Pig slaughter involves billions of animals annually worldwide. The welfare implications of different stunning and killing methods vary enormously and choosing better methods could prevent vast amounts of suffering.
CO2 stunning represents a significant ongoing welfare problem in commercial pig slaughter because of clear evidence that pigs find the initial exposure highly aversive. Research demonstrates that pigs respond to CO2 with distress behaviors including panic, squealing, and attempts to escape, before losing consciousness. This means billions of pigs experience a period of conscious distress during the killing process. The welfare alternative - inert gas stunning using nitrogen or argon - causes unconsciousness without apparent aversion. The barrier to adoption is cost: CO2 is cheaper than inert gases, and high-throughput commercial systems would require modification. But this is a welfare cost being borne by pigs to save money, and that trade-off deserves serious scrutiny from welfare advocates and regulators.