The Better Chicken Commitment (BCC) is widely recognised as the leading framework for higher‑welfare indoor broiler chicken production. One of its most important requirements is the use of Controlled Atmosphere Stunning (CAS) rather than traditional electrical water‑bath stunning. While CAS is not without flaws, it offers several animal welfare advantages over electrical water baths. For instance, CAS does not require conscious inversion of live birds, reduces or eliminates pre-slaughter handling, and is generally more consistent in delivering an effective stun. However, not all CAS systems are created equal. The effectiveness and humaneness of a CAS system depends on several factors, including the system design (e.g., tunnel, pit, or closed cabinet), the type of gas used, the number of gas phases, and the gas concentration within each phase.

Moving beyond electrical water-bath stunning

Electrical water-bath stunning has been the industry standard for decades, yet it carries deep, unavoidable welfare flaws. It involves inverting conscious birds, shackling them by their legs, and immersing their heads in an electrified water bath. This process is inherently stressful and painful, with risks of pre-stun shocks and inconsistent stunning effectiveness due to variations in bird conductivity and equipment parameters. Inadequate electrical settings can result in birds being immobilized but still conscious, posing a significant welfare concern. Electrically immobilized birds remain fully conscious and will experience the severe pain, fear, and distress of the throat cut and subsequent bleed-out until death.

What CAS provides — and why the BCC requires it

CAS uses controlled gas mixtures to render birds unconscious while they remain upright in their transport crates. When properly designed and managed, this approach eliminates live shackling, reduces handling, and creates a calmer, more predictable process. Birds are spared the distress of inversion, and the system delivers a more consistent and reliable loss of consciousness across the flock. Multi‑phase systems allow operators to control the rate of change in gas concentration, ensuring a gradual, smoother induction curve and reducing the likelihood of aversive reactions. Because unconsciousness can be maintained through to slaughter, the risk of birds regaining awareness is significantly reduced. These advantages are why CAS is a core requirement of the Better Chicken Commitment.

CAS isn’t automatically more humane

Despite its benefits, CAS is not a welfare guarantee on its own - it depends heavily on the gases used, the rate at which birds are exposed to them, the design of the system, and the quality of monitoring.

Carbon dioxide (CO₂) is most used for CAS slaughter given its more readily available, less costly, and easier to container being heavier than oxygen. However, CO₂ is aversive to poultry. CO₂ activates chemoreceptors in the mucous membranes of the respiratory tract, which can cause discomfort, pain, and breathlessness. Research has shown CO₂ can be detected by laying hens at concentrations as low as 6.6%. With hens showing breathlessness (heavy breathing) starting at 8% CO₂ and the first signs of aversion (head shaking and gasping) at 13-14% CO₂.^[1]

When CO₂ is used, it is fundamental that birds are first exposed to a low concentration (<30%). Within the European Union, it is a legal requirement that the level of carbon dioxide in such mixtures does not exceed 40%, and in the United Kingdom, it must legally ≤30%. If only CO₂ is used, a multi-phase system with five gradual phases is strongly recommended. This approach increases the CO₂ concentration gradually, helping to minimize aversive reactions and ensuring that birds are unconscious before higher concentrations are introduced.

Alternatives to CO₂-only CAS systems are the use of inert gases, such as argon or nitrogen. These gases are colourless, odourless, tasteless, and non-irritating, and can also reduce bird aversion during the induction of unconscious. However, inert gases are generally more expensive. Like CO₂-only CAS systems, vigorous wing flapping can also occur in unconscious birds with inert gas and can also injure and stress the birds within the crates who are still conscious. Mixed-gas approaches, such as combining CO₂ with nitrogen or argon, have been shown to reduce aversive effects. Suggested mixtures include 30% CO₂ with 60% argon in air, or up to 90% argon alone.

Why CAS system design matters

CAS systems vary significantly in design, and these differences have direct welfare implications. Pit and tunnel systems are pre‑filled with gas, meaning birds are exposed immediately to a fixed gas concentration at each new phase. This sudden exposure can cause strong aversion, particularly when the concentration differs sharply from that of the previous phase. These designs also rely heavily on crate geometry and airflow patterns; mismatches can create pockets of variable gas concentration, leading to inconsistent stunning.

Closed‑cabinet systems, by contrast, introduce gas only after birds are inside the chamber. This allows for more gradual, controlled changes in gas concentration. These systems provide greater consistency and control and are considered more favourable from a welfare standpoint.

Monitoring and process control is essential for CAS systems

Even the best‑designed CAS system requires continuous monitoring. Real‑time visual assessment—via windows or cameras—is essential for detecting behavioural indicators of distress, such as gasping, escape attempts, or prolonged wing flapping. Gas concentration sensors must be calibrated regularly, and airflow patterns must be validated to ensure uniform distribution. Staff must be trained not only in equipment operation but also in interpreting bird behaviour during induction, as behavioural cues remain the most immediate indicator of welfare compromise.

And crucially, welfare begins long before stunning. Birds should be crated on the farm in containers compatible with the CAS system to avoid unnecessary re‑handling, which adds fear and stress.

Transport conditions, loading density, and thermal environment all influence birds’ physiological state at the time of stunning and therefore affect how they respond to gas exposure.

In summary

CIWF believes that CAS systems are the most humane commercial alternative currently available for stunning and/or killing poultry. However, while CAS offers many welfare advantages to traditional water bath electrical stunning, we acknowledge certain CAS systems are less humane in practice due to the system design and gases used and can lead to suffering if not well managed to ensure bird welfare is prioritized.

When using CAS slaughter systems, it is paramount that thoughtful design and good management is in place to reduce any risk of welfare problems. For humane CAS slaughter, CIWF strongly recommends:

The use of inert gases or a mixture of inert gases + CO₂ versus CO₂ alone
For CO₂-only systems, a minimum of five gradual phases should be used
The use of closed cabinet vs. pit or tunnel CAS systems for better control and uniformity in application across all birds
All CAS systems should have windows or cameras to allow for active monitoring of the birds’ welfare in the CAS system, and to bring attention to any distress behaviours shown by the birds.
Any pre-slaughter handling should be eliminated by crating birds at the farm that are compatible with the CAS system to prevent re-crating.

For more information on CAS systems, please check our resource “Controlled Atmosphere Systems for Broiler Chickens”

^[1] Gerritzen M, Lambooij B, Reimert H, Stegeman A, Spruijt B, 2007. A note on behaviour of poultry exposed to increasing carbon dioxide concentrations. Applied Animal Behaviour Science 108(1–2), 179–185.

Why the Better Chicken Commitment requires CAS systems