IHEEM DIGITAL WEEK – INFECTION CONTROL
Professor explained, and to be done using a specialist sampler. She showed pictures of two such machines – one ‘a fairly crude’ cough aerosol sampler designed to sample for TB, with a tube to capture exhaled breath. The other was a ‘far more sophisticated sampler’ used for viral sampling, designed by Sir Professor Don Milton at the University of Maryland – the ‘Gesundheit’ machine. The user of this places their head inside the cone, and the condensation aerosol sampler then size-segregates particles into below and above 5 microns in diameter, allowing identification of the virus within.
Fine and coarser fractions Giving some examples, via slides, of data from Professor Milton on Influenza, Prof. Noakes said: “What you can see here are the fine fractions of under 5 microns, and the coarser fractions. Both show quite significant virus amounts. You can see that with Influenza, there is an increase, but also a huge variation in the number of RNA copies per sample between individuals, which may also be attributable to their stage of the disease.” Professor Noakes said the scientific/ engineering community did possess some knowledge about how different activities have a varying impact on exhalation, and what is contained in exhaled breath. Data on inert aerosols, for example, demonstrated that when somebody was singing, breathing, or speaking, both the activity and the volume mattered. Pointing to a slide of breathing, she said: “You can see here that there is a very large distribution between individuals, so some people breathe out aerosols as much as other people would cough, but with very loud speaking or singing, there is an increase with volume. We thus know we get variations in droplets and aerosols produced, but not yet the extent to which this correlates to virus.”
By now, scientists knew that the viral load seen in an individual’s samples changed with the course of the disease. Prof. Noakes said: “This very early data relates to symptomatic cases, but you can see that with the throat and nasal swabs, you get a drop-off in the number of RNA copies over the day of the symptoms,
Depends on: •Lamps – number, location, intensity •Airflow – determines duration of UV exposure •Microorganism susceptibility
The Wells-Riley Equation Nc = S 1 – e Q Relates new infections (NC []( Iqpt ) ) with time (t) to disease, occupant and ventilation characteristics
• S = number of susceptibles • I = number of infectors • Q = room ventilation rate • P = occupant breathing rate • q = Quanta, number of infectious doses generated per unit time
Linking airflow and risk.
and the proportion with positive cultures falls quite significantly.” From a hospital perspective, this suggested very sick COVID patients may well not be very infectious. ‘The people ‘to worry about’ were individuals at the very start of the symptoms, such as asymptomatic healthcare workers, and anyone contracting coronavirus while in a hospital setting.
The physics explained
Having discussed aerosol and droplet sizes, the Professor moved on to ‘what happens to them’. She explained: “We think of an aerosols as particles in the air, subject to forces such as gravity and drag force, so they will settle depending on mass, but air velocity also comes in.” Explaining the evaporation process, Prof. Noakes emphasised that the aerosols were not water, but were rather contained in respiratory fluid comprising salts, surfactants, and proteins ‘of quite big mass’. When the droplets evaporate, the result is not ‘naked virus 100 nanometers in size’; instead, they will evaporate to a smaller particle concentrated with virus and other components of the respiratory fluid. Calculations suggested that at that juncture, the aerosols were between 20 and 50 per cent of their original diameter. She said: “The process is really quick; a 20 micron particle will evaporate down to its minimum size in under second, and even a 100 micron particle to 50 microns within five seconds.” Very small particles take anything up to 45 minutes to fall, but bigger particles, settling in still air, will settle much quicker, in anything from
three minutes to about 30 seconds. However, some of these evaporated particles might not settle that quickly. At three minutes, the evaporated aerosols might fall at of .015 m/s. Prof. Noakes added: “The air velocity in a room, however, is typically between about .05 and 0.1 m/s, and in an operating theatre, much higher. Even bigger particles have a falling speed of under 0.1 m/s, so, depending on the air velocities, even these particles might be affected. Sometimes a higher velocity will see them deposit out faster, but at others they may be carried further in the air.”
Managing the risk
Professor Noakes next moved to managing the risks of aerosol/droplet transmission. She said: “One of the basic principles we have advocated is based around using a hierarchy of risk controls, starting with elimination, i.e. not undertaking a work activity, or doing it online. We are predominantly then focusing on engineering controls – screens, barriers, doors, surfaces, and ventilation. These should sit above marking out signs on the floor, your cleaning regime, and PPE as a priority, since they provide the background control that enables these other things to happen effectively.” Showing a pyramid of hierarchy of risk controls, Prof. Noakes noted that as one descended it, the greater the control related to individual behaviour. “So, for instance,” she said, “PPE’s effectiveness depends very much on individual behaviour, while engineering controls, and the building and environment, are much higher up the pyramid.”
In-duct UVC light systems.
Turning to ‘some of the protective mechanisms’, the Professor said: “We talk a lot about masks – which serve to illustrate what happens with the air around the person.” She showed a slide of a woman coughing, firstly without a mask, and how it disturbs the airflow around her, with a thermal plume of about 0.2 m/s. Putting her mask on, however, very significantly lowered the distance the plume of air is carried, from around one metre, to ‘much less’.
November 2020 Health Estate Journal 37
©Dr Azael Capetillo/Tecnologico de Monterrey
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