most reliable and effective (they could also remove some pollutants), although they could be noisy in use. They were often characterised by a ‘clean air delivery rate’, which encompassed both the flow rate, and the system’s efficacy with different particle sizes and pollutants. Prof. Noakes said: “I will also touch on germicidal ultraviolet systems – which use light in the UVC range to damage microorganisms’ DNA, but depend for their efficacy on the species,


30 Health Estate Journal April 2021


the climate conditions, and the UV dose. Most germicidal ultraviolet,” she continued, “is at a wavelength of about 254 nm, although there is emerging technology with far UV, at 222 nm; the latter is very promising, but has not yet been tested at scale. There are lights which cover the wavelengths, but I’d be cautious, as they produce ozone as a by- product, so, UV-wise, 254 nm is probably the most effective currently.” With a number of laboratory studies already undertaken on the germicidal effectiveness of UV, the Professor said there was substantial data available, including on its effect on the coronavirus. She added: “There are some real-world studies, including some demonstrating an effectiveness in a clinical environment, so, of all the air cleaning devices, UV is probably the one with the most potential benefit in a clinical setting.”


Factors impacting effectiveness Such systems could be applied ‘within a box’ or in a duct (Fig 6), with the effectiveness depending on the number of lamps, the air flow, and the susceptibility of the particular microorganism; bacteria and viruses tended to be more susceptible than fungi and sporiforms.” Here the Professor showed a model of a germicidal UV system in a duct, with a single lamp creating a surrounding UV field. She said: “We modelled the dose this lamp would give using computational fluid dynamics (Fig 7), and if you look at the table, the CFD data is pretty similar to the test data, which is the UVA data. What was


Table 1: What is in a device? Device 1


interesting is that the model allows us to explore what happens, and if you were just to do an experiment, you’d get a dose of about 10 joules per square metre; what you’ll see from the CFD model is that quite a lot of particles that go through get less than that average dose, and a small number more, so it’s very unevenly distributed. So, some microorganisms get over-exposed, while others are under- exposed, indicating that this particular device is not very efficient in design.”


Upper UV room units The speaker next showed a slide demonstrating the use of upper room UV units in a homeless shelter in New York. She said: “On the bottom right, you can see a study we set up to test some of these devices experimentally and computationally. They are quite complex in operation – you have interaction between the airflow, the light itself, and the microorganisms. We can measure and model the UV distribution from the light, combining this with an airflow model, to show where the UV light interacts, and how it distributes the dose, and thus inactivates microorganisms in a room. What we showed is that they perform better at lower ventilation rates – as with all air cleaning devices – but also that the airflow pattern in the room changes the device’s effectiveness. We consistently see that it’s is lower when we have the air introduced at high level, and removed at low level. We couldn’t say this was consistent for all rooms – this was just for this chamber model – but it tells you that the airflow pattern matters.”


HEPA filter only, no ionisation HEPA filter plus ionisation


Ionisation only, HEPA removed 2


HEPA filter only, no ionisation HEPA filter plus ionisation


Ionisation only, HEPA removed


Reduction (%) 60.2 62.1 25.2


52.9 28.1 1.6


©Dr Louise Fletcher, University of Leeds


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