All current, peer-reviewed, validated studies say that people are generally under the influence at 5 nanograms(ng) of delta-9 THC per milliliter of whole blood (mL).
A person with 5 ng/mL delta-9 is 6.6 times more likely to cause a crash than a person with no detectable delta-9.
At 5 ng/mL, most users will exhibit objective signs of impairment, including impairment of psychomotor skills. Highly automated behaviors, those not requiring conscious monitoring (i.e., driving), are most affected as well as critical tracking, ability to complete divided attention tasks, and complex tasks. Signs of impairment begin between 1-2.5 ng/mL. Global impairment is observed between 2.5-5 ng/mL.
Chronic users may partially compensate for some impairing effects of THC, but only for short periods of time and for simple tasks, not for unexpected or high-demand circumstances. Heavy users may develop partial tolerance for some of the impairing effects of THC but insufficient for the complex tasks required to drive.
When smoked, peak delta-9 THC levels are reached while actively smoking and range from 25-100s of ng/mL. This level drops to less than 5 ng/mL within three hours after smoking. Heavy users may have residual levels of less than 2-3 ng/mL within twelve hours after smoking. Very low levels of THC (less than 1 ng/mL) may be detected in heavy users for weeks following smoking.