Is it permissible for a woman who wears the niqab to remove her khimar due to an illness that causes severe headaches and extreme shortness of breath, to the extent that it becomes a burden for her?

A woman is required to cover her face in front of non-mahram (definition) men. If she experiences a severe headache or shortness of breath due to covering, she is permitted to uncover it out of necessity, as "Allah does not burden a soul beyond its capacity." Some jurists do not consider the face to be 'awrah (P.N. something that must be covered), and it is permissible to take advantage of this concession to avoid hardship, provided she strives to avoid being seen by men.