Ohalot, Chapter Ten, Mishnah Four
1) If there are multiple hatchways, one above the other, and they each have an opening of one handbreadth [square], if there is uncleanness in the house, what is directly [below] the hatchways remains clean.
2) If the uncleanness is directly [below] the hatchways, the house remains clean.
3) [In the case] where the uncleanness is either in the house or directly [below] the hatchways, if something susceptible to uncleanness was placed either in the upper or the lower [hatchway], everything becomes unclean.
4) If the article is insusceptible to uncleanness, what is below becomes unclean, but what is above remains clean.
Section one: Our mishnah deals with a multi-story building, with hatchways one over the other, all of them one handbreadth wide. If the uncleanness is in the house (bottom floor), it remains in the house. The space below the hatchways remains clean.
Section two: If the uncleanness is below the hatchways, it doesn’t enter the house.
Section three: The mishnah now discusses what happens if something that is susceptible to impurity blocks up one of the hatchways, be it the upper or lower one. In this case no matter where the impurity is, under the house or under the hatchways, everything is impure. Seemingly, if the blockage was on the bottom hatchway, the contents of the upper stories should be pure, because the impurity would remain down there. However, since the article is susceptible to impurity, it doesn’t block the impurity. It does, though, turn the bottom floor into one ohel, so that even if the impurity is under the hatchway, the whole floor is impure. Furthermore, the sages said that just as the bottom floor is impure, so too all of the floors are impure, even if the article is below the hatchway and would seem to escape through the upper hatchways. We look at all of the floors as if they were all closed up.
Section four: If the article is not susceptible to uncleanness, then it blocks the impurity from spreading to the upper floors. Only the bottom floor is impure.