The are several factors which determine the quality and effectiveness of the vapor that's inhaled.
When vaporizing cannabis the aromatic components with a minimal bioactivity will turn into vapor at temperatures around 127°C (261°F) whereas the cannabinoids with a higher bioactivity are released at temperatures around 200°C (392°F). Research has determined that the efficiency of vaporizers is highest around 226°C (439°F) and only half as efficient between 150 (302) and 180°C (356°F) depending on the plant material used.
Type and quality of the vaporizer
The wide range of test results suggests that the type of vaporizer is one of the most important factors determining the amount of active components that are released, as well as the amount of harmful byproducts that might be produced.
Density of the product that is to be vaporized
When a product is grinded, the hot air can flow through the material rather than around it, releasing more of its components.
Type and potency of the herb that's used, its weight, humidity and amount of volatile oils
There are many types of cannabis (the main division being C. sativa and C. indica), each having a somewhat different ratio of cannabinoids and thus a different medical and psychoactive effect. Concentrations of THC may vary significantly from one plant to another depending on the how, when and where it was grown, harvested etc. The same applies to all other herbs one may wish to vaporize.
The type of preparation, for example leaves, crude flower tops, hashish, hash oil, pure THC etc.
Certain parts of the plant have very high concentrations of THC whereas others contain very little. For example, if a preparation contains stems and leaves it's going to be less potent than a preparation made from the flower tops only.
The purest preparations have the highest level of efficiency: 54% for pure THC preparations versus 29% for plant material (weed) with a THC-percentage of 12%.
The amount of time the vapor is stored (for example in a balloon) before it's inhaled
The longer it takes before the vapor is inhaled, the more THC or other active ingredients will settle on the inside of the vaporizer or balloon. This loss may be negligible if it's only a couple of minutes, but can be up to 50% if the vapor is stored for more than 90 minutes.
For how long the vapor is kept in the lungs, and the amount of unused vapor that is exhaled again
The research done by the University of Leiden determined that between 30 and 40% of inhaled THC is not absorbed by the lungs but exhaled unused. There were no significant differences between individuals in this regard.