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?
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*.
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?
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.
Better air quality at a cost of 1 p per hour per room
I typically leave a bedroom and a kitchen window open a crack, for ventilation. This time of year this lets in cold outside air that has to be heated to keep the flat warm. This costs money, but we can estimate that cost. See if it is large or small.
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).
AI-powered chatbot that can be scarily good, can be just as bad as the World Health Organisation
A new chatbot called chatGPT* is making waves. Some of its answers are scarily good. For example:
Question: What is a phase transition?
ChatGPT: A phase transition is a change in the physical state of a substance, such as a change from a solid to a liquid or a gas, or vice versa. Phase transitions are driven by changes in temperature, pressure, or other physical factors, and they can occur in both pure substances and mixtures. For example, water undergoes a phase transition from a liquid to a solid (ice) when it is cooled below 0 degrees Celsius, and it undergoes a phase transition from a solid to a gas (water vapor) when it is heated to its boiling point. The study of phase transitions is important in many fields, including chemistry, materials science, and meteorology.
ChatGPT (4th December 2022)
A morning in the life of my personal ventilation status
Yesterday I went to the University of Bristol to give a seminar, on some of the physics of COVID-19 transmission. It was great to see the people there and chat, and I took along a CO2 monitor so my seminar could include some real-time data on how well ventilated the seminar room was. It turns out that the ventilation was not that great, but I then opened a window in the room and it got better. I am sure many more intelligent and better delivered seminars have been given at the University of Bristol Physics Department. But they can’t be many seminars during which the seminar a potential health hazard was identified by the speaker, who then partially addressed it during the same seminar.
Evidence-based social distancing guidelines
Early on in the pandemic, social distancing measures were introduced, with various distances. The UK went for 2 m, the USA for 1.8 m (6 feet), Hong Kong 1 m, etc. None of these were based on much evidence, but we now know a lot more. COVID-19 is an airborne disease. Although more data is badly needed, it seems likely that somewhere between most, and almost all, infections are caused by breathing in virus, that an infected person breathed out seconds or minutes earlier. The virus is carried by tiny droplets suspended in the air. And most transmission is indoors.
A day in the life of my CO2 exposure
Over the summer I am continuing to play around with my new toy: a CO2 meter. This measures the carbon dioxide aka CO2 in the air. A plot of the CO2 concentration over one complete day (Thursday 11th August 2022) is shown above. This is in my bedroom. The units are ppm = parts per million, so 400 ppm means that out of every million molecules in the air, 400 are CO2.
Chance & Necessity 