Indeterminacy of cannabis impairment and ∆9-tetrahydrocannabinol (∆9-THC) levels in blood and breath

Previous investigators have found no clear relationship between specific blood concentrations of ∆9-tetrahydrocannabinol (∆9-THC) and impairment, and thus no scientific justification for use of legal “per se” ∆9-THC blood concentration limits. Analyzing blood from 30 subjects showed ∆9-THC concentrations that exceeded 5 ng/mL in 16 of the 30 subjects following a 12-h period of abstinence in the absence of any impairment. In blood and exhaled breath samples collected from a group of 34 subjects at baseline prior to smoking, increasing breath ∆9-THC levels were correlated with increasing blood levels (P < 0.0001) in the absence of impairment, suggesting that single measurements of ∆9-THC in breath, as in blood, are not related to impairment. When post-smoking duration of impairment was compared to baseline ∆9-THC blood concentrations, subjects with the highest baseline ∆9-THC levels tended to have the shortest duration of impairment.