CO2, Fear and Slaughter

08 August 2019

CO2 stunning is the main method used in the slaughter of pigs and chickens in the UK. The gas is also utilised by researchers of the human brain to invoke feelings of fear and panic. Animal slaughter is a necessarily unpleasant business, but just how unpleasant does it need to be?

CO2 Slaughter

Back in the summer of 2018 a shortage of CO2 in the UK, gave pig and poultry producers cause for concern. The gas is used to ‘stun’ (kill) animals by asphyxiation before they are drained of blood and processed for meat production. In large factories this means many hundreds of pigs or many thousands of chickens per hour. In the U.K. 86% of pigs and 70% of broiler chickens are killed by CO2 gas. The remaining pigs and chickens are killed by electric stunning and Halal. Electric stunning is a particularly unpleasant method of slaughter as it necessitating the rapid, manual handling of vast numbers of animals. This creates a great deal of stress and injury to the animals and an unpleasant and hazardous working environment.

As an alternative to electric stunning, there is little argument that CO2 stunning has drastically improved the welfare of both the animals and the workers involved. For this reason it has gained the endorsement of PETA; People for the Ethical Treatment of Animals. It is even hailed by meat producers as a quick, painless method to kill animals, but this is far from the truth.

The Amygdala and CO2

Scientists studying the fear response in human beings often study the amygdala. The amygdala is an almond shaped set of nuclei clusters in the brain. It is associated with the processing and regulation of emotions and induction of the fear response. By comparing patients with brain damage to healthy subjects, researchers can study how those damaged areas normally function. One of these patients, dubbed 'SM’ by researchers, has a genetic condition resulting in bilateral amygdala damage. She operates perfectly normally in all areas including IQ tests and facial recognition but has no fear response. Her lack of fear has been ably demonstrated from a series of violent encounters. She has been held up at knifepoint and gunpoint, physically confronted by much larger women, almost killed in an act of domestic violence and on several occasions threatened with death. SM claimed to have felt only upset and angry during these occasions but never fearful. On subsequent interviews by two psychologists who had no knowledge of her condition, she was described as being very resilient and even heroic. It was thought by the researchers that SM’s inability to recognise dangerous situations greatly contributed to her encountering so many violent incidents. Researchers call such violent encounters external threats. In an experiment designed to provoke the fear response via an internal mechanism, SM and other similar volunteers were given masks through which a mixture of air and carbon dioxide were passed;

‘Each reported feeling extreme amounts of fear – a sensation they had not experienced in many years.’

Medical Xpress

‘We found that inhalation of 35% CO2 evoked not only fear, but also panic attacks, in three rare patients with bilateral amygdala damage.’

"They gasped for air, their heart rate shot up, they became distressed, and they tried to rip off their inhalation masks. Afterward, they recounted sensations that to them were completely novel, describing them as "panic."


The researchers had predicted that none of the group would experience such a reaction because the amygdala is also involved in the detection of excess CO2. When CO2 is mixed with water it creates carbonic acid. When too much CO2 is breathed in, it acidifies the blood, which is detected by chemosensors in the amygdala. What the researchers are now led to believe is that the amygdala is not the ‘seat of fear’ as many see it, but an area of the brain where emotions are moderated by present conditions. It is in fact thought to be the reason that all three subjects with damaged amygdala did panic. They had no ability to moderate their feelings given the context. In a similar experiment on healthy subjects only three of twelve experienced the same response. Both mammals and birds are shown to have the same chemoreceptors sensitive to CO2 and they are similarly averse to the gas.

So some animals will not experience the same intense panic as others, but animals have been shown to demonstrate empathy; the ability to feel what others are feeling. Pigs especially are intelligent, group orientated animals and have a strong aversion to the gas and to seeing others in the group killed.

Other effects of the gas include breathlessness, and feelings of suffocation, which can cause great distress.

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