On Wednesday I tuned into a 15-minute segment on RTE Radio 1 discussing the role of ventilation in managing COVID-19 risk with Prof. Orla Hegarty, School of Architecture in UCD and Kim Roberts, a Virologist in TCD and would encourage anyone with an interest in this topic to have a listen.
Perhaps the most surprising thing was Prof. Hegarty stating that while risks of contamination from surfaces represented the initial focus in March, it is now known that these risks are ‘overstated’ and airborne spread is now thought to be the primary factor in super-spreading events. If conditions are right, ie buildings with poor ventilation, it is likely that even following the 2m social distancing and 15-minute in-company rule, that the COVID-19 infection risk is still significant. She states that ‘the key [to minimising spread of the pandemic] is not to tell everyone to have half their contacts but to de-risk the locations which can give rise to super-spreader events.’ Further to this, were these factors considered, it would enable much safer reopening of the economy because assessing ventilation and air quality is measurable. Currently, at least in Ireland, NPHET (Ireland’s answer to SAGE) are not factoring ventilation into their guidance in any meaningful way.
The k Number
However, we must understand where Prof Hegarty’s comment is coming from. When talking about SARS-CoV-2 transmission, the strain of virus that causes COVID-19, we talk about the R0 (pathogen reproduction number) which as of October 24th was 1.06 in Ireland and Germany, 1.02 in Italy, 1.04 in the UK, and 1.1 in the USA. However, when estimating the transmission rate, we must look deeper into the characteristics of this virus and consider the k number (pathogen conversion). The k value tells us whether a virus will be spread in a linear manner or whether there are more likely to be clusters of outbreaks. Influenza for example has a median R-value of 1.28, and a k value of 1 [1] – it has a linear transmission making it easier to estimate infection rates. SARS-CoV-2, however, has an R-value of 2-3 but a k value of 0.1, this low k-value means that around 80% of COVID-19 cases are caused by 10% of people, and results from several studies support this. [2] This low k value is the reason behind the, now unfortunately famous, COVID-19 cluster outbreaks.
This k value also means that COVID-19 transmission is influenced by several general factors; biological, environmental, and behavioural.
Behavioural Factors
We can’t change our biology, but we can reduce the risks of COVID-19 transmission by adjusting our behaviour and ensuring our surroundings adhere to certain rules. We obviously cannot know who is within that 10% of super-spreaders, so we should act like everyone is. As such, we must, at a minimum, ensure that we all follow the environmental and behavioural guidelines that were designed to minimise the chance of infection and be strict on the ‘Covidiots’ that we’re seeing everywhere.
This fantastic article [3] shows how COVID-19 is spread in different indoor scenarios. It looks at behaviour, showing that talking increases the transmission rate by a factor of ten over the infected person remaining silent, and by singing or shouting you are fifty times more likely to transmit the virus. One of the settings for the analysis of potential transmission is a bar – after four hours (in a worst-case scenario) fourteen people would be infected, but if they are wearing masks then only eight people would be infected. However, if the people spent less than four hours in the bar and the bar was adequately ventilated then the infection risk drops down to one person.
Environmental Factors
This brings us neatly to environmental factors.
Let’s look at the widely publicised outbreaks in meat plants in Ireland in the summer, and outbreaks in Israeli schools in September which contributed to the government implementing a severe lockdown. So, what is the link between them? Well in both cases, warm outdoor air led to high levels of indoor air recirculation, allowing virus particles to contaminate a significant number of people. The key then is to stop recirculating air and to pump in fresh air instead.
However, there is a trade-off with comfort, not to mention the cost, given that in the Northern Hemisphere, pumping in fresh air during the next 6 months (who am I kidding, 8 months) will dramatically increase building energy costs.
Dr Roberts states in the piece that there is a lack of hard data about how best to strike that balance, with one paper stating that windows should be opened for 5 minutes every hour, depending on the occupancy. We know, however, from speaking to teachers and principals all over Ireland that they are opting to leave windows and doors open permanently, but some building owners and facilities managers are actually at a loss on how they should proceed.
Data is the Missing Piece
It is clear (at least to me) that data is the missing piece of information. Yes, it may be required to leave windows and doors open permanently in some classrooms and yes, it may be required to pump fresh air continuously through HVAC systems in others, but we should be able to provide data to prove or disprove this and to help schools, commercial buildings and healthcare facilities to establish appropriate, location-specific rules and help them de-risk their premises and provide a safe working and teaching environment as we head into these critical six months.
References
[1] https://bmcinfectdis.biomedcentral.com/articles/10.1186/1471-2334-14-480
[2] https://medium.com/data-in-the-time-of-corona/you-are-the-weakest-link-796d9eb5d560
[3] https://english.elpais.com/society/2020-10-28/a-room-a-bar-and-a-class-how-the-coronavirus-is-spread-through-the-air.html
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