CO or N would be completely painless. CO is often called the "silent killer" as many humans die from it every year. Doesn't need to be pure CO or anything. Nitr

CO or N would be completely painless. CO is often called the "silent killer" as many humans die from it every year. Doesn't need to be pure CO or anything. Nitrogen would need to be pure but is similarly silent and painless. There is a video online of a pig lured into a nitrogen filled chamber by food. He loses consciousness, recovers, and goes straight back to the food like nothing has happened. As for CO_2, I recommend trying to breathe the gas at the top of bottle of pop without wanting to violently withdraw from the pain.