You sure can. And most people are understanding and will either put out a cigarette or move somewhere else. If they refuse, it is best for you to move somewhere else if this is possible. If you are in an area where smoking is prohibited, you should point this out to them and if they ignore it, find someone in charge to ask them to leave. You are really doing them a favor to make it difficult for them to smoke.
A Answers (2)
Vincent T DeVita Jr, Oncology, answered
Ruth White, MPH, Social Work, answered
Say simply, "Smoking is really bad for me, so do you mind smoking where I won’t be affected by your smoke? Thank you so much for understanding". Most people will positively respond to such a request. If that doesn't work then if you are in an area where they should not smoke make that a point. And really it may just be easier for you to move if they have a legal right to smoke in that area.