Why does second-hand smoke cause health problems?

It's a Myth: I am not an advocate of smoking and there are lot's of reason to quit smoking but "Second Hand Smoke" is not one of them. The expression "second-hand smoke" is a very effective piece of anti-smoking propaganda used all the time by public health officials and physicians yet there is absolutely no scientific evidence to support it's existence. It's logical to conclude but not proven in any study.