Never heard of any 'nail biting deaths'; yes, it's gross, but there're reasons' people do it & just can't help it! It's a singular affect situation: you & you, alone. Doesn't impinge on anybody else, unless it's their eyes' seeing the nails!
Smoking! Gross, filthy, smelly, stinky, harmful to all & clothes & downright dangerous! It KILLS!
Surely published research has been around long enough by now to discourage this foul habit! But obviously not, & people are starting smoking at even a YOUNGER age!
Hate it when you get into a lift & a smoker has got out before you! It stinks! Where're the extractor fans?
When a work colleague returns from 'smoko' & the smell wafts in the office & they smell to the point of sickly. Don't they know this, or do they have a 'couldn't care less about anybody else' attitude? Foul, just foul!
I dislike both equally. I couldn't live with either. Although having said that, I would be less worried about smoking as that is just an addiction but nail-biting is usually a symptom of something deeper so would worry me more. But I would be extremely unlikely to ever get close enough to a nail-biter or a smoker to be giving it too much thought anyway....