What are the circumstances in which the niqab may not be worn?

Scholars differed on the obligation of wearing the niqab. The fatwa issued is that it is obligatory to cover the face, like the rest of the body. However, a woman in ihram for Hajj or Umrah is not permitted to wear the niqab; rather, she should draw something down over her face if she fears the gaze of foreign men. It is permissible to uncover the face out of necessity, such as for testifying or for medical treatment. A doctor is permitted to look at whatever part of her body is necessary, and a witness may look at the face of the person they are testifying against. A person dealing with a woman in a sale or rental transaction may look at her face. It is also permissible to remove the niqab if wearing it causes harm due to illness or other reasons, provided that outings are limited to what is necessary.