Hand-washing is permitted for a mitzva. Therefore, kohanim may wash their hands (up to the wrists) before Birkat Kohanim (Rema 613:3; SA 128:6). However, one who had a nocturnal emission on Yom Kippur should not immerse himself, even if he would normally do so, because the pious practice of immersing after a seminal emission does not override the prohibition on washing. Similarly, a nidda whose time to immerse coincides with Yom Kippur should postpone visiting the mikveh until the night after Yom Kippur (SA 613:11-12).
After waking up in the morning, one should use a cup to wash his hands three times to the base of the fingers (where the fingers meet the palm), because a ru’aḥ ra’a remains on the hands after a night’s sleep, and it can harm the body’s orifices. To remove it, the hands must be washed three times, alternating between left and right. After using the toilet, one washes the hands again and recites the berakha of “al netilat yadayim.” This washing is a mitzva, as the Sages ordained hand-washing before praying Shaḥarit (Peninei Halakha: Prayer 8:4 n. 2). Even though normally we enhance this mitzva by washing the entire hand, on Yom Kippur we wash only to the base of the fingers, because technically this is adequate for both cleanliness and the removal of ru’aḥ ra’a (SA 613:2). While it is true that when one tries to wash beyond the base of the fingers, the palm can get a little wet, this is not a concern, since it is not his intention.
If one touches an area of the body that is usually covered and sweaty, he is considered to have touched something dirty. If he wishes to recite sacred words afterward, he should wash his hands, as he is washing them for a mitzva and not for pleasure (MB 613:5-6; Kaf Ha-ḥayim ad loc. 6; Peninei Halakha: Prayer 5:2). There is uncertainty regarding the status of one who relieves himself without touching any part of the body that is usually covered, as perhaps he does not need to wash, since he did not touch any filth. In order to avoid this uncertainty, when one relieves oneself it is best to touch a part of the body that is usually covered. This way, all agree that one may wash his hands until the base of his fingers, including the knuckles, in order to recite the berakha of Asher Yatzar in a state of cleanliness (SA 613:3, MB ad loc. 4).
Regarding immersion to become tahor, according to most Rishonim, immersing at the proper time is a mitzva and overrides the prohibition of washing. Rabbeinu Tam disagrees. However, nowadays everyone is tamei from exposure to corpses, so even immersion does not purify for the purpose of eating taharot. Therefore, even those who maintain that it is a mitzva to immerse on time would agree that nowadays it is not (Tosafot, Beitza 18b). Furthermore, nowadays women do not actually immerse on time (according to Torah law), since we follow the rabbinic stringency of having every nidda count seven clean days following the conclusion of her period (which Torah law requires only of the zava). Beit Yosef OḤ 554:8 summarizes the issue, and SA 613:12 rules that a nidda does not immerse on Yom Kippur.
. Many of these laws are the subject of disagreement. We will mention some of them and then clarify the halakha. According to most poskim, one is only required to wash each hand once after using the bathroom, but many have the custom of washing each three times. (See MB 4:39.) Similarly, on Yom Kippur, according to most poskim one washes once, while others say three times (R. Mordechai Eliyahu, Hilḥot Ḥagim 45:25). See Peninei Halakha: Prayer 8:3-5 n. 2. Some say that one who touches an area of the body that is usually covered should wash only that hand until the base of his fingers (Ḥayei Adam 40:18, MB 613:6). Others maintain that he must wash both hands (Shlah, Yafeh La-lev; see also Kaf Ha-ḥayim 4:86). They also disagree about a case in which one touches his shoe – even if it is made of cloth – with one finger. Some say he must wash just that hand, while others say he must wash both hands. There are many other uncertainties regarding these laws. For example, does one who touches an area that is usually covered, but which nonetheless is not sweaty, need to wash his hands? (See Peninei Halakha: Prayer, ch. 5 n. 2.)
There is an apparent question pertaining to the view that one may wash on Yom Kippur in these circumstances. Technically, one who touches a sweaty part of the body may simply rub his hands on any sort of cloth and then recite sacred words (SA 4:23, MB ad loc. 61). Why, then, do we not do so on Yom Kippur? It seems that whenever a person would wash his hands ritually under normal circumstances, he may wash them on Yom Kippur as well, even if technically it would be sufficient for him to rub his hands clean, because he is washing for the sake of a mitzva. If, however, he sometimes suffices with rubbing his hands on his clothing or the like, he may not wash on Yom Kippur, since for him this hand-washing is not a mitzva. Consequently, most discussions of this subject deal with people’s behavior during the year. Accordingly, we may ask: Why does Shulḥan Arukh permit washing only to the base of the fingers in the morning and after using the bathroom on Yom Kippur, if during the rest of the year, people normally wash to the wrist? I would like to suggest the following answer: Unlike most of the disagreements raised in the previous paragraph, where the halakha itself is the subject of dispute, according to Shulḥan Arukh washing to the wrist during the year is not a law but rather a mere stringency. Thus, on Yom Kippur we revert to the basic law and do not wash to the wrist. Most Aḥaronim agree with Shulḥan Arukh. Arizal, though, takes a different approach. According to him, washing the entire hand is required all year to remove ru’aḥ ra’a. However, on Yom Kippur, the power of the ru’aḥ ra’a is weakened, so it is sufficient to wash to the base of the fingers (Ben Ish Ḥai, Toldot 2; Kaf Ha-ḥayim 4:14; see Minḥat Yitzḥak 10:45).