My b/f and I discovered this same thing one night when we were younger, putting our mouth right onto the pillows and exhaling right into them to avoid crazy pot smell in his parents basement. It was a nasty brown/resin stain left on the pillow, and yes, this shit IS in your lungs. And Chowder, no matter how much you might want to beleive otherwise, pot smoke is not totally harmless.
you can reduce this nasty build up shit by using the volcano. It elimintates 49 of the 50 present carcinogens found in marijuana. That's pretty fucking good. It;s the only vapo out there that does. It cost us an arm and a leg, but totally worth every penny.