After watching this Tom Scott video https://www.youtube.com/watch?v=1Nh_vxpycEA I bought this CO2 monitor: https://amazon.com/gp/product/B00MB93CK8 It worked quite well and showed that I don't have anything to worry about. Now I'm trying to pass it on to someone else. I'll be willing to sell it for half price if anyone is interested.
If anyone is interested it seems you can find (more or less reliable) sensor modules for about $20, such as the MH-Z19, and read it from to an arduino or esp.