User blog comment:FairyRaven/Fairy Tail Chapter 397: Steel, Review and Commentary/@comment-7418318-20140825132453/@comment-24800928-20140825171314

I guess it could kinda falls in the prnciple of slayers can't consume their own element if it comes from them.

well carbon isn't really poisonous unless you comsume too much (which is why they replaced the lead in our pencils for graphite), which is the reason of the 5 minute limit, and I'm guessing the fight was under 5 minutes, so they all should be fine soon or later. Since levi is still up from ingesting the least amount, she could just give natsu fire to get him back to himself.