As expected, many countries are now relaxing their lockdowns as case numbers and deaths move past the peak. In the UK on Sunday, the PM took 15 minutes to explain what was going on although spectacularly failed to explain anything. The debacle, watched by 30 million people, has left us with vague and often contradictory public health messaging about how we should all behave, which is the worst possible approach.
This article provides a summary of the advice and attempted “clarifications” from the past 3 days: https://www.huffpost.com/entry/how-boris-johnsons-stay-alert-message-unravelled-in-24-calamitous-hours_n_5ebb4d71c5b6b58e4cc98f7c
With the government failing to provide credible expert guidance, we have been left largely on our own. Tempted to rely on our “common sense”, humans are terrible at intuitively understanding risk, especially where statistics and brand-new problems are involved. We love to draw analogies e.g. “it is just a flu” even if these analogies are not very good. My approach is to research the virus as much as I can and share my current thinking on what I’m doing to stay safe.
I have been disappointed by the output from governments, health bodies and academia, but have found a few bright people able to communicate the science. Prof Erin Bromage of Dartmouth University is excellent, and I have incorporated much of his recent post in my thinking. https://www.erinbromage.com/post/about-the-author-professor-erin-bromage
Please be careful and do not imagine I am giving you all health advice or that there is certainty on any of these items. This is simply my current understanding of the science and how it applies to behaviour.
How the virus is transmitted
My understanding of how we become infected through surfaces has not changed very much and I have nothing to add to the official guidance. The advice on handwashing and not touching your face, combined with regular cleaning of surfaces seems a highly sensible way to reduce the risk of transmission.
I have changed my thinking around infection through the air. It seems that most cases of transmission come via the air rather than surfaces and given this is the main risk it should be the focus of our public health advice on return to work as it is much harder to manage as an individual. In particular the risk comes from aerosol i.e. the very small particles which last far longer in the air and move further than the 2m social distance guidelines.
Initially I made the mistake of thinking that viral transmission is a bit like a game of tig – if you got too close to an infected person you were “it”. It is often portrayed that you are safe from being “it” or safe from infection if you stay more than 2 metres away from other people, “social distancing”. The more I have read, the more this seems a poor metaphor for the transmission of a virus.
To become infected with a virus, there is a threshold number of virus particles required to enter your system. If there are just a few particles, then your immune system recognises them, bats them away and you are not infected. If there are many, the system is overwhelmed, and the virus freely replicates.
This number must vary by person but let’s assume a threshold of 1000 particles as a reasonable benchmark. Your risk of infection is determined by the number of particles in the air and how long you are exposed for:
Infection = Exposure x Time
If your exposure were 50 particles per minute and you spent 20 minutes in that environment, then you would reach the threshold of 1000 particles and be at serious risk of infection. Higher numbers of particles in the air or longer exposure both of course lead to higher risk.
How many virus particles does an infected person put into the air?
Initially let’s assume we are outside in good airflow and let’s ignore how long the particles stay in the air. The number of particles released by an infected person will vary depending on whether the person is:
- Heavy breathing/singing
Considering each in turn:
- Breathing– not very risky
There is a low amount of viral material, perhaps only 3-20 virus RNA copies per minute. In addition, the water particles you breathe out upon which the virus sits are low velocity and so drop to the ground quickly. This means that even without good social distancing it would take a long time to become infected perhaps many hours, so in this situation social distancing should be very effective.
- Speaking – more risky
Speaking generates as many as 10 times as many droplets as breathing i.e. up to 200 per minute which means we could reach our 1000 particle threshold in as little as 5 minutes. Of course, in most conversations we each speak about half the time with pauses, so it seems more reasonable to think of 10-15 mins as a reasonable threshold for dangerous exposure levels. Here the good news is that the velocity of the particles is again low and so social distancing of at least 2m should be effective. Adding more distance of course, would make this even safer and so extended conversation outdoors should be safe.
- Heavy breathing/singing – be far more careful
Playing sport or singing in a choir is significantly more dangerous than simply breathing or speaking. Here the number of particles and their radius of spread is far greater. If someone is simply walking past, then the time of exposure is so short so as to not be too risky, but any activity where you are within a few metres for an extended period will be dangerous.
