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.
But because I had the CO2 monitor with me, it recorded the CO2 levels during my journey to Bristol. This is plotted above and I found it quite interesting. The plot starts at 8 am in the morning when the CO2 monitor and I were at home, and I was having breakfast. The CO2 levels are around 600 ppm, which is pretty good*. There is no strict cutoff but often less than 1000 ppm CO2 is considered good ventilation.
The CO2 level dipped at 9 am as I walked to Guildford train station and was outside. I got on a train to Reading at about 9:15 am. Initially the train was quite empty but it filled up and was pretty full by the time we approached Reading. This shows up very clearly in the CO2 level which increased as the train filled up and reached quite high levels by the time we got to Reading.
The ventilation in this GWR train seems only adequate when the carriage is maybe a quarter full, beyond that the ventilation is maybe not really adequate for the number of people in the carriage.
Then a little after 10 am I got to Reading station and was outside again, so the CO2 level dropped. I got on the train to Bristol Temple Meads and the CO2 level went back up to higher-than-ideal levels. The ventilation in the trains on GWR’s London to Swansea line is also less than ideal. Then at 11:30 am I got to Bristol and the CO2 level dropped as I walked up the University.
I was a bit surprised by how high the CO2 levels are in the trains. I would guess that this is a choice, in the sense that the ventilation is adjustable and GWR could run it at a higher level but have chosen not to. I don’t know why, except that at this time of year fresh air brought in from outside needs to be heated, which does cost energy. With not only COVID-19 around but also flu, better ventilated trains would be better for the health of passengers and staff.
* The equation is that for every 400 ppm of CO2 concentration above 400 ppm, the air contains about 1% of other people’s exhaled air. So for example 800 ppm is 1% other people’s exhaled breath, 1,200 ppm is 2%, 1,600 ppm is 3%, and so on. I discuss this in an earlier post. Caveat: This assumes: 1) only CO2 source in train carriage is passengers. This should be OK. Think Guildford to Reading train was a diesel which will produce CO2 but I would hope diesel exhaust is well away from any ventilation inlets. 2) That trains don’t filter and recirculate air. If air is filtered (assuming a good filter this should remove 99%+ of any virus) and recirculated, then the CO2 concentration can be quite high but there will still be little risk of infection from airborne virus. The filters remove the virus particles not the CO2.
Chance & Necessity 