Carbon dioxide (CO2) gas stunning is now the most common method of rendering pigs unconscious before slaughter in many countries. Its welfare implications — particularly the distress caused during induction — are significant and contested.
The welfare problem with CO2 stunning is the induction period: pigs are conscious for 15-30 seconds while experiencing the painful and distressing effects of high CO2 concentration. They show clear behavioural signs of aversion — attempting to escape, vocalising, head-shaking, and gasping — indicating that the experience is highly aversive before unconsciousness occurs. This represents a preventable welfare harm at the final moment of the animal's life, affecting billions of pigs globally each year. The existence of low-aversion alternatives (inert gas mixtures, controlled atmosphere stunning with argon) demonstrates that the choice to continue using high-concentration CO2 is economic, not technological. Regulatory reform requiring less-aversive stunning methods is the most impactful welfare intervention available.