Praise be to Allah.
The Arabic phrase al-qashaf al-mayyit (translated here as “dead skin”) refers in linguistic terms to dirt that accumulates on the skin, in such a way that it becomes stuck to it with the passage of time. This is what is meant by describing it as “dead”.
It says in al-‘Ayn by al-Khaleel ibn Ahmad (1/375): Qashaf refers to dirt on the skin. A rajul mutaqashshif is a man who does not take care of himself by washing himself and keeping himself clean.
This word qashaf refers to one who does not care what gets onto his body of dirt. End quote.
Al-Murtada az-Zubaydi (may Allah have mercy on him) said: Qashaf is dirt that builds up on the bottom of the feet. End quote from Taaj al-Aroos (24/259); see also Lisaan al-‘Arab (9/282).
As that is the case, undoubtedly this dirt that accumulates on the skin should be removed, once it begins to form a layer that prevents water from reaching the skin, if it is possible to remove it by putting in effort until it is gone.
It says in Haashiyat as-Saawi ‘ala ash-Sharh as-Sagheer (1/132), speaking of the conditions of wudoo’ being valid:
… The second [condition] is that there should be no barrier that prevents water from reaching the skin, such as wax or grease that forms a layer on the limb. That includes crusty matter in the corner of the eye, ink on the hand of a scribe, and the like.
The words “and the like” include dirt that accumulates and forms a layer on the body, which includes dead skin. End quote.
In Haashiyat al-Jamal ‘ala Sharh al-Manhaj (1/113), it says:
One must remove things like dead skin and dirt that gathers under the fingernails, which prevent water from reaching the skin. End quote.
This applies if it is possible to remove it without harm or what is regarded as too much effort.
But if it has become part of the body, in the sense that harm would result from removing it, then it does not affect the validity of wudoo’, because it is regarded as part of the body.
It says in Haashiyat al-Bujayrimi ‘ala al-Khateeb (1/128): al-Qaffaal said: Accumulation of dirt on the limb does not affect the validity of wudoo’, and removing it is not a must if it has become like part of the body, because in that case it is not possible to remove it from the body. What is meant by it becoming like part of the body is that it cannot be distinguished visibly from it. End quote.
The phrase “al-qashaf al-mayyit (dead skin)” may sometimes refer to actual dead skin on the body, as occurs in some places on the feet, and occasionally on the hands, or after wounds and the like have healed. This does not have to be removed, and it is not required to make water to reach what is beneath it, because it is part of the body and it is usually not possible to remove it without causing some kind of pain or harm. Therefore it is not required to remove it at all.
It says in Haashiyat al-Baajoori ‘ala Ibn Qaasim (1/51): Anything on the body which forms a barrier [to water reaching the skin] must be removed, such as dirt that accumulates from the outside, if it is not too difficult to remove it; otherwise it may be left, because it has become like part of the body.
As for dirt that is exuded from the body, such as sweat, that does not affect wudoo’ at all.
The same applies to the skin covering a boil, even if it is easy to remove it. End quote.
Shaykh Sulaymaan al-Maajid (may Allah preserve him) said: What is meant by dead skin is when new skin grows because of the accumulation of fluid due to a wound and the like, but the original upper skin remains as it is. This upper layer is the dead skin; it is called dead because it has no sensation.
Some of the fuqaha’ are of the view that this dead skin does form a barrier that prevents water from reaching the new skin that is attached to the flesh, and that wudoo’ is not valid unless it is removed and the water allowed to reach the new skin. However, they stipulated that removing it should not cause harm.
What is more likely to be the case is that wudoo’ is valid if that upper layer – and not what lies beneath it – is washed, because this is not a barrier caused by the individual’s actions; rather it is the person’s skin. Based on that, if the person affected leaves it until it forms a crust and drops off by itself, there is no blame on him. The ruling only has to do with the uppermost layer of the skin.
End quote from the shaykh’s website
With regard to dandruff, it does not affect the validity of wudoo’, because purification of the head (during wudoo’) is a matter concerning which there is leniency, so there may be concessions granted concerning it that are not granted in other cases.
It says in Fataawa Noor ‘ala ad-Darb by Shaykh Ibn ‘Uthaymeen: But with regard to what is applied to the head, such as henna and the like, it does not matter if the woman wipes over it, because it is proven that the Prophet (blessings and peace of Allah be upon him) stuck his hair together during the Farewell Hajj in a process called talbeed [when the hair is stuck together using gum and the like, which creates a layer over the hair and may prevent water from reaching it], and talbeed prevents water from reaching the hair when wiping over it. Moreover, purification of the head is a matter concerning which there is leniency, based on the evidence that it is not obligatory to wash it; rather what is required is to wipe over it, even if the hair is thin, and indeed even if there is no hair on the head at all. So purifying the head is is a matter concerning which there is leniency, and it consists of simply wiping over it. Hence it is allowed to wipe over anything that is put on the head. So it is permissible for a man to wipe over his turban, even though it is possible for him to take it off and wipe his head, but that is by way of making things easy. By the same token, according to the view of many scholars, it is permissible for a woman to wipe over her head cover if it is wrapped under her chin. End quote.
And Allah knows best.