User blog comment:Miskos3/Fairy Tail Chapter 486: The Fourth Guest, Review/@comment-7418318-20160525151848/@comment-5303512-20160527221748

Well there is more to it than just the shape and size of the lungs but other components that make of the lung as well.

Do you honestly think that actual Iron Lungs are normal, no human in existence can survive on actual iron lungs. Except for Iron Ling machines that help people to breath, but only when their breathing starts to fade.

Well when we see all of Gajeel's Iron Dragon Slayer Spells' they all have some sort of nuts, bolts, gears that operates like any other machinery and such so there is no doubt that there is some more special components the lungs that allows him to do what he is able to do with the MBP to not let it poison him.