The classic examples of the melacha of gozez (shearing) on Shabbat are the removal of hair and nails from a live or dead human or animal.[1] However, one gemara[2] mentions removing warts and another[3] mentions removing strands of partially detached skin from around the nails as Torah-level prohibitions. This would ostensibly also be the case regarding removing dry skin, as the Shemirat Shabbat K’Hilchata[4] confirms. Similarly, it is forbidden to purposely remove dandruff (a form of dry skin) that is still attached to the scalp.[5]
If it were not definite that the scratching would cause skin to come off and it was not your intention that it would, it would be permitted to scratch, as a davar she’eino mitkaven[6] is permitted on Shabbat.[7] However, we will take you at your word that it is definite that the unintentional removal of skin will occur, making it a p’sik reishei[8] and ostensibly forbidden.[9]
However, in your case, there are several mitigating factors. First, the gemara states regarding warts that there is at most a Rabbinic violation if they are dried out.[10] This mitigating factor likely applies to dry skin as well. Even if the warts are not dried out, if one removes them by hand, he violates only a Rabbinic prohibition, as the normal way to remove warts is to use a utensil. This second point probably does not apply to dry skin. The Bi’ur Halacha[11] raises the possibility that it is normal – and therefore forbidden from the Torah – to remove certain strands of skin by hand. However, if one used his elbow or otherwise did things unusually, there would once again be no more than a rabbinic violation.
A further mitigating factor in the case of dry skin relates to gozez’s basic definition. We accept the predominant opinion that gozez applies whether one needs that which is removed[12] or whether the removal improves the surface from which it is taken.[13] The melacha is not transgressed on a Torah level if there is neither a use for that which was removed nor an improvement of the area from which it was removed. In your case, you do not plan to use the removed skin flakes. Moreover, we have been told by a dermatologist that the removal of dry skin is not good for the remaining skin. Thus, there is another reason that there is no Torah prohibition involved.
In order to turn the mitigating factors into a lenient ruling, we need to employ an important general rule. Some authorities are lenient in cases of p’sik reishei when the problematic result is forbidden only rabbinically,[14] but the more accepted opinion is that such acts are still forbidden.[15] However, when there are two reasons why there is no Torah prohibition, most authorities permit a p’sik reishei, at least when the result is not desired.[16] We have demonstrated that there are at least two reasons that scratching dry skin is not a Torah prohibition. Since, as mentioned, it appears that it is not in the interest of the person who itches to have the skin removed, it is permitted to scratch the area even if some flakes will certainly fall off.
If one wants to be particularly stringent, he can refrain from scratching or scratch lightly, especially considering that it is better not to scratch in the first place. However, in a case where the discomfort of the itch makes it difficult to control himself, the laws of Shabbat need not hold him back.