The Official Website of Grand Ayatollah Makarem Shirazi

Just as eating and drinking invalidate fasting, thick dust which reaches the throat and turns in to mud there also invalidates fasting. Therefore, the individuals such as street cleaners whose jobs require them to work in dusty environment must be careful not to invalidate their fast. In fact, if a fasting person believes that they might run in to thick dust or smoke in a certain place, they must be cautious and not go there unless they make sure that thick dust or smoke do not enter their throat. If a fasting person forgets that he is fasting and he allows dust to reach his throat or has been careful enough but inadvertently dust reaches his throat, his fasting will not be invalidated. What is meant here by dust is anything that looks like dust whether it is created by something that is prohibited to eat such as dirt or something that is permissible to eat like flour. Smoke is also another invalidator of fasting. For example, smoking is prohibited and in addition to its general prohibition, the smoke taken in while smoking will invalidate fasting as well. Therefore, if a fasting person smokes while fasting, they must both observe the qada' for that they is fast and pay expiation of the deliberately broken fast. Also breathing in thick steam or vapor also invalidates fasting. Even steam and vapor that is not thick as an obligatory precaution invalidates fasting too. But taking a shower will not invalidate fasting even if steam permeates the shower and it is breathed it intentionally. But if taking a shower makes a fasting person weak, it is considered to be abominable during the time of fasting. The individuals who suffer from asthma and need to use inhalers during the day do not need to worry because inhalers release a sort of gas which is not an invalidator of fasting. Breath sprays will not invalidate fasting if, like inhalers, they too produce a substance that enters the mouth in the form of a gas. Naturally, there is no problem with using perfumes for a fasting person but it is considered abominable makrūh for a fasting person to smell fragrant plants.