Author Archives: Richard Sear

Computational physicist at the University of Surrey. My research interests are in COVID-19 transmission, especially masks, soft matter & biological physics

Which is harder to kill? Bad news wrapped in protein, or in soap?

Viruses have been described* as “bad news wrapped in protein” because many of them are just the genes of the virus – the bad news bit which takes over an infected cell – wrapped in a protein shell**. The wrapping is an elastic shell a few nanometres thick that is assembled from proteins. The image just below shows a reconstruction of such a protein shell:

The colours are false, they just use different colours for different proteins. This protein shell protects the “bad news” inside. But not all viruses are like this. Some, such as SAR-CoV-2 shown at the top of this post, are “bad news wrapped in soap”. Instead of a shell made of an array of protein molecules, it is (mostly) made of soapy/fatty molecules called lipids (shown in grey).

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Health risks of elevated carbon dioxide in the air we breathe

On Sunday the MailOnline published an alarmist article on what they say are the risks of wearing a face mask, due to elevated carbon dioxide (CO2). Masks are an obstacle to breathing and so although when you wear a mask you just breathe a little bit harder and so get all the oxygen you need, they do retain a bit of the carbon dioxide you breathe out. When you breathe in, you tend to breathe in more carbon dioxide then if you are not wearing a mask. This is a well-known problem and the FFP2 standard for masks specifies that the carbon dioxide measured should be no more than 1%, which is 10,000 ppm* (parts per million). If you look at an FFP2 standard certificate you see they measure the carbon dioxide (via a method I don’t quite follow but I assume people have worked out a good way). For that case they measured carbon dioxide at 0.6% or 6,000 ppm.

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Viruses can infect animals across tens of kilometres, and the English channel, can they infect humans across these distances?

Particularly in the early stages of the COVID-19 pandemic, there were still medics who thought that because the virus was confined to some sort of mythical fast dropping, droplets, the virus could only be transmitted over distances of a metre or two. The virus could not go further. This can’t be true as it conflicts with basic aerosol physics. And there is pretty strong, pretty direct, evidence for transmission of COVID-19 across large rooms. But what I had not appreciated until this week was that people studying transmission of the virus that causes foot-and-mouth disease*, have pretty good evidence that the virus can carry over tens of kilometres. For example, from northern France across the English Channel to the Isle of Wight. The distance from the Isle of Wight to France is of course a lot more than two metres.

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Viruses need to survive a large but slow shock, to spread across a room

Someone infected with COVID-19, flu, measles, maybe mpox, or any one of a number of a bunch of viruses, breathes out the virus in tiny (micrometre) droplets of mucus. The droplets start in the infected person’s breath, which is saturated with water, i.e., at 100 % relative humidity (RH), because the breath has come straight from the infected person’s lungs. As the breath mixes with the air of a room (or of the surrounding air if they are outdoors), the humidity drops from 100% to the typical 40 to 60% of the air in and outdoors. This mixing takes a few seconds, and driven by this change of humidity the droplet dries out. The virus needs to survive this sudden drying intact, in order to go on to infect a new person. Drying in a few seconds may sound quick, but viruses are small so this may actually be felt as slow drying by the virus.

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Masks when you are breathing: A hostile environment for viruses?

Early on in the pandemic there were all sorts of schemes going around to allow FFP2 and N95 masks to be reused; in hospitals they are traditionally worn only once (for say a single shift) and discarded but then there was a shortage so people wanted to extend the period a mask could be used. Were these schemes to try and clean a mask needed?

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Tyndall showed us that dust is repelled by heat and attracted by cold

In 1870 John Tyndall was a prominent Irish scientist; he is now perhaps less known, possibly because he studied unglamorous things like dust. In a presentation that year he noted that a heated wire repelled dust particles. Note that this is separate from the convection of the air itself that the heated wire also causes. In addition to the convection the particles move relative to the air. Shortly afterwards his contemporary James Clerk Maxwell gave the following explanation, which is probably mostly right*.

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COVID-19 infections come in three to four-month-long waves, for some unknown reason(s)

The title of this blog post kind of says it all, see the plot above. The plot is for the calendar year 2022 and for each week shows what percentage of COVID tests come back positive, saying that the person is infected. Data is from ONS. Note that 2022 starts with the back end of a wave. At the start of the 2022 infections were high but dropping, then there are peaks around March, July and October, before infections start climbing again at the end of the year. So the data are as clear as day, the question is: What is going on? Why the wave every three to four months?

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With all windows shut, the air in my home turns over about once per ten hours

Over part of Christmas I was away for a few days. As you can see from the plot, this was long enough for the CO2 concentration inside my flat to relax to the atmospheric value (or close to that) of a bit over 400 ppm. The plot has time as the x-axis, starting at midnight of the day I left. I left at about midday of the first day and the CO2 concentration then relaxed back to about 450 ppm, and stayed there. The relaxation is well fit by an exponential function with a time constant of close to ten hours. The fit is the dashed cyan curve.

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Keeping the air you breathe at home reasonably fresh in winter

On Wednesday 14th December 2022 it was around freezing (0C) all day, and night. I worked at home in the morning to around midday, then was in work from midday-ish to five-ish. The plot shows the CO2 levels in my home for the 24 hours of that day, midnight to midnight. When I was home I kept small windows in my bedroom and kitchen open a few centimetres to keep some air turning over. As you can see from the plot, when I was at home the CO2 level was mostly between 800 and 1000 ppm (parts per million).

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