BBC News - A World Health Organization study in 192 countries states that second hand smoke causes 600,000 dearths world wide annually.
The problem with the "proof" idea is that you can not say that had these specific people not been exposed to smoke they would still be alive, because they were and are now dead. Personally I think the comparative analysis provides compelling proof and I take the view that the WHO are not a bunch of propagandists publishing falsified studies.
IMO those that take the attitude that their 2nd hand smoke is harmless to others are just choosing to believe what is comfortable to them.
I am prepared to consider the possibility that my opinion is wrong and that 2nd hand smoke is fine, in which case the impact on smokers has been at a social and convenience level. However, if one considers the possibility that it is right the impact on non smokers is on a medical level. So which is more important?