Gas chambers

Last updated 28 March 2023

Most Australian slaughterhouses now use carbon dioxide (CO2) gas chambers to render pigs unconscious prior to slitting their throats. Referred to by the industry as "controlled atmosphere stunning" this process involves lowering pigs into a chamber which is pumped full of deadly CO2 gas. The pigs are exposed to the gas for 2-4 minutes depending on the chamber. In most cases, this is long enough that they are functionally dead, unable to be woken up - making gas chambers a form of killing, rather than purely a stunning method.

While the industry claims that gas chambers are the most humane method of stunning when dealing with a large number of pigs, numerous investigations show how pigs frantically fight to escape the chamber, gasping for air and screaming in agony. In 2023, investigators exposed themselves to CO2 gas inside one of the industry's gas chambers to experience what the pigs would be feeling. Even after a short time with low concentrations of gas, they experienced burning eyes and lungs, extreme panic and light-headedness. In reality, gassing pigs only has one benefit to the industry - that it's the most efficient method of immobilising pigs for slaughter, meaning that they are able to kill more pigs at a faster rate. 

A pig inside a gas chamber at Benalla abattoir

Inside the Chamber

As part of our most recent investigation, a Farm Transparency Project investigator hid himself inside of these chambers to reveal exactly what happens to pigs in their final moments.

Most gas chambers are built around a similar design. A pit is dug into the ground or built above it, this forms the shell for the chamber into which Co2 gas is pumped through a system of pipes. The pigs are herded from the holding pens, where they have likely been waiting overnight after being transported on trucks from a farm, through a series of pens and doors and up the ‘race’ which is a tight passageway leading to a door. They are then forced through the door using paddles, electric prodders and boots and enter a suspended cage referred to as a gondola. The number and size of the gondolas differ depending on the slaughterhouse. Some have only two while other, more modern chamber designs can have as many as five gondolas, each fitting 8-10 pigs inside. In most cases, sows will enter the gondola alone or in pairs while pigs and piglets will enter in larger numbers.

An investigator stands inside the gas chamber at Diamond Valley Pork

A pig is gassed to death inside the gas chamber at Australian Food group slaughterhouse while a hidden investigator films from inside

Once the pigs are inside, the gondolas are lowered down into the chamber by a pulley system. Depending on the chamber, the gas will hit them at around 1.5 - 2 metres down. Footage shows that as soon as they are exposed to the gas they begin frantically searching for air, pushing their noses up through the top bars of the gondola, seeking the oxygen that they know is just above them. Pigs violently spasm and shake as they are lowered into the chamber and can be seen gasping for air even after they have fallen still. 

The gondolas continue their rotation and tip pigs out the front of the chamber onto a metal bench in the ‘kill room.’ Workers then slit the pig's throat before hanging them up by their feet and sending them along a shackle line to the scalding tanks and further processing. While our investigator was inside one of these chambers filming, he witnessed many occasions where pigs would become stuck inside the gondola, unable to be tipped out. Other pigs would then be forced over the dead one, showing visible distress as they stopped to sniff her face before being kicked from behind and shoved deeper into the gondola. Once, a worker was witnessed climbing on top of the gondola to try and dislodge a pig, an act that demonstrates that, as well as being torturous for pigs, they are also dangerous to those operating them. In some chambers the only access point is a maintenance door at the top and a precarious ladder meaning that if a human or a pig were to fall inside, retrieving them would be incredibly dangerous and difficult. 



Marel is the leading manufacturer and supplier of CO2 gas chambers in the world. Most new chambers are designed by Marel or Butina. Butina is a previous brand name of Marel Meat. 

A diagram of the Butina Backloader G3 Relax. A similar chamber design is used at Diamond Valley Pork in Victoria

CO2 Gas

CO2 is a colourless, non-toxic gas which is produced naturally in the environment. Most animals, including humans and pigs, emit CO2 as a byproduct of respiration. It has many applications in food, construction and storage. While not toxic to mammals, CO2, breathing in Co2 is the reverse of what our lungs need to operate, making it a powerful and effective asphyxiant.

According to the Wisconsin Department of Health Services “exposure to CO2 can produce a variety of health effects. These may include headaches, dizziness, restlessness, a tingling or pins or needles feeling, difficulty breathing, sweating, tiredness, increased heart rate, elevated blood pressure, coma, asphyxia, and convulsions.”

Activists who have experienced CO2 gas inside the chambers also report burning eyes, chest pain, a feeling of panic and a cough for hours after the exposure occurred.

The largest suppliers of CO2 gas are BOC, Air Liquide and Air Products. Air Liquide and Air Products formed a merger in 2000 to purchase BOC, however they remain separate businesses. Air Liquide and BOC have both been connected to the construction of a new CO2 production facility at the Longford power plant in Victoria. 

Gas chambers in Victoria use CO2 from either Air Liquide or BOC.

BOC gas cylinder outside Diamond Valley Pork kill room

An air liquide gas cylinder outside Australian Food Group slaughterhouse