The lands of Mordor have long been tainted by the evil that dwells there, and is a treacherous place to travel through for the Ringbearer, not only because of the dozens of orcs and Sauron’s servants who reside there, but also because of the actual atmosphere itself. Boromir even says at the Council of Elrond “The very air you breathe is a poisonous fume.” So it comes as little surprise that by the time Sam and Frodo reach the edges of Mount Doom, they are exhausted beyond recall, and delirious with the sickening intoxication of the place.