Board Thread:False Info and Speculative Discussion/@comment-24.130.28.230-20150721180826/@comment-25242503-20150721222150

It's also possible not all supernatural creatures are affected in the same way by the stuff. Maybe some are more sensitive to it (i.e weres and the like) or maybe Lydia and Meredith, by their very nature as Death's messengers  are, plainly put, inmune to  a lot of supernatural things ( remember, they are also inmune to terianthropy,  for example).

I don't think it works as a ward against evil intents only. For instance, scott couldn't come across a mountain ash circle to save deaton when he was hanged at the bank vault