From common sense, a non-smoker has more time (no need for smoking breaks), 1 less distraction when trying to work on something (the distraction of wanting to smoke), and 1 less item on their to-do list (buy more cigarettes)
There is the fitness/health aspect as well; people consider it part of productivity to run/workout (etc.), and smoking can hinder that.
There is a strong link to absenteeism (as in the article posted by Brian), here is another article I found on Scientific American: Quitting Smoking Can Boost Productivity
- Smokers take on average nearly three times as much sick leave as did non-smokers, and significantly more time off than ex-smokers as well.
- The more recently an ex-smoker had quit, the more absenteeism they still showed. So too, it took quitters 12 months to become more productive than current smokers. Past that one-year mark, though, ex-smokers were on average five percent more productive than smokers