A recent study published in the American Journal of Medicine, entitled “The Association between Smoking and Low Back Pain: A Meta-analysis,” concludes:
Our findings indicate that both current and former smokers have a higher prevalence and incidence of low back pain than never smokers, but the association is fairly modest. The association between current smoking and the incidence of low back pain is stronger in adolescents than in adults.
The obvious reason for this result is that adolescents slouch (in their chairs, and otherwise), and adolescents who smoke are likely slouch more than those who do not smoke. I used to slouch, but I hardly ever got near a cigarette. Go figure.