The fuqaha’ differed as to whether boar bristles are najis (impure). The majority of Hanafis, Shaafa‘is and Hanbalis are of the view that they are najis (impure). The Maalikis are of the view that they are taahir (pure).
According to the view that they are najis, it is not permissible to use them when they are wet or to let them touch anything that is wet, because najaasah (impurity) is transmitted by wetness.
In al-Mawsoo‘ah al-Fiqhiyyah (20/35) it says: The majority are of the view that boar bristles are najis, so it is not permissible to use them because that is using something that is najis in and of itself.
According to the Shaafa‘is, if a khuff (leather slipper) is stitched with thread made from boar bristles, the place where the stitches are cannot be made pure by means of washing or using dust, but it may be overlooked and forgiven, so the individual may offer obligatory and naafil prayers in them because it is too difficult to avoid this thing. According to the Hanbalis, if it was stitched when wet it must be washed, and it is permissible to use a sieve made of impure bristles if it is dry, because the impurity is not transferred, but it is not permissible to use it if it is wet because the impurity is transferred by wetness.
The Hanafis regarded it as permissible for those who do stitches to use boar bristles in cases of necessity.
The Maalikis were of the view that boar bristles are taahir (pure), so if they are cut off with scissors it is permissible to use them, even if they are cut when the pig is dead, because the matter has nothing to do with whether the animal is alive or dead. But it is mustahabb to wash them because there is some doubt as to whether they are taahir (pure) or najis (impure). But if the bristles are plucked, then they cannot be taahir (pure). End quote.
It also says (26/102):
The Maalikis are the only ones who say that boar bristles are taahir because they (the bristles) are taahir when the boar or pig is alive. This applies if the bristles are cut off and not plucked. If they are plucked then the roots are najis but the ends are taahir.
They quoted as evidence for that the verse in which Allah, may He be glorified and exalted, says (interpretation of the meaning): “…and of their wool, fur, and hair (sheep wool, camel fur, and goat hair), a furnishing and articles of convenience (e.g. carpets, blankets, etc.), a comfort for a while” [an-Nahl 16:80]. This verse appears in the context of reminding people of Allah’s blessings, so the apparent meaning is that it includes animals both dead and alive.
They also quoted as evidence the hadeeth of Maymoonah (may Allah be pleased with her) according to which the Messenger (blessings and peace of Allah be upon him) said concerning a sheep of Maymoonah’s (that had died) when he passed by it: “It is only haraam to eat it.” According to another version: “Only its meat is haraam for you; you are allowed to make use of its skin.”
On the basis of reasoning they concluded that the dead animal is regarded as pure when alive, and issue of whether the hair (or bristles) is pure has nothing to do with whether the animal is alive or dead.
The most correct view is that the bristles of pigs and the hair of dogs and other animals are taahir (pure).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: With regard to hair growing on a body that is impure, Imam Ahmad had three reports:
1.That all kinds of hair are pure, even the hair of dogs and the bristles of pigs. This is the view favoured by Abu Bakr ‘Abd al-‘Azeez.
2.That all kinds of hair are impure, as is the view of ash-Shaafa‘i.
3.That if the hair of the dead animal was pure when it was alive, then it is pure, as in the case of sheep and mice; but the hair of animals that are regarded as impure when they are alive is also impure, as in the case of dogs and pigs. This is the view favoured by the majority of his companions.
The most correct view is that all hair is taahir (pure), whether it comes from a dog, a pig or other animals. This is unlike the saliva.
Based on that, if the dog’s hair is wet and it gets onto a person’s clothes, there is no problem with that, as is the view of the majority of the fuqaha’, Abu Haneefah and Maalik, and of Ahmad according to one of the two reports from him. That is because the basic principle concerning substances is that they are pure, and we cannot regard anything as impure or haraam except on the basis of evidence to that effect, as Allah, may He be exalted, says (interpretation of the meaning):
“He has explained to you in detail what is forbidden to you, except under compulsion of necessity”
[al-An ‘aam 6:119]
“And Allah will never lead a people astray after He has guided them until He makes clear to them as to what they should avoid”
The Prophet (blessings and peace of Allah be upon him) said in the saheeh hadeeth: “One of the most serious sins a Muslim may commit is to ask about something that was not forbidden, then it becomes forbidden because of his asking.”
In as-Sunan it is narrated from Salmaan al-Faarisi in a marfoo‘ report that he said: “What is permissible is that which Allah has permitted in His Book, and what is forbidden is that which Allah has forbidden in His Book. And what He has said nothing about is that which He has forgiven. As that is the case, the Prophet (blessings and peace of Allah be upon him) said: “The purification of the vessel of one of you, if it is licked by a dog, is to wash it seven times, the first of which should be with dust.” All versions of this hadeeth mention nothing except licking; they do not mention any other parts of the dog, so regarding the dog itself as impure is a conclusion that is reached only by analogy… With regard to every animal that was said to be impure, the argument concerning its hair or feathers is like the argument concerning the hair of the dog. If it is said that every wild animal that has fangs and every bird that has talons is impure – except cats and whatever is smaller than them, as is the view of many of the scholars, the scholars of the people of Iraq, and it is the better known of the two reports from Ahmad – then the issue of the hair or feathers of such creatures is subject to the same argument, whether it is impure or not. There are two reports from Ahmad, one of which says that it is pure. This is also the view of the majority, such as Abu Haneefah, ash-Shaafa‘i and Maalik. The second view is that it is impure, which is the view favoured by many of the later companions of Ahmad. The view that it is pure is the correct view, as stated above.
End quote from al-Ikhtiyaaraat al-Fiqhiyyah dumna al-Fataawa al-Kubra, 5/264
Based on that, there is nothing wrong with using a hairbrush made out of boar bristles, and it does not matter if it touches wet hair, but avoiding that is preferable, so as to avoid an area of scholarly difference of opinion.
And Allah knows best.