Not nearly as much as the anti smoking lobby would have you believe...they are zealots just like the greenies with global warming.
As an example there is currently NO scientific evidence that second hand smoke is harmful...despite ENORMOUS amounts spent in studies trying to prove that it is. Look at the laws being promulgated on that lack of scientific rigor.
That is not to say non smokers don't find the smoke smell unpleasant but unpleasant is not the same thing as unhealthy.
There is actually some evidence that suggests second hand smoke might actually minimise sundry lung/breathing maladies but like the 'evidence' that suggests it is harmful it is statistically irrelevant because the studies (both ways) cannot come up with statistical indications better than 1.1 which is MILES short of the statistical likelihood of 2.0 needed to even begin thinking about using the data for Govt policy making. The fact that up until the 60s you could smoke pretty well anywhere and most people smoked cinemas (as an example) were chock full of cigarette smoke yet the incidences of childhood asthma was remarkably low. Since smoking has been progressively banned nearly everywhere in the last 30 years asthma has become much more prevalent than it was. Proof of anything? No - but interesting non the less and the subject of medical research.
Other studies have found a correlation between smokers and things like Alzheimers...smokers suffer remarkably less from this disease than non smokers. Also the subject of ongoing medical research.
Did you know that up until 1960 cigarette filters contained Blue asbestos?
Deadly by itself and the medical literature suggests that in concert with the chemicals contained in cigarette smoke the combination is far worse than either by itself. What do you suppose that combination did for the lung cancer statistics up until the mid/late 80s?
Now I am a smoker and one who wants to give it up...again...because it is just intuitively sensible that it cannot be good for you. But I also believe it is not NEAR as bad as the anti smoking lobby zealots would have you believe. My father smoked until he was 60 and is now a healthy 76. My first DAME (who was over 70 and in semi retirement) had been a smoker all his life until his late 50s. He told me the following;
Cancer is a disease of the elderly and as the population demographic ages more people die of cancer. 100 years ago people rarely lived beyond 50-60 (and few died of cancer) now it is commonly closer to 80 and more die of cancer.
Things like heart disease and emphasema were definately directly related to smoking and for that reason alone I should give up sometime...and I will...hopefully soon.
If it is true that smokers live shorter lives then the burden they make on the health system later in life is shorter than those who have never smoked.
It is an unfortunate fact of modern life that we are at the mercy of sundry single issue advocasy groups all pushing agendas. Global Warming is another example, there are many others.