IHEEM DIGITAL WEEK – INFECTION CONTROL
Professor Milton’s work using the Gesundheit machine, which focused on both Influenza and a seasonal coronavirus (not SARS-CoV-2), had shown that masks will block both the fine and larger particles quite significantly.
The two-metre rule
Noting that much of the past six months’ discussions had been dominated by ‘the two-metre rule’, Prof. Noakes added: “When you are very close to somebody you are exposed to this very high concentration cloud of droplets and aerosols, but as they get further away, this disperses, and some of the deposition onto surfaces is lost. If you are in the same room, the level depends on what remains in the air, which depends on the ventilation rate. Potentially, you could also be exposed down the corridor and in the neighbouring room, depending on the ventilation set-up.” Currently, there was very little evidence of the latter, apart from potentially in chilled food processing settings with very high levels of recirculated air. Prof. Noakes explained that how far something such as exhaled virus projects depended both on the ejection rate, and the ‘violence’ of it; thus sneezes and coughs travelled much further than when somebody simply breathes. She said: “Some very nice recent work has shown that when somebody is singing or talking continuously, they actually create disturbances in the air that propagate and keep going.” This phenomenon could sometimes be seen on watching somebody smoking or vaping, with a resulting cloud ‘hanging’ for longer.
Engineering factors
From an engineering perspective, the goal was to break the transmission chain, to prevent the source reaching the susceptible person – achievable in a number of ways. She explained: “From an airborne perspective. we can look at ventilation and filtration, and things like surface technology. We can also examine how the engineering influences human behaviour.”
On ventilation, Prof. Noakes said: “A key question is: ‘How much ventilation do we need, and how much is important?’ One way we can start thinking about this is to ask whether we can relate ventilation rate to risk.” This was ‘not very easy to do’, although there were modelling tools available, such as the Wells-Riley model, which relates how many infections one might get with time, taking into account infectious dose (the ‘quanta’), how much people breathe, exposure time, and the room ventilation. These factors determined a concentration in air that people breathe in over time.
38 Health Estate Journal November 2020
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4.20 m Upper room UV application.
Some unknown parameters Prof. Noakes said a key ‘difficulty’ with the coronavirus was that many of the parameters were unknown. She said: “We do know, however, that there are some very big ranges.” Here she showed a ‘fairly complex’ graph illustrating results from running various models for different emission rates – ranging from a ‘normal’ COVID case, to a ‘slightly higher shedding’ case, to one involving somebody singing or shouting. She said: “What we are looking at is what percentage of people are affected, with ventilation rate, and built into this model is a lot of variability of parameters. Relating the evidence we have so far to people, and particular ventilation rates and rooms, what we see is that when the room ventilation rate is from about 1-3 litres/second/person, the risks really shoot up. Once, however, it is 5 L or 10 L/s/person ideally, the risk is, in most cases, controlled.” Thus, based on current evidence, Prof. Noakes said if a space is ventilated at its design flow rate – typically for a hospital ward, six air changes per hour – the transmission risk is very low, but increases with unventilated spaces, such as staff offices, with all the windows shut, waiting areas, or corridors.”
Corridor study
Prof. Noakes referred to an experimental study involving an individual walking up and down a corridor, where a contaminant is released, and the person inadvertently ‘mixes’ with it via the air. Her slides showed the extent to which the contaminant ‘dispersed out’ as the person ‘mixed’ with it, depending on the speed of walking, corridor width, the ‘width’ of the person, and time. She explained that as a result, it would be possible to model what this activity meant ‘in terms of breathing’. “If, for example,” she said, “you took 5-6 breaths during your walk down the corridor, some may contain very little contaminant, and others a much higher
amount. With a fully mixed airflow, however, you would be breathing a more consistent amount. One inhales more cumulatively in a fully mixed airflow, compared with a transient one, but gets a much higher dose in the latter.” This study evidence pointed to the need to consider corridors and how well ventilated they are, ‘and that our interactions with a space are quite complex, and can potentially change our exposure without us realising’.
Germicidal UV
The speaker’s next focus was germicidal ultraviolet (UV technology), and its potential effectiveness as a control method against the virus. She explained that germicidal UV devices use UVC light to lethally damage the DNA of microorganisms, but that their effectiveness depends on the species, climactic conditions, and the amount of UV the bacteria/virus is exposed to. To date, there was very little evidence on the impact of germicidal UVC on coronavirus, but some laboratory data indicating that it will kill SARS and MERS. Discussing its potential use in a healthcare setting, Prof. Noakes explained that germicidal UVC light technology could be incorporated into the ventilation supply air – via ‘a very simple plug-in device’, a more sophisticated version, akin to an air- conditioning unit, and requiring proper installation, or the most effective – using an upper room UV unit, which creates an open UV field above people’s heads. The latter was, however, ‘much harder to install correctly’; the installer needed to ensure no residual UVC in the occupied zone below, given that it is ‘quite hazardous to health’.
Another potential ally against airborne viruses was UV robots, although these were for disinfection only, for unoccupied spaces. The Professor said: “I am not sure they are going to do much for coronavirus, beyond what normal cleaning can do.”
•Complex 3D airflow patterns
•3D UV light fields
•Microorganism source/dispersion
•Microorganism susceptibility
Small UV device
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©Dr Carl Gilkeson/University of Leeds
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