IHEEM DIGITAL WEEK – INFECTION CONTROL
Machines used for sampling for microorganisms.
aerosols, while the larger droplets could deposit onto the eyes, nose, mouth or face, and onto surfaces.
Scientists were not yet certain of the balance between the various transmission routes, nor how effective the mitigation measures for each were – a key goal over the next 12 months would be ‘to try to unpick this’. One known risk factor was proximity to others, especially face to face. Prof. Noakes added: “We also know that from an aerosol perspective, the risk increases where ventilation is poor, and people are closer. Even with contact transmission, the risk increases with a shorter distance, because you’re more likely to be touching the contaminated surfaces; equally, without good cleaning and hand hygiene, the risks rise.” Also, the longer an individual is close to another, the higher the risk. It was also known that several activities – such as shouting and singing – increased ‘viral shredding’, while laboratory data indicated that the virus favours cool, dry conditions, and that exposure to high sunlight levels results in it ‘dying fairly quickly’. In dark environments, it can survive for very long periods. Although currently all based around laboratory data, some of these findings would almost certainly translate to real-life environments; for instance a number of chilled food processing plants had seen outbreaks.
Aerosols vs. droplets
Looking next at aerosol and droplet ‘mechanisms’, Prof. Noakes explained that an aerosol was defined as ‘a suspension of fine solid or liquid droplets in a gas’, and a droplet as ‘a very small drop of liquid’. She used her slides to give an idea of comparative sizes. She said: “The circle on the screen is a 100 micron droplet, a little bit bigger than a human hair, which is typically some 60 microns wide. A 10 micron droplet, inhaled, will go quite
36 Health Estate Journal November 2020
a long way into your lungs. So,” said the Professor, “we are principally concerned about aerosols and droplets ranging in size from under 1, to over 100, microns, and possibly even bigger. We have to think how these particles will behave in air.”
Looking at ‘classic infection control’, Prof. Noakes said particles under 5 microns tended to be considered as airborne, and of over 100 microns as droplets. She said: “These airborne droplets are the ones with which, for example, we associate with tuberculosis, a classic true airborne disease that has to infect the lung.” SARS-CoV-2 viruses were ‘a little bit different’, in that it is believed they do not necessarily need not to reach the lung to infect; instead this can happen via receptors in the nose, mouth, and upper respiratory tract. Turning to droplets landing on surfaces, and while a very fine aerosol would take ‘a very long time’ to settle, a droplet sized from 100 microns upwards would settle very quickly and potentially contaminate the
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0 10 The fate of aerosols. 20 30
surface. The Professor said: “Given that humans cannot see anything under about 40 microns, if you can see it, it is a droplet, or a very large aerosol. The importance of these different sizes depends on the transmission route.”
Slides from 1946
Looking at some interesting data, via her slides, the speaker first showed some historical information from 1946, which ‘nicely showed what is in someone’s exhaled breath’. Others illustrated the impact of somebody counting to 100, coughing, and sneezing. Prof. Noakes said: “What these show is the number of particles produced, and the size distribution. In all cases, most of the particles are under 50 microns, and many are under 10 microns in diameter. More recent data shows more fine particles, because back in 1946 they couldn’t measure those. We now use optical counters for this.”
Sampling for microorganisms within such particles was challenging, the
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Falling velocity (m/s)
©GII: Don Milton\University of Maryland
©CASS: Fennelly et al 2012 Time to fall 2 metres (min)
©Professor Cath Noakes, University of Leeds
©CASS: Fennelly et al 2012
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