Come now...smoking causes cancer? That's so trite. How about:

Don't get me wrong. I think everyone has the right to do dangerous and lethal things to themselves. I am a steadfast proponent of attrition. The more smokers die messily, the less there are to avoid in my daily life.

Of course, if you would encourage children to stay away from smoking I'd appreciate it. It's so very tiring to keep explaining and asking politely to not have to smell the taint of burned poison on my skin and clothes and hair.

Thank you for your time. I know there's not a ton of it left.