AIRBORNE INFECTION CONTROL 250 1200 200 1000 800 150 600 100 400 50 200 0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Sample number
Steady state test – continuous occupancy • Room is subject to a continuous source of contamination • Samples with the device switched off and on • Difference is reduction due to the device - % or log reduction
Figure 5: Steady state versus decay.
often here we are actually referring to microbial reductions, so one log reduction equates to a 90 per cent removal, and a 2 log reduction, a 99 per cent removal. That is worth noting, because if an air cleaning device manufacturer states a log reduction, you can identify what that means in terms of equivalent air changes.
Deposition’s impact
“We also, of course,” the Professor continued, “lose quite a lot through deposition, particularly over longer time periods, certainly with the biggest aerosols.” She demonstrated this via a graph (Fig 3) showing the time it takes for different-sized particles in still air to fall. For example, a 20 μmparticle would take some three minutes, and a 50 μmparticle about 30 seconds. Conversely, a 5 μm particle could still be in the air after 45 minutes, and potentially, considerably longer. She said: “The red curve here plots the fall in velocity – you can see that the 20 μmparticle that takes three minutes to fall, and has a falling velocity of less than 0.025 m/s, and that even a 50 micron particle falls at about 0.075 m/s. Anyone knowledgeable about hospital room air velocities will know we typically see velocities of up to 0.05 m/s, and quite often up to 0.1 m/s. Air is not still, and so these particles may well take longer to fall, and indeed travel further, than you think.” The Professor said healthcare engineers also needed to think about individuals how got exposed, showing a graphic illustrating how different-sized particles could be inhaled into the different parts of the lung. She elaborated: “The ‘respirable convention’ refers to particles that can be inhaled right to the deep lung, the ‘thoracic convention’ to those reaching the top of the lung, and the ‘inhalable convention’ to the matter that deposits in the nose, at the very top of the respiratory
28 Health Estate Journal April 2021
tract. With tuberculosis,” she added, “we only really care about the respirable connection, but for SARS-CoV-2, all of the particle sizes potentially matter.”
Aerosol-generating procedures Here she showed another graph illustrating the relationship between air change rate and particle removal, that she had put together for aerosol- generating procedures in dentistry. She explained: “In a well-mixed room, looking at how long it takes to remove 90 per cent of the aerosols, the blue line shows a completely unmitigated procedure, and the red what happens with some mitigation – in this case dental dam and high volume section at source reduces the source concentration.” Next showing a slide relating to oral bacteria measured in two dental surgeries, in exactly the same way, she said: “You see spikes, but also that things come down to a background level within about half an hour of generation, so what happens in a real- world setting can be quite complicated. Go into a real hospital environment (she showed data of bacteria counts in in air measured with a laser particle counter in a respiratory ward), and the bacteria and particle counts don’t necessarily correlate – which tells us that some of these particles are not bacteria. Equally, it confirms that things fluctuate with activity; for example whether the windows are open, and the number of people in the room. So, we are not trying to contain something really uniform, but something that varies all the time.”
Air cleaning in more detail Having set the context, Prof. Noakes next moved to the crux of her address – air cleaning devices. She said: “Air cleaning is based around some form of technology to remove or inactivate microorganisms;
there are many different technologies available.” She explained that when considering using such a system, thinking about its efficacy, and the accompanying evidence, were key, as was the energy consumed by the device, and whether the technology affected air quality in another way, and any accompanying risks. For example, some systems produced secondary aerosols and pollutants. Looking at where air cleaning systems would be applied, the speaker said some form of air cleaning technology could be put into the supply air in the healthcare facility’s ventilation system, which was particularly likely to prove effective in locations housing high-risk patients, or where recirculated air was in use (Fig 4). She explained: “Some of these air cleaning systems have benefits for HVAC system performance, for example in keeping the cooling coil clean, and improving energy efficiency. What they won’t deal with is a contaminant source in the room. The first question you should ask is: ‘Where is the source of the infection I must address, and does this inform where we install the system?’”
Looking at the room, there were a number of potential different locations for an air cleaning device – depending on its technology and design. Many comprised ‘some form of box with a fan in it’, that draws through the air and treats it. Air cleaning systems could either be installed in say, a ceiling, or on a high wall, like an air-conditioning system, making them part of the building services, or could be local, ‘plug-in’, portable units. Both types were of a single pass design.
More specialist designs The Professor said: “We then have another couple of more specialist unit types – the first being upper room UV, where you create an open field of
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 9 20 Elapsed time (min)
Decay test – removal rate • Short-term contamination event • Samples during decay with device off, and device on • Difference in decay rates indicates the efficacy of the device
— Control — With Device
Number of cfu/m3
Number of cfu/m3
©Dr Louise Fletcher, University of Leeds
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