What I need is a hallucinogen (or mixture of them), the presence of which in the atmosphere could cause somebody to have flashbacks.
The character was abused as a child, emotionally and physically (not sexually though), and therefore has some quite horrific memories. Hence the cave. (Come to think of it, he's also a chemistry graduate, so he ought to know. I only wish I could ask him...sigh.)
I read a lot of stuff about hallucinogens, and tried Googling 'inhaled hallucinogen memory flashback' and lots of variations on the same theme; it only succeeded in befuddling me more. Any help, please?