Carbon dioxide (CO₂) stunning is used to make an animal unconscious just before slaughter. ‘Stunning’ is probably the wrong word as animals are not responding to a shock, which is the case with mechanical or electrical stunning methods.

CO₂ stunning, like all methods involved in humane slaughter, must meet the following criteria:

• death of an animal without panic, pain or distress
• instant unconsciousness followed by rapid death without regaining consciousness
• reliability for both single or large numbers of animals
• simplicity and minimal maintenance
• minimal impact on operators or observers
• a high level of operator skill.

In Australia and many other countries, CO₂ stunning is mainly used on pigs. A number of pigs enter a chamber that is then sealed and filled with 80% to 90% CO₂ in air. It has been shown that pigs lose consciousness within 13 to 30 seconds. Following this, immediate slaughter is essential so the animal does not start to recover.

There have been welfare concerns about CO₂ stunning because it irritates the respiratory tract. However, research has produced conflicting results with some showing pigs tolerate CO₂ stunning and some showing they do not. A sample of test results show:

• a large variation in pigs’ reactions — ranging from none to violent attempts to escape
• that excitement can occur while the pig is still conscious
• in some breeds, excitement starts after the pig is unconscious
• different pig genetics probably explain the different reactions. 

A variation on CO₂ stunning is a controlled atmosphere chamber that contains a mixture of nitrogen, argon and CO₂ gases but no oxygen. Controlled atmosphere stunning is used pre-slaughter in some poultry-processing plants and on some poultry farms for euthanasia. Hens are unconscious within 20 seconds of being in the chamber. Argon and nitrogen are important for painless gassing, as using CO₂ only may cause distress to the hens.

For further information, see: Model Code of Practice for the Welfare of Animals: Livestock at Slaughtering Establishments