Ruling on a woman in ihram wearing a face mask
Question

By Allah’s leave, I intend to do Hajj. Is it permissible for me to wear a face mask instead of niqab, because my eyesight is weak and I want to be able to see clearly?
Praise be to Allah.Firstly:
The woman in ihram is not allowed to wear anything that is tailored to fit her face, such as the niqab, burqa‘ and litham [types of face veils], because of the report narrated by al-Bukhari (1838) from ‘Abdullah ibn ‘Umar (may Allah be pleased with him), who said: A man stood up and said: O Messenger of Allah, what garments do you instruct us to wear in ihram?
The Prophet (blessings and peace of Allah be upon him) said: “Do not wear a chemise, or trousers, or a turban, or a burnoose, except for one who does not have sandals, in which case he may wear khuffs (leather slippers that come above the ankle), but let him cut them so that they come lower than the ankles. And do not wear anything that has been dyed with saffron or safflower, and the woman in ihram should not wear a niqab or gloves.”
The jurists disallowed wearing a burqa‘, and some of them disallowed wearing a litham, because they are face veils (niqab) or come under the same ruling as face veils.
Ibn al-Qayyim (may Allah have mercy on him) said: … The Prophet (blessings and peace of Allah be upon him) said: “A woman should not wear a niqab or gloves” meaning in ihram. So her hands and face are equal with regard to the prohibition on wearing what has been made to fit the shape of any part of the body.
The correct view is that anything that comes under the general meaning of the wording of the hadith and the same reason for prohibition is also prohibited. Although the burqa‘ and litham are not called niqab, there is no difference between them and niqab. Rather if the niqab is prohibited [in ihram], then it is more appropriate that the burqa‘ and litham should also be prohibited."(I‘lam al-Muwaqqi‘in  2/393-395).
It says in Kifayat al-Talib ar-Rabbani (1/554): She may cover all of her face and hands with a cloth that she lets down from the top of her head, but she should not use any pins.
She should not wear a niqab, burqa‘ or litham; if she does any of that, then she must pay a penalty (fidyah). End quote.
The litham is something that is worn over the mouth.
It says in al-Misbah al-Munir (2/549): The litham is something that covers the lips.
And it says (2/556, under the root la fa ma): The verb talaffama refers to taking the end of the turban and putting it over the mouth, like a niqab, except that it does not reach the bridge or tip of the nose.
If it covers part of the nose, then it is called a niqab. This was stated by Abu Zayd.
Al-Asma‘i said: If the niqab covers the mouth [only], then it is a lifam or litham. End quote.
Based on that, the face mask is a niqab, so it is more appropriate that it should be prohibited than the litham.
The only difference is that the face mask does not cover the entire face; rather it covers part of it, but it is tailored to cover that part of the face.
It says in al-Insaf (3/466): Tailored clothes include that which is tailored to fit the shape of the body part in question. This is according to scholarly consensus. 
A number of scholars said that it refers to that which is tailored to fit the shape and is intended to do so.
Al-Qadi and others said: Even if something is worn in a manner other than what is usual, such as wearing socks on the hand or a khuff on the head, the penalty (fidyah) is still due. End quote.
So wearing a face mask is not permissible for a woman [in ihram], just as a niqab is not permissible.
But if she needs to wear it because of sickness, or because of foul smells, then it is permissible for her to wear it, but she must pay the penalty [fidyah] for doing something that is ordinarily prohibited in ihram because of an ailment.
Shaykh Zakariya al-Ansari (may Allah have mercy on him) said in Asna al-Matalib (1/507): If someone wears when in ihram something that is prohibited to wear in ihram, or if he covers something that is prohibited to cover when in ihram, because he needs to do that due to extreme heat or cold, or for the purpose of medical treatment and the like, it is permissible, but he must pay the penalty (fidyah). End quote.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said:
There are three scenarios for doing actions that are prohibited when in ihram:
i. The first scenario is when he does the prohibited action for no essential reason and with no excuse. In this case, he is sinning and must pay the penalty (fidyah).
ii. The second scenario is when he does it for an essential reason. In this case, he is not sinning but he must pay the penalty (fidyah).
If he needs to cover his head because of cold or heat from which he fears harm, it is permissible for him to cover it, but he must pay the penalty (fidyah).
iii. The third scenario is when he does it but is excused because he was unaware of the ruling, or he forgot, or he was forced to do it, or he fell asleep. In this case, he is not sinning and he does not have to pay the penalty (fidyah)."(Majmu‘ Fatawa wa Rasa’il al-Uthaymin 24/433).
The penalty (fidyah) is: to fast for three days; or to feed six poor persons, giving each one half a sa‘ of foodstuff; or to sacrifice a sheep. The pilgrim in ihram may choose any one of these three options.
As for a man, there is nothing wrong with him wearing a face mask, because it is not prohibited for him to cover his face [when in ihram], according to the more correct scholarly view. Please see the answer to question no. 106560 .
Secondly:
The woman in ihram should cover her face in front of non-mahram men with something that she lets down from the top of her head over her face.
Ibn Qudamah said: Ibn al-Mundhir said: The view that the burqa‘ is makruh (disliked – when a woman is in ihram) is soundly narrated from Sa‘d, Ibn ‘Umar, Ibn ‘Abbas, and ‘A’ishah, and we do not know of anyone who disagreed with that. Al-Bukhari and others narrated that the Prophet (blessings and peace of Allah be upon him) said: “and the woman [in ihram] should not wear a niqab or gloves.”
But if she needs to cover her face when men pass by near her, then she should let down the cloth from the top of her head over her face.
That was narrated from ‘Uthman and ‘A’ishah, and it was the view of ‘Ata, Malik, ath-Thawri, ash-Shafa‘i, Ishaq and Muhammad ibn al-Hasan, and we do not know of any difference of opinion regarding that. That is because of the report narrated from ‘A’ishah (may Allah be pleased with her), who said: The riders used to pass by us when we were in ihram with the Messenger of Allah (blessings and peace of Allah be upon him). When they came alongside us, one of us would let down her jilbab from her head over her face, then when they had passed us, we would uncover our faces. Narrated by Abu Dawud and al-Athram. Al-Mughni (3/154).
The hadith of ‘A’ishah was classed as sahih by al-Albani in Risalat Jilbab al-Mar’ah.
You can make this jilbab of light material that you can see through without any difficulty.
If that is too difficult for you, then there is nothing wrong with you wearing niqab and paying the penalty (fidyah); that will be better than wearing a face mask which does not cover the entire face.
And Allah knows best