The studies that have been done when we're looking at actual driving studies, or we're looking at laboratory studies of the impairment effects of cannabis demonstrate that the impairment of driving skills lasts approximately two to four hours after the ingestion of cannabis. At the very beginning of the testing procedure, you may have a THC level of 25 to 50 nanograms per millilitre of blood. That level falls quickly. In approximately 90% of the population, within 160 minutes, the level will be below 2.5 nanograms per millilitre. People who are heavy consumers of cannabis may have residual blood alcohol levels, after not having smoked for several days, of two or three nanograms per millilitre in their blood.
Trying to pick a level, and saying that is the impairment level, is an educated exercise in trying to come up with that level. Some states, such as Colorado and Washington, have picked five nanograms per millilitre.