One who touches his feet, or [other normally covered] parts of the body, with his hands is required to wash his hands one time [immediately] afterwards. [This applies even if one only touched the area with his small pinky. One who touches the normally revealed areas of the body is not required to wash his hands.]
Must one wash both hands if only one hand touched the normally covered area?
Some Poskim rule one is only required to wash the hand that touched the covered area and not the other hand. Other Poskim rule one is required to wash both hands. Practically the main opinion follows the lenient opinion although one who is stringent is blessed.
Must one wash his entire hand if only part of it touched the normally covered area?
One must wash the entire hand, up until the wrist, even if he only touched the covered area with a single finger. [If however his fingers did not touch the area, but rather only his palm or back of the hand, some write washing is not required.]
What is defined as the “covered areas” of the body?
This is dependent on the normal practice of each community and follows the area of the body that is normally covered majority of the time. Thus the face, neck area up until the chest, and from the elbow and down, is today considered normally uncovered areas and don’t require washing of the hands upon being touched. In those areas that it is common to walk barefoot, the feet are considered an uncovered area. [Nevertheless one is to wash his hands upon touching the feet even in such a case, due to the evil spirit.]
Must one wash his hands if he touched the uncovered area of a child?
This depends on the area of the body of the child. Those areas that are normally always covered require washing hands. Thus if one changes the diaper of an infant one is to wash his hands after. However those areas that are commonly left uncovered, such as the legs of toddlers, do not require washing. However there are those that are stringent by all areas normally covered by an adult.
Are the orifices of the body considered “covered areas”?
Yes. Hence if one inserts his finger into his ear or nose he is required to wash his hands.
Must one wash his hands if he touched sweat?
One who touched sweat that came from the covered areas of the body is to wash his hands afterwards. Thus if one touched sweat found under a Kipa or hat or by the underarms of a shirt, he is to wash his hands. If however he touched sweat of the uncovered areas, he is not required to wash his hands. Thus touching perspiration of the face, hands and neck does not require hand washing.
Must one wash his hands upon touching the covered areas even if he is clean from sweat, such as if he just took a shower?
Some Poskim rule that in such a case one is not required to wash his hands, with exception to one who touches his feet. However practically it is suggested to be stringent and wash the hands even in such an instance.
Must one wash his hands if he touched the covered areas using a cloth?
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 Kama 4/18
 As for the reason why the feet were specifically excluded from the rest of the covered parts of the body, Kaf Hachaim 4/73 explains that according to the Mekubalim the evil spirit which resides on the feet is very strong and does not move from its place even after being washed. [Kaf Hachaim in name of Yifei Laleiv 1/22 and Mor Uketzia 4; Rav Poalim 2/4] Thus perhaps its requirement of washing is irrelevant of whether it is considered a normally covered area. See Chapter 6 Halacha 1B.
Regarding how the Kohanim were allowed to touch their feet while washing before service: Some explain the evil spirit did not reside in the Temple. [Rav Poalim ibid] or that they did not actually touch their feet. [Levushei Mordechai Even Haezer 47]
 Ketzos Hashulchan 2/11
 The reason washing hands is required: The normally covered areas of the body contain sweat which dirty the hands. [92/7; 164/2] This sweat is considered poison. [Yoreh Deah 116/4; Admur Shemiras Haguf Vihanefesh 8] It appears from Admur that due to this there is an evil spirit that resides on the hands after touching these areas, as explained in length in Halacha 2.
Other Opinions: Some Poskim rule that touching the covered areas of the body does not bring an impure spirit and hence does not require washing with water. Rather the hand requires cleaning in any fashion in order to be allowed to learn Torah. [Machatzis Hashekel 4/17; Nivei Shalom 4/12; Machazik Bracha brought in Shaareiy Teshuvah 4/12; Soles Belula 4/14 in name of Makor Chaim; Ben Ish Chaiy Toldos 17] Admur however rules that it causes impurity, and hence he rules only washing with water helps. So also rules Kisei Eliyahu ibid; Yifei Laleiv 4/18 in name of Mor Uketzia; Peri Megadim 4 M”Z 14; and so is implied from many Poskim. [brought in Kaf Hachaim 4/60]
 Ketzos Hashulchan 2/11; Kaf Hachaim 4/64 in name of Peri Megadim
 Michaber 4/21
 Kaf Hachaim 4/86
 Chayeh Adam 40/18; Admur in Seder Netilas Yadayim 17 regarding one who touched a covered area during a meal that he must only wash that hand [However Tzaruch Iyun if this can be used as a source as perhaps regarding removing the Ruach Raah washing both hands is required.]; Other Poskim listed in Kaf Hachaim ibid
 Yifei Laleiv 1/27; Leiv Chaim 2/7; There he concludes that so rules the Shlah Hakadosh and so is the custom of the world.
 Ketzos Hashulchan 2/11; Eretz Chemda brought in Kaf Hachaim 4/86; M”B 4/57
 Piskeiy Teshuvos 4/23 in name of Chazon Ish
 Machatzis Hashekel 4/23 [brought in M”B 4/53; Kaf Hachaim 4/99]
 Michaber 4/21; M”B 4/53; Kaf Hachaim 4/99 in name of Beis Yosef 92; Kol Bo; Taz 92/2; M”A 4/23; Elya Raba 4/14; Soles Belula 4/2
 See Kaf Hachaim 4/73 brought in footnotes.
 Ben Ish Chaiy Toldos 17; Kaf Hachaim 4/99; Opinions in Piskeiy Teshuvos 27/2
This applies only to the arm which one lays Tefillin as it is normal to reveal it daily for putting on Tefillin. However the other arm, and both arms of women, are commonly always covered and hence require washing of the hand. [ibid]
 See Piskeiy Teshuvos 27/2 for various opinions on this matter.
 Or Letziyon 2/44-6; See Piskeiy Teshuvos 4/23 for a list of rulings from today’s Poskim on this matter.
 Piskeiy Teshuvos 4/29; Az Nidbaru 14/28
 Sheivet Halevy 8/2; However see Kaf Hachaim 4/98 in name of Ruach Chaim that one is not required to wash his hands after touching a sweat filled shirt, even if the sweat came from covered areas of the body. Ashel Avraham Butchach 4 rules likewise that one is not required to wash the hands, although he concludes it is an attribute of piety to do so. Halichos Shlomo 20/17 in name of Rav SZ”A is lenient and so rules Piskeiy Teshuvos 4/30
 Ashel Avraham Butchach 4; Kaf Hachaim 4/98 in name of Ruach Chaim; Piskeiy Teshuvos 4/23; The reason for differentiating between sweat of the revealed parts of the body and covered parts of the body is because only the seat of the covered parts is considered contaminated and brings the evil spirit. [Kaf Hachaim ibid]
 Kaf Hachaim 4/85 in name of Ruach Chaim 2
 Regarding the feet Kaf Hachaim 4/73 explains that according to the Mekubalim the evil spirit which resides on the feet is very strong and does not move from its place even after being washed. [Kaf Hachaim in name of Yifei Laleiv 1/22 and Mor Uketzia 4; Rav Poalim 2/4; Torah Leshma 13] However see M”B 4/54 in name of Peri Megadim 92 which implies that the feet has the same law as other parts of the body.
 Piskeiy Teshuvos 4/56; Vayeishev Moshe 2/12; Halichos Sholomo 2/15 in name of Rav SZ”A.
 Chayeh Adam 22/9; Yifei Laleiv 1/27; Poskim brought in Kaf Hachaim 4/87; See Admur 97/3