On the one hand, it's hard to see how something (smoke) that definitely does kill people (smokers) can be harmless once it's exhaled (do the smokers' lungs filter out all the nasties, or something? They certainly don't filter out the stink - you MINGERS).
On the other, it's the kind of thing for which data collection is extraordinarily difficult.
Intuitively, I simply do not believe the bleatings of smokers who say that second-hand smoke is harmless. However, I have no science to back that up whatsoever.