Smoking has become a secular "sin" to the Left, and the predictable result has been a mountain of "studies" "proving" that second-hand smoke causes all sorts of problems. This is a common phenomenon on the Left, but I won't go into other examples as they will simply be a distraction.
SHS is unpleasant in a number of ways, and it seems that former smokers are most susceptible to the petty irritations. Understood and agreed. It is rude to smoke in places where other humans cannot avoid the smell, etc., and considerate people do not do so.
In order to "prove" scientifically that SHS causes one or more diseases, it is necessary to compare the general population with the population that is regularly exposed to SHS, and to compare the incidence of the target diseases between the experimental group and the control group. What you are looking for is a STATISTICALLY SIGNIFICANT difference between, say, the lung cancer rate of those exposed regularly to SHS and the lung cancer rate of the GENPOP. There have been many broad studies that have tracked, for example, the non-smoking spouses of heavy smokers vs the GENPOP, and as a general proposition, the lung cancer, emphesema, and other expected diseases are no more prevalent in the non-smoking spouses than they are in the GENPOP.
Of course, some non-smoking spouses do contract lung cancer, just as some non-smokers in the GENPOP contract lung cancer, for reasons that are not entirely understood. The assertion that, "My Aunt Sadie got lung cancer," is not proof of a statistically significant difference in society. In fact, it is a pointless, irrelevant observation.
The CDC has been preaching for decades the "fact" that SHS causes cancer and a number of other diseases, while the relevant studies have NOT SHOWN that this is a fact. In fact, they are inconclusive at best. Pregnant women shouldn't smoke if there is even any question that the carcinogens in cigs can get into the baby's bloodstream. Easy enough.
But the fact is that smoking in public is no more harmful and no more irritating than someone passing gas in an elevator or a bus. Less harmful than teenagers dropping "F-bombs" is a crowded theater. Some people are rude, and sometimes we have to take action to avoid various sorts of unpleasantness. Deal with it. The Gub'mint needs to stay the fuck out of the discussion until it is scientifically proven that SHS causes diseases. Which it isn't yet.