I think Titanium should be one of the closest things to safe we have, they have been using it for years for various medical implants in conjunction with another metal called Tantalum which is in the same family.
The problem is who knows if any metal, quartz, or even boro glass is safe used in the manor that is used for BHO. There are simply no studies on it, and besides, there have been various studies proving that marijuana is not fully safe. Fact is MJ is a drug and is not needed to live so it could have potential side effects. There are various reports online about quartz toxicity, stainless steel which some vapes use in their heat chambers is made of Chromium and Iron which are both toxic to a degree.
My point with this rant is that if studied enough, people will find something toxic or wrong with anything and it seems like everyone is so concerned about the various heating elements and Ti pads that they overlook the fact that marijuana or BHO might not be doing your body any favors.