- Cough and sneeze – very risky
The number of virus particles expelled in coughs and sneezes is enormous and they travel at high velocity so social distancing will not protect you as the particles will go a lot further than 2m. If someone is coughing or sneezing, then you simply have to try to be nowhere near them and hope they sneeze in the other direction.
What does this mean for what we can safely do?
For both breathing and speaking, it takes time to get a dangerous level of exposure and social distancing outside will work. Passing people in the street or someone running past you is not dangerous and I feel less paranoid about passing contact with strangers. I can also be quite confident about talking to people more than 2m away outside, especially in the sunshine, where UV does a fantastic job of killing the virus in the air and on any surface.
But this is notably different from the current government advice which has been very restrictive on people having socially distanced interactions outside in the sunshine. It seems now you are allowed to sit 2m away from people outside but only if you don’t know them. I have found there to be a large difference between what I think is safe and the current government advice. They are still very restrictive on outdoor activities I think are safe and promoting indoor activities I think are still very dangerous.
What about Indoors?
The situation indoors is very different from outdoors and inherently riskier. Let’s consider 5 risk factors:
- Type breathing (normal breath, speaking, heavy breathing, cough, sneeze)
- Number of people
- Size of space
- Amount of ventilation
- Length of time
As described above socially distanced breathing and speaking are lower risk, but in an enclosed space, particles will continue to circulate in the room so you will not be protected from prolonged exposure. The guidance today from the government on workplace safety, suggests that simply having workers and commuters at least 2m apart on trains and offices will provide a significant level of protection. I do not think this is true.
Here is a sample office floorplan with social distancing, courtesy of WeWork
Here we have an example of an office with people all 2m apart and no open windows. The air in the room will slowly circulate and everyone in the room will receive a high level of exposure.
There are well documented examples of where this has happened. The restaurant below had airflow from right to left and over a 90-minute dinner, Diner A1 infects many people even at other tables, well beyond the 2m threshold.
This example from a call centre is even more compelling.
From only one infected employee over a single week, we see very high level of infection (note only 2 out of 94 infected were asymptomatic). The air circulates around the room over a long period and they are all exposed.
The issue of airflow distributing infection is certainly well known in the medical profession. The following diagram is from the Department of Health HTM 01-05 and shows how instruments and people should move in a sterile environment. But government are not recommending this concept being applied to other workplaces.
Simple advice on a 2m separation indoors is far from sufficient, but this has been overwhelmingly the focus of the government’s return to work advice. If a simple 2m rule indoors is not enough, then we need to better understand what matters and how to make indoor spaces safer. Simply opening public transport, offices, restaurants, and cinemas with social distancing is not going to work.
I would like to find research which gives parameters to turn the risk factors above into an operational risk model.
20 people, 140 sqm, speaking, low ventilation, 8 hours = what risk level?
Socially distanced office:
10 people, 140 sqm, speaking, low ventilation, 8 hours = what risk level?
How about a squash court?
2 people, 60 sqm, heavy breathing, low ventilation, I hour = what risk level?
I imagine that the relationships are non-linear and non-intuitive which means I cannot say how much safer a socially distanced office is from a normal one. Nor can I be confident if they are safer or worse than an hour of squash – one could easily be an order of magnitude more dangerous than the other. I need a lot more information before I can make good risk decisions.
Even though our government has given up we cannot.
- How far do particles travel and how long do they stay in the air? https://www.youtube.com/watch?time_continue=39&v=WZSKoNGTR6Q&feature=emb_logo
- Size of particles: amount of virus, travel, persistence, types of breathing
- If someone else has been in a room for an hour how long should I wait before I enter it?
- Does aircon and airflow make it worse by distributing the particles?
- How much ventilation is required to make indoor risk more similar to outdoor risk?
- Do plexiglass or masks actually help much?
- What level of risk are we aiming for in workspaces and why?
Medical practice uses the concept of Universal Precaution, but I cannot see that we can all wear PPE
2 thoughts on “What is safe?”
Most useful article I have read for a while, with my imminent return to my office looming…
This looks almost a month old, but is the best collection of studies on (potential) transmission pathways that I’ve found: https://www.worldometers.info/coronavirus/transmission/
It seems to be a mix of situation analysis from places were actual transmissions were identified (esp in hospital settings) and controlled physical studies (like the air duration and duration of surface contamination).