read this, it has a lot of interesting infomation about them
http://en.wikipedia.org/wiki/Vaporizer

"A NORML-MAPS study published May 2, 2003 found that the Volcano Vaporizer, a high-end vaporizer, was remarkably clean, its vapor consisting 95% of THC with traces of cannabinol (CBN), another cannabinoid. The remaining 5% consisted of small amounts of three other components: one suspected cannabinoid relative, one suspected PAH, and caryophyllene, a fragrant oil in cannabis and other plants."