If intending to use CO2 as a ctb method, I'd personally advise against it. CO2 is the gas that causes the 'hypercapnic alarm response', that intensely unpleasant panicked sensation that occurs when blood CO2 rises. Not only that, but the CO2 gas will react with the water in your mucous membranes (nasal passage, eyes, mouth, throat) forming carbonic acid, which will cause a painful burning sensation.
CO2 is used to stun pigs prior to slaughter in the wickedly cruel meat industry, and there is footage of this you can watch on YT. You will observe that the pigs suffer greatly, screaming and thrashing about violently before they become incapacitated.
An experiment was once conducted wherein a pig that experienced CO2 stunning chose to starve for several days instead of eating, when its only food source was placed in the same room in which it had previously been gassed. The video of this experiment is also on YT.
With all of that said however, ultimately I recognise that it's up to you, and that pain is less of an issue for some people. If you decide to utilise CO2 for ctb, then that's your choice and I fully respect it.
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