Praise be to Allah.
Does swallowing saliva break the fast?
“If a fasting person swallows his saliva that does not invalidate his fast even if there is a lot of it and if that happens in the mosque or elsewhere. However, if it is thick like mucus, he should not swallow it , rather he should spit it out into a tissue or something else if he is in the mosque.
And Allah is the Source of strength. May Allah send blessings and peace upon of Prophet Muhammad and his family and Companions.” (Al-Lajnah al-Daimah li’l-Buhuth wa’l-Ifta, 1-/270)
Is it permissible to swallow mucus?
If it is asked, is it permissible to swallow mucus deliberately?
The answer is:
“It is haram to swallow mucus whether one is fasting or not, because it is considered to be dirty and it may carry diseases produced by the body. But it does not break the fast if a fasting person swallows it, because it does not come from blood and swallowing it is not regarded as food or drink. If he swallows it after it reaches the mouth, it does not break the fast. (Al-Sharh al-Mumti’, 6/428 by Shaykh Muhammad ibn ‘Uthaymin, may Allah have mercy on him)
And Allah knows best.