It's genetics....
Some people are genetically predisposed to tolerating tobacco and others are not.
I've said this before....just in a different way.
Some people will get emphysema and others barely get a scratchy voice after decades of smoking. And unless you know for certain....it's a gamble...with your life.
Some people get emphysema who have never even smoked too. I'm sure Genetics play a part, but perhaps there are other variables that we may not be aware of. I'm also wondering if it's all the additives they put in modern cigarettes that are the biggest problem.