Thanks for the compliments guys!
And I just used a P100 particle filter. It's not volatile or anything, so no risk of fumes. But it did get over everything and stained my shoes a nice dark yellow. I'd say what it is, but that would make it pretty obvious which place it is :p.
I will say, my friend may or may not have gotten a small amount in his mouth while adjusting his filter. He said it tasted both metallic and salty. This should be enough for you to find it, if you're good with chemistry ;) .