The use of CO chambers is inhumane to animals and dangerous to workers. If the animal doesn't die the first time they go back in for a second try. If they are healthy they die not from lack of O2 but instead from organs shutting down. Click to read more
A needle and syringe containing sodium pentobarbital is passed through the chest wall and several layers of muscle into the heart. It's not to miss and accidentally punctures the lungs and causes them to fill up with fluid.