User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-173.210.154.172-20131109073606/@comment-7418318-20131109074359

Anyone can travel to the spirit world (assuming they have a method to enter it, like grabbing a spirit that de-summons), but without special protection they'd be unable to breathe and thus suffocate unless they are sent back in time.

The reason why Natsu was unharmed was because he "stayed" there only a few seconds.