Both my mother and her father were still smoking frequently when they were on oxygen. In other words: When they should have known better. Her mother was looking really bad at 50 and when the Dr told her "it's killing you, and not in some abstract way", she cut it out cold turkey and pretty much instantly looked 10 years younger.
Grandfather was struggling in general, but it was twisted up intestines that did him in. My mom, one nurse told me, her lungs were "just shot". COPD. Funny thing was that she was getting enough oxygen (with O2), she just couldn't get rid of CO2. Basically took her outta the game 20 years early.
Hopefully, my kids who watched this, can use the experience to steer clear of some things. If you can't set a good example, show what a bad example leads to. :-)