It can take up to 30 minutes to end an animal's life, and that's IF the gas chamber is successful. Sometime animals have to go through the gassing process twice. There are also several other issues with gas chambers. Gassing causes an animal's organs to shut down before the brain does, therefore the animal feels pain. With lethal injection, the brain is the first thing to go, so no pain is felt. Since several animals can be placed in a single gas chamber at once, fights between the animals can break out due to stress and anxiety. Also, if gas chambers are not regularly inspected, carbon monoxide/dioxide can leak into the air making it dangerous for shelter staff members. Lethal injection is the more humane, and safe, alternative to gassing.