Carbon dioxide stunning before pig slaughter is the dominant method in European slaughterhouses, but its welfare implications are contested, with CO2 causing distress before unconsciousness and alternatives gaining research attention.
CO2 stunning causes a period of aversive experience before unconsciousness that has been clearly demonstrated in both behavioral and neurophysiological research. Pigs show distress responses during CO2 exposure that are not present with argon stunning, and prefer to avoid CO2 in choice experiments. The EFSA has recommended that alternatives to CO2 be developed and adopted, but economic inertia and high conversion costs have slowed change. The scale of the welfare concern is significant - tens of millions of pigs are CO2 stunned annually in Europe. Research into palatable nitrogen and argon alternatives, and into optimal delivery systems, is the priority for improving this widespread welfare problem.