High-concentration CO2 controlled atmosphere killing for broilers is considered welfare-superior to water-bath electrical stunning, but its own welfare characteristics — including aversiveness of CO2 — require evaluation.
Broiler CAS welfare reflects the complexity of comparative welfare assessment — comparing two imperfect systems to identify the better one. High-concentration CO2 causes an aversive sensation before unconsciousness, which is a genuine welfare cost. However, water-bath electrical stunning requires hanging conscious, pain-sensitive broilers upside-down by their legs — a stressful restraint — before electrical stunning that itself has significant failure rates. On balance, CAS wins welfare comparison because it eliminates handling stress before killing, produces more reliable unconsciousness, and avoids the significant failure rate of water-bath systems. Low-oxygen inert gas CAS (argon or nitrogen) avoids the CO2 aversiveness and is arguably the most welfare-friendly commercial killing method currently available, though equipment cost has slowed adoption. The direction of travel in broiler welfare is clearly toward CAS.