Machshirin, Chapter 2, Mishnah 2. This chapter is occupied with the question of which liquids can make food susceptible to tumah, and it gives special attention to perspiration: the sweat of a person and the moisture that collects on walls and in enclosed spaces. Our mishnah applies that discussion to two settings, a bathhouse and a house that contains a pool.
A word about the terminology. Here, as throughout these mishnayos, tamei does not mean tamei in the plain sense. It means that the liquid functions as a hechsher: food that comes into contact with it now becomes susceptible to receiving tumah. And tahor means the opposite, that the liquid is not machshir le'kabbel tumah.
The bathhouse
The opening clause reads: "Merchatz teme'ah, ze'asah teme'ah." Where the bathhouse itself carries tumah, the perspiration found in it carries tumah as well. It helps to picture the institution as it existed then: bathing facilities of that era doubled as steam chambers, rooms one entered precisely in order to sweat, so that heavy moisture clung to the walls and hung in the air.
The Rav explains what must be true for a bathhouse to bear tumah at all: its water has to be mayim she'uvim, water that was drawn. Water that remains joined to the earth is beyond the reach of tumah entirely, so the mishnah's case has to involve water that was drawn off. Once we are dealing with such water, the vapor and the droplets that collect in the room carry tumah of themselves, and any food that meets that moisture is thereby made capable of receiving tumah.
"U'tehorah": if the bathhouse is tahor, the moisture falls into the ordinary category of ki yutan. As we have seen many times, for a liquid to make food susceptible there must be ratzon, the owner's desire that the food get wet. So with a tahor bathhouse everything depends on his wishes: if he wants the food to become wet, the moisture is a hechsher and the food becomes susceptible; if he does not want it, or is indifferent to it, the food is not rendered susceptible at all.
The pool inside a house
The mishnah continues: "Ha'breichah she'ba'bayis, ha'bayis meze'ah mechamasa." There is a pool inside a house, and the house perspires on account of it. The warmth and moisture rising from the pool cause condensation to form on the walls of the house.
The ruling follows: "Im teme'ah", should the pool carry tumah, then "ze'as kol ha'bayis", the sweat of the whole house, so long as it is "she'mechamas ha'breichah", brought about by that pool, is tamei. Any dampness in the building generated by the pool is judged like the pool's own water: it serves as a hechsher, and food that touches it becomes capable of receiving tumah.
The Rav spells out the flip side. Should the house be sweating from some other cause, with the pool not responsible for it, that moisture remains tahor and confers no susceptibility. The deciding factor is where the dampness comes from: whatever the tamei pool itself produces inherits the pool's status, while dampness traceable to something else does not.