An Affordable CO2 Monitor
I found a seemingly-high-quality CO2 monitor here for less than $50, or on Amazon or from the manufacturer’s website for $70.
The most recent discussions of different CO2 monitors on the market I could find on LessWrong[1] were a few years old, and it seems that prices have come down somewhat since then. I found this positive review of a CO2 monitor manufactured by Qingping,[2] and then found it on AliExpress for about $40 (though it seems to have gone up by about $5 since I ordered it). The review found that its measurements agreed quite well with the more expensive Aranet4, and it uses a Sensirion SCD40 CO2 detector, which is apparently well-respected.
I’m not entirely sure what the think of the evidence around CO2 negatively impacting cognition, but it seems plausible enough to be worth spending <$50 on a monitor.
You can buy a even cheaper one from taobao, this one is $20 before shipping (but I expect the buying experience to be quite complex if you’re outside china)