from Wiki, not necessarily the most reliable source out there, but better than what I could dig up in a public library.
"Few vaporizer studies/reviews have really addressed the quality of the vapor extracted and delivered. Most studies/reviews have tended to focus on the mode of usage of the vaporizers and not the quality of the vapor extracted and delivered. When one considers that the active compounds in Cannabis number at least 60-70 in quantity and that the aromatic terpinoids begin to vaporize at 126 °C, but the more bio-active
cannabidiol (CBD),
Cannabinol (CBN), and
delta-9-tetrahydrocannabinol (THC) do not vaporize until near their respective flash points: CBD 206.3 °C
[6], CBN 212.7 °C
[7], THC 149.3 °C
[8] then it becomes apparent that the only way to get a full spectrum vapor inhalation is to extract and deliver rapidly from a small sample at a time. Because most commercial vaporizers are slow in extraction and delivery, the vapor inhaled is first aromatic, but only minimally active; and then as the apparent temperature rises, the vapor becomes increasingly bio-active, but minimally aromatic as most of the aromatics already released. Only rapid extraction and delivery approaches such as the sequential Venturi vaporization chamber bowls with a hot air gun or heat wand enable inhalation of fuller spectrum vapor. By utilizing a sequential
Venturi effect, the actual level of condensation of the vapor can be greatly improved over hoses or balloon capture approaches because the vapor extracted and delivered is more condensed, without needing higher temperatures that tend to lead to hybrid smoke/vapor."