My wife adheres to purdah (hijab). She regularly looks at the women of this country and that of the Arab countries, who wear the purdah (hijab) but do not conceal their faces. So, she always ask me if it is necessary to cover ones face for purdah (hijab)?
The original rule is that it is necessary to cover the face for purdah (hijab). However, in cases, where a woman is in severe need, for instance if there is a large crowd and she fears falling down, or she needs to open her face for some necessary task, for instance in front of a doctor, then there is permission for her to expose her face. One should always remember that no person’s action is regarded as evidence in the Deen.
And Allah Subhaanahu knows best.
(Hazrat Mufti) Muhammad Taqi Usmani (May Allah preserve him)
Fatawa Usmani Vol.4 Pg.399-400