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When is carbon dioxide stunning used in abattoirs?

Article ID: 118
Last updated: 09 Sep, 2015
Revision: 4
Views: 13242

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. Rather, the animal gradually loses consciousness through exposure to the gas.

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

  • death of an animal without panic, pain or distress
  • 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 and poultry. For example, with pigs, a number of animals enter a chamber that is then sealed and filled with 80% to 90% CO2 in air. Exposure to CO2 does not result in immediate insensibility and requires exposure to a high concentration of the gas. In pigs, exposure of at least 15 seconds is needed before body collapse occurs. Following exposure to the gas, the animal or bird should be dead or irreversibly unconscious.

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. For example, when used for the euthanasia of spent hens, birds 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 bird.

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

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Attached files
file Use of carbon dioxide for euthanasia - Jan 2007.pdf (124 kb)

Also read
document What do we mean by humane killing or slaughter?
document RSPCA Policy G1 Humane killing
document RSPCA Policy H2 Standards, codes of practice and guidelines
document How are animals stunned prior to slaughter

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