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The Shulchan Aruch (4: 18) records a list of activities that require one to wash his hands.

אֵלּוּ דְּבָרִים צָרִיךְ נְטִילָה בְּמַיִם...וְהַנּוֹגֵעַ בְּרַגְלָיו...וְהַחוֹפֵף רֹאשׁוֹ...וְהַנּוֹגֵעַ בְּגוּפוֹ בְּיָדו

"These things require washing with water...one who touches his feet, one who rubs his head, one who touches his body with his hand."

The example is clarified (ibid: 21)

צָרִיךְ לִזָּהֵר בִּתְפִלָּה אוֹ בַּאֲכִילָה שֶׁלֹּא לִגַּע בְּשׁוֹק וְיָרֵךְ וּבִמְקוֹמוֹת הַמְכֻסִים בָּאָדָם, לְפִי שֶׁיֵּשׁ שָׁם מִלְמוּלֵי זֵעָה. וְכֵן שֶׁלֹּא לְחַכֵּךְ בְּרֹאשׁוֹ אֲבָל מְקוֹמוֹת הַמְגֻלִּים בְּרֹאשׁוֹ וּבְפָנָיו וּמָקוֹם הַמְגֻלֶּה שֶׁבִּזְרוֹעוֹתָיו אֵין לְהַקְפִּיד

"One must be careful while praying or eating not to touch one's leg or thigh, or places that are generally covered since those places are full of sweat. Similarly, one shouldn't rub his head. But regarding uncovered parts of his head, face, or arms, one need not be stringent."

This aversion to sweaty parts of the body is apparently predicated on the assumption that sweat is deadly poison (Shulchan Aruch YD 116:4):

צָרִיךְ לִזָּהֵר מִזֵּעַת הָאָדָם, שֶׁכָּל זֵעַת הָאָדָם סַם הַמָּוֶת, חוּץ מִזֵּעַת הַפָּנִים

Are there any later poskim who rule that these rules are no longer operative either because of increased hygiene, or because of better scientific understanding of the poisonous nature (or lack thereof) of sweat?

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To borrow from the answer of @Mefaresh here:

Tzitz Eliezer, part 7, 2:14 concludes that if he washed his head and his hair is clean, even a person who scratches the roots of his hairs does not need to wash his hands.

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