User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-122.174.97.73-20140615140118/@comment-7418318-20140615140544

They explained that. Without the special protection of the Spirit Clothes you can not breathe in the Spirit World.

When Virgo passed through on her way to answer Everloo's summons, she only was in the Spirit World for a few seconds at most, which wasn't long enough for Natsu to suffocate.