Should a Muslim avoid actions that might invalidate his fast or raise doubts about its validity, such as going out for a stroll and being exposed to smoke or dust, or washing his face without performing ablution, or swimming where water might enter his throat, or sitting near a craftsman who produces dust?

It is permissible for a fasting person to go out for recreation, and they are not harmed by any road dust or smoke that may precede them to their throat. In fact, the Shafi'is went so far as to say that if one intentionally opened their mouth until dust entered their stomach, their fast would not be invalidated. Consensus has been reported that the precedence of dust and the like into the stomach of a fasting person does not invalidate their fast. It is permissible for a fasting person to wash their face, as well as to swim, as long as they do not predominantly believe water will enter their stomach. As for something reaching the stomach through the eye, there is a difference of opinion on this matter, and the most correct view is that the fast is not invalidated thereby.