Board Thread:Episode Discussion/@comment-5430952-20181103172633/@comment-25936766-20181118125727

Cinder becoming inmortal would be quite ridiculous. About as much as making every member of RWBY a Maiden. Unless you meant inmortal as in "can't die naturally", which would be confusing since Salem is also inmortal and in her case that comes with Invincibility (or at least the power to shrug off everything).

Now, a more mundane explanation: Since she's part Grimm now, and Grimm are known to not need to eat or shit and etc., Cinder doesn't have the same desperate need for air that normal people do; she either can survive without breathing or doesn't need much air to function at average capacity.