I'm pretty sure it would be swamped by changes in precisely how big the average testing hall is, and how its heating system works. I would not be surprised if

I'm pretty sure it would be swamped by changes in precisely how big the average testing hall is, and how its heating system works. I would not be surprised if typical school-air CO2 level has changed by 50% over the last century, but which direction I would struggle to guess. We have more indoor space per person now, but also seal buildings tighter. Especially in summer.