Sholom Rav.

Your definition of Hevel is correct, as is your portrayal of the case in hand.

To be honest with you, whenever I learn this Gemara, the same question that bothers you bothers me. Bear in mind however, that what Chazal are saying is, not that every animal that falls into a shallow pit face downwards will die, or even most animals, but that in the event that it did, it was the foul air that caused its death. That seems to me quite feasible.

Be'Virchas Kol Tuv

Yours sincerely

Eliezer Chrysler

Here is an earlier response to the same question:

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Talmid Rabeinu Peretz (in the Shitah Mekubetzes 22a) says it is something that is caused by the walls of the Bor. It sound like he understands it to be a presence of a trait rather than the absence of air.

This trait might be warmth as we find in Shabbos (48a and elsewhere) - see also Rashi 51a DH Mayim "water adds Hevel (to a Bor)," and Rashi 53a DH d'Ahani "Hevel that enters (its nostrils) at the beginning of its fall can kill it (even if no more Hevel enters at the end of the fall)."

But none of these are definitive proofs and it may still be posited that Hevel is an absence of oxygen.

However, see Zevachim 113b where we find that the Hevel in Eretz Israel at the time of the Great Flood killed its inhabitants (even though there was no water there). The Gemara proves the existence of Havla by the Flood from Rav Chisda's statement that the flood waters were boiling hot. This implies that the "killer-Havla" is heat.

Once again, though, Havla may be a general term for unbreathable air due either to heat or rarification of its oxygen.

Best wishes,

Mordecai Kornfeld

Kollel Iyun Hadaf