I have been on an Air Canada flight from Vancouver to Hawaii when a cat relieved itself. The smell was horrendous and the flight was miserable. No air filtration system can handle that smell.The poor cat was upset and it was obivious from his constant meowing that it was in a great deal of distress. If an animal is not used to being caged or confined to small spaces, than it can be cruel to subject an animal to the flight; especially if it is a long haul. And yes, iI own two cats!