פירוש על זבים 5:9

והוא בבית הבליעה (and it is in the esophagus/gullet) – all the time that it (i.e., the food) is in the esophagus it defiles clothing, and since it defiles clothing, it defiles two things (i.e., it’s law is like that of a primary source of ritual defilement – that it makes something first degree of ritual impurity and a first makes non-holy produce second-degree of ritual impurity) and invalidates/makes unfit one (i.e., something second degree of ritual impurity makes priest’s due/heave offering unfit).

Introduction Today's mishnah talks about a person who eats the nevelah (carrion) of a clean bird. In Toharot 1:1 we learned that this carrion defiles him once it is in his gullet. There are more details about the laws governing the carrion of a clean bird in that mishnah, for those of you who are very interested.

הכניס ראשו באויר התנור – even though the carrion of the bird exists with the esophagus/gullet of the person who eats it, everything is in the airspace of the over, nevertheless, it (i.e., the oven) is ritually pure. For the oven is not defiled on account of the person, for everything that defiles clothing does not defile humans and earthenware vessels. But because of the carrion of a clean bird it does not become defiled, for it lacks defilement other than the person in his esophagus/gullet.

He who ate nevelah of a clean bird, and it still is in his gullet, defiles at two [removes], and disqualifies [terumah] at one [more remove]. When the nevelah of the clean bird is in his gullet, he is a father of impurity.

הקיאה או שבלעה (he vomited it out or swallowed it) – that is he separated from it. That from when it (i.e., the food particles of the carrion of the clean bird) descended to the stomach, it has no defilement, and when it is in his mouth, it has no defilement, for it is written (Leviticus 17:15): “[Any person, whether citizen or stranger,] who eats what has died/"[וכל נפש] אשר תאכל [נבלה]" for it is written, “who has not eaten” – food in the esophagus/gullet.

If he put his head within the air-space of an oven, the oven remains clean. Despite the fact that he is a father of impurity, if he puts his head into the air-space of an oven, he doesn't defile the oven. This is because a person who is defiled such that he must wash his clothes does not defile earthenware vessels. Note that the oven can generally be defiled by something impure entering its air space. Furthermore, once the carrion is in the gullet of the person, it only defiles the person. Because it's inside him it doesn't go out of him and defile the oven.

But if he vomited or swallowed it, he defiles at one [remove] and disqualifies [terumah] unfit at one [more remove]. The nevelah only makes him into a father of impurity when in it is in his gullet. If he expels it, or swallows it, he goes down a notch in impurity to having only first degree impurity.

But as long as it is still in his mouth, until he swallows it, he remains clean. When it's just in his mouth, he is clean. It only begins to defile him once it has entered his gullet.