Most of the studies on this issue are heavily biased against public smoking.

Most of the measurements taken are done so straight from the tip of the lit cigarette. Whereas most smoke from a cigarette, which is being smoked, is not only filtered through the tobacco and filter, but also the lungs of the smoker before the majority of the smoke gets into the air.

In short, 2nd hand smoke is likely not good for you, yet all the Doom and Gloom hypotheses are most certainly inaccurate to plausible effects.