Sweat-Its toxicity and danger of consumption:
One must beware from [eating] human sweat, as all human sweat is poisonous with exception to the sweat of the face. This is hinted to in the verse “With the sweat of your brow you shall eat bread.” As well, eating sweat is included in the prohibition of “Al Tishaktzu Es Nafshoseichem/Do not do repulsive things to yourself.”
Placing food in armpit: One is to beware not to place a loaf [or slice] of bread [or other food] under his armpits, due to the sweat. Likewise, one is not to place bread between his upper and lower clothing if it is sweaty. If, however, the lower clothing is not sweaty then one may place cold bread on it by the armpit, being that the food does not directly touch one’s skin. However, by hot bread one is to beware from doing so even if the clothing is dry, as the heat causes the sweat to penetrate through the clothing into the bread.]
Food on body or sweaty clothing: One is to beware not to place food between his clothing and his skin, due to the sweat. [Likewise, one is not to place them between his upper and lower clothing if it is sweaty. If, however, the lower clothing is not sweaty then one may place food on it, being that the food does not directly touch one’s skin.]
Washing hands after touching sweat of body: 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. It is forbidden to learn Torah or recite blessings until his hands are cleaned. [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 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 if he touched the covered areas using a cloth?
 See Admur Shemiras Haguf Vihanefesh Halacha 8; Michaber Y.D. 116:4-5; Kaf Hachaim 116:36-37; Kitzur SHU”A 33:3; Shemiras Hanefesh Kehilchasa 7:18
 Admur Shemiras Haguf Vihanefesh Halacha 8; Michaber Y.D. 116:4; Rambam Rotzeiach 12:4; Rif Avoda Zara 10b; Rosh Avoda Zara 2:13; Yerushalmi Terumos 8:3
Sweat of other uncovered parts of body: From the fact that only the sweat of the concealed parts of the body cause one’s hands to become putrid and require washing implies that the sweat of all normally uncovered areas does not contain danger, and not just the sweat of the face. [See Admur Kama 4:18; 92:7; 164:2; Michaber 4:21; Beir Heiytiv 4:26; M”B 4:52; Kaf Hachaim 4:98; Shemiras Hanefesh Kehilchasa ibid Biurim 18] However, from the wording above of the word “except for the sweat of the face” it is implied that all sweat is considered toxic. Likewise, its consumption would anyways be prohibited due to Al Tishkatzu, as stated in Michaber ibid.
 Bereishis 3:19
 Admur Shemiras Haguf Vihanefesh Halacha 8; Shach Y.D. 116:2; Kaf Hachaim 116:37
 Michaber Y.D. 116:6
 Michaber Y.D. 116:6
 The reason: This is forbidden due to it being repulsive. [Michaber ibid]
 Admur Shemiras Haguf Vihanefesh Halacha 8; Michaber Y.D. 116:5; Yerushalmi Terumos 8:3
 Shemiras Hanefesh Kehilchasa ibid
 The reason: As all human sweat is poisonous with exception to the sweat of the face. [Admur ibid; Michaber Y.D. 116:4; Rambam Hilchos Rotzeiach 12:4; Yerushalmi Terumos 8:3]
 Kaf Hachaim 116:36 and 39
 Peri Toar Y.D. 116:5; Zivcheiy Tzedek 116:27; Darkei Teshuvah 116:34; Kaf Hachaim 116:39
 Chochmas Adam 68:2; Kaf Hachaim 116:36
 Kaf Hachaim 116:36
 Kaf Hachaim 116:36 and 39
 Admur Shemiras Haguf Vihanefesh Halacha 8; Michaber Y.D. 116:5; Rambam Rotzeiach 12:4; Yerushalmi Terumos 8
 Admur ibid; Rambam ibid
 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]
 See Admur 92:5-7; 164:2 [such as the thigh and leg]; 7:2; Kama 1:5; 4:3; Basra 1:6; 4:1; Siddur
 Ketzos Hashulchan 2:11; Kaf Hachaim 4:64 in name of Peri Megadim
 Michaber 4:21
 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]
 Chayeh Adam 22:9; Yifei Laleiv 1:27; Poskim brought in Kaf Hachaim 4:87; See Admur 97:3