Smoking causes lung cancer, but exactly which compounds cause what amount of cancer?
Identifying the most carcinogenic could help produce cigarettes that are less dangerous. Or mass produce just the most carcinogenic compounds to use as a chemical weapon. (Not sure how useful it would be as a weapon which takes years to kill, maybe a surreptitious murder.)
Intriguing is the possibility that polonium, highly radioactive, accounting for only a trace of the mass of cigarette smoke, accounts for a large portion of cancer caused.