They are not that air-tight... see this cat on one of the pictures (https://pics.livejournal.com/igor113/pic/001g7xxp). He was a part of a cat unit used to test submarines for air-tightness. Soviets would lock such unit up on a submarine for a week... if cats are dead, submarine is air-tight. One of the picture survived! Though I heard that cat survivability was close to 100%. Only reported cat fatalities were due to bad food.