They were too expencive to buy recreationally, at least for me. Evikon, small company that does science instruments. Sorry, cant find the model anymore, I think it was discontinued.

I do belive now though that even having recreational level sensor will give you at least understanding overall co2 levels and this something we should look into more.

I founs a paper using the same sensors that measured co2 in schools. In all cases, at certain times the measurement was higher than norm. occassionally over 2000. I can now understand much better kids getting sleepy and unable to concentrate in classroom.

Paper sadly in Estonian only: https://www.google.com/url?sa=t&source=web&rct=j&url=http://...