Reference:
Audio Tape: Taking Advantage of the Last Ten Days of Ramadaan
We know that smoking is prohibited, however what is the evidence that it breaks your fast? What about second hand smoke, does it break ones fast?
Whatever enters your throat may Allaah bless you; is either in the form of food, drink or whatever takes their ruling. Smoke is in reality, something that was previously solid that turned into smoke [by burning in this case]. So if a person breathes in this smoke, [some of it] turns back into a liquid form, and any liquid that enters a person’s throat invalidates his fast.
Smoking is prohibited to begin with, because it is harmful. The scholars state that this smoke turns into a liquid after it is breathed in, and it is this liquid that brakes your fast.
Likewise even Bakhoor (incense), which is a fragrance in the form of smoke, the scholars say that it is not permissible for a fasting person to breathe it in because it turns into a liquid.
As far as second hand smoke, if a smoker is next to you, you must take the necessary precautions by closing your mouth and nostrils, or ask him/her to stop smoking, just as you would do if you came across a small sand storm [i.e you would cover your mouth and nostrils].
