Colorado has recently added a new section to its DUI laws that sets legal limits on marijuana levels in the bloodstream. Under the new DUI laws, which took effect in May of 2013, a driver is presumed to be under the influence of marijuana, if at the time of driving the driver’s blood contained 5 nanograms or more of active THC (delta 9-tetrahydrocannabinol) per milliliter of blood.
There appears to be no readily accepted consensus on the exact amount of marijuana a driver must consume before he or she is considered to be under the influence. This is due impart to the fact that frequent smokers can build up a tolerance to THC and THC is absorbed differently into the blood stream than alcohol. Some critiques of Colorado’s new law argue that the 5 nanogram threshold is too low because medical marijuana users always have some level of THC in their blood. Remember, a nanogram is one billionth of a gram.
Experimental studies evaluating the impact of the active THC contained in marijuana on driving skills, demonstrates that marijuana use can impair the physical and mental skills need to operate a motor vehicle. THