Cathe Park worked at the Victoria pound for three years and part of her job was "tanking", which was what the staff called putting animals in the gas box.
Here Cathe describes the death of a cat in a gas box:
From firsthand experience, I am about to describe the process of euthanizing a domestic or feral cat by Carbon Dioxide gassing.
For the feral cat I will set a weight sensitive coon trap and wait until he enters and becomes snared. The domestic cat I merely place in a galvanized metal carrying crate. I make my way to the killing room. While I prepare the gas chamber I leave the cat and his crate uncovered on the floor or on top of the freezer.
Once I have filled the gas chamber with Carbon Dioxide I place him, crate and all, into the chamber, closing the plexiglas lid. He immediately starts gasping for air climbing as high as he can in the restricted space. I can only describe his writhing as panic. I press the release button to inject another burst of Carbon Dioxide. He reacts violently to the loud hissing noise. His mouth is gaping open; his tongue is reaching out trying to grab something. He is clawing at the sides of the cage to try and escape. Using his powerful hind legs, he tries to jump. In this abnormally small space, with only 3 inches of head room, he bashes against the top of the crate. He repeatedly leaps up in an attempt to gain an escape route. This continues for a few seconds. His hind leg pads and nails get stuck in the wires of the bottom of the cage. Violently struggling and thrashing about in an effort to get a breath of oxygen that will keep him alive, this instinctively clean animal now loses control of his bladder and bowels. In his subsequent writhing he will cover himself with his own feces and urine. Desperately trying to bite his way through the galvanized metal he clamps his teeth around the bars surrounding him. If he has any energy left within him he explosively vomits and begins to froth at the mouth, adding to the detritus he has previously expelled from his 10 pound body. Claws become dislodged adding blood to the mixture. Exhausted from the struggle to remain alive, his body sags and goes limp. He has passed out but his heart is still pumping the Carbon Dioxide rich blood through his veins and arteries. His teeth remain firmly clamped to the top of the cage. Death will not occur for a few more minutes, although his body appears lifeless. His body remains in the position it was in when he lost consciousness. We don't know for sure if he is dead yet, as in her efficiency the staff attendant has continued her daily routine while the cat slowly dies. Later, she will come to remove the body. It could be 10 minutes or an hour. Rigor mortis sets in quickly, within minutes of expiration. To remove him from the wire carrier, his jaw must be broken and several claws need to be pulled or cut off. Now he is put in an ordinary green garbage bag and put in the freezer awaiting removal to the landfill. The gas chamber, after a quick wipe down, is ready for use immediately.