The stunning process ensures the animal is unconscious and insensible to pain before being bled out at slaughter. Common stunning methods include electrical stunning, captive-bolt stunning or the use of carbon dioxide gas. Following stunning, the animal is bled and remains unconscious until it dies due to blood loss.
Persons responsible for the stunning animals must be appropriately trained and competent in their required tasks to ensure that every animal is humanely stunned.
A captive-bolt stunning device administers a blow to the animal’s skull of sufficient force to render the animal immediately unconscious. A blow of sufficient force will damage the nerves and blood vessels in the brain causing brain dysfunction/destruction and diminished blood circulation, resulting in immediate unconsciousness. Captive-bolt stunning devices where the bolt enters the brain (penetrative bolt) will result in greater stunning efficiency than non-penetrative stunners (‘mushroom-headed’ bolt) that rely on the percussive blow on its own to achieve immediate insensibility.
An electrical stunning device (applied to the head only) must pass sufficient current through the brain of the animal to interrupt normal brain activity and render the animal immediately unconscious. Electrical stunning (or electronarcosis) is reversible as it disrupts normal brain function for a short time only. An electrical stunning system requires the correct voltage, current and application time to be delivered for the stun to result in instantaneous and painless unconsciousness.
Carbon dioxide gas is used to induce unconsciousness. As the animal inhales the carbon dioxide gas, blood oxygen level decreases resulting in a loss of brain function and eventual brain death. Animals must remain at the point of maximum gas concentration for a specified period of time to ensure they are irreversibly unconsciousness. There are welfare concerns about carbon dioxide stunning because it irritates the respiratory tract. For more information see the article Is carbon dioxide stunning of pigs humane?