....anyway, rodway had been using what looked a bit like photographic film....which had gone black at the end....as a carbon dioxide 'tester strip' (lol - something that was a deliberate confusion, let us say - dioxide/monoxide - i wonder 'why'?...one of the 'disgusting plants' programs?)....and so i would guess that a more uptodate version....would be the following - dazzlenation....see previous post....go figure:

Carbon dioxide (CO₂) | Johnson Test Papers