Above is a plot of prevalence of COVID-19 in children versus the concentration of carbon dioxide (CO2) in their classrooms. Prevalence is defined as being number of children who test positive for COVID-19 per 100, so 0.01 means one in 10,000 children has COVID-19. Cumulative CO2 concentration is obtained by integrating CO2 concentration over something like a school day, I think. The data is from a study by Empa (part of the Swiss university ETH), of classrooms in Graubünden, a canton of Switzerland. As they say, this is very preliminary, but although of course the data is noisy there does seem to be a trend. The R in the plot is Pearson’s correlation coefficient, and a value of 0.72 suggests a significant correlation between the amount of CO2 in the air children are breathing in classrooms, and how many are becoming infected with COVID-19.
It is pretty clear that COVID-19 transmission is (at least predominantly) via small aerosol particles breathed out by an infected person, then breathed in by another person, who becomes infected. If you open the windows of a classroom, then air flows out and is replaced by fresh air from outside. We all breathe out lots of CO2 – produced by our metabolism – and so opening the windows allows this CO2 out. If the classroom contains an infected person, then with the CO2, aerosol particles containing virus also leave the classroom. This lowers the concentration of virus in the classroom air and so the risk of infection.
It is nice to see studies of this sort, although I am not sure why after almost two years we are still at the preliminary data stage in Switzerland while in the UK we seem to be not even at this stage. Increasing ventilation does seem to be rather easy way to reduce transmission, and not just in schools. A crowded room such as a party would also benefit from better ventilation, to reduce the risk. Unfortunately our government seems more interested in organising their own parties than in making our parties safer.