Reference for Makhshirin 2:1

זֵעַת בָּתִּים, בּוֹרוֹת, שִׁיחִין וּמְעָרוֹת, טְהוֹרָה. זֵעַת הָאָדָם, טְהוֹרָה. שָׁתָה מַיִם טְמֵאִין וְהִזִּיעַ, זֵעָתוֹ טְהוֹרָה. בָּא בְמַיִם שְׁאוּבִים וְהִזִּיעַ, זֵעָתוֹ טְמֵאָה. נִסְתַּפֵּג וְאַחַר כָּךְ הִזִּיעַ, זֵעָתוֹ טְהוֹרָה:

Moisture [on the walls of] houses, cisterns, trenches and caves is pure [it does not make food susceptible to impurity]. A person's sweat is pure. [If] one drank impure water and [then] sweated his sweat is pure. If he went into drawn water and [then] sweated his sweat is impure [makes food susceptible to impurity]. If he dried himself off and then sweated his sweat is pure.

Jastrow