What is the ruling on one who swallows phlegm that has not exited to the letter kha’ (throat) during prayer, and is he obliged to prevent it?

The ruling concerning the purity and impurity of regurgitated food (qals) has been explained in Fatwa no. 143277, and the effect of swallowing it on prayer in Fatwa no. 139785. If the regurgitated food does not reach the outer part of the mouth such that it can be spit out, then swallowing it does not cause harm. However, if it does reach the outer part of the mouth, then it must be spit out. Prayer is not invalidated by swallowing it unintentionally. One is not obligated to forcefully prevent the regurgitated food from reaching the outer part of the mouth; rather, it is obligatory to spit it out once it reaches a point where that is possible.