Indeed, it says exactly there in the quote: > riddled with fire and ash and dust, the very air you breathe is a poisonous fume As carbon monoxide is lighter t

Indeed, it says exactly there in the quote: > riddled with fire and ash and dust, the very air you breathe is a poisonous fume As carbon monoxide is lighter than air, orcs and other creatures on the ground are less affected and probably more adapted to the poisonous atmosphere of Mordor; whereas the eagles flying high and accustomed to the clean air of Middle Earth would simply succumb to the prolonged exposure to CO. Of course, the destruction of Mt. Doom and subsequent paralysis of Sauron's military capabilities allow the eagles to fly in at much lower altitudes in the end.