Pigs, chickens, rats, and humans all have different thresholds and responses to hypoxia, and CO2 in particular. What might be humane for humans can be extremely distressing and painful for a particular animal, and vice-versa. There's a significant amount of research exploring these responses. The gassing protocols are driven by solid science; alot more solid than nitrogen asphyxiation for humans. Though, how well the protocols are followed is a different matter.

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