Board Thread:False Info and Speculative Discussion/@comment-24.130.28.230-20150721180826/@comment-4091815-20160130220207

Scott did not "cross mountain ash" he busted through it. It affected him. It stopped him. It does none of that to Lydia. Scott is an exception only in that he was able to brute his way through a barrier.

Lydia can not only cross Mountain Ash without noticing, she can handle it and use it as other non-supernatural's can. She is completely unaffected by it which puts her squarely in the company of the other non-supernaturals on the show.