Praise be to Allah.
The one who picks skin from his lips whilst he is fasting must eject it. If he swallows it absentmindedly, or there is no trace of it, or it is difficult for him to eject it, so he swallows it with his saliva, then his fasting is still valid.
But if he deliberately swallows it when it is possible to eject it, then he has invalidated his fast.
It says in al-Mughni (3/126): “If someone has food between his teeth, one of the following two scenarios must apply:
- It is very small and he cannot eject it, so he swallows it. This does not break his fast, because it is not possible to avoid it, so it is akin to saliva. Ibn al-Mundhir said: The scholars unanimously agreed on that.
- It is a large amount and it is possible to eject it. So if he ejects it, there is no blame on him, but if he swallows it deliberately, he has invalidated his fast according to the view of most scholars.
But Abu Hanifah said: It does not invalidate his fast, because it is inevitable that some of what he has eaten will remain between his teeth, so it is not possible to avoid it. Therefore it is like saliva.”
Our view is that if he swallows food that he could have ejected voluntarily, knowing that he was fasting, then he has invalidated his fast, just as if he began to eat. This is different from something that is mixed with saliva, because it is not possible to eject it.”
So if you used to swallow these bits of skin when you were able to eject them, then your fast is not valid, and you must make up the days on which you did that.
And Allah knows best.