Mishnah Machshirin 2:1: 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.

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:

Mishnah Machshirin 2:2: Moisture from an impure bathhouse [that is, the water in it is impure] is impure. And [moisture from] a pure [bathhouse] achieves BeKhi Yutan [a state where liquid put upon food makes it susceptible to impurity]. [If] there is a pool in the house and the house [walls] exude moisture because of it, if [the pool] is impure, the moisture of the whole house caused by the pool is impure.

מֶרְחָץ טְמֵאָה, זֵעָתָהּ טְמֵאָה. וּטְהוֹרָה, בְּכִי יֻתַּן. הַבְּרֵכָה שֶׁבַּבַּיִת, הַבַּיִת מַזִּיעַ מֵחֲמָתָהּ, אִם טְמֵאָה, זֵעַת כָּל הַבַּיִת שֶׁמֵּחֲמַת הַבְּרֵכָה, טְמֵאָה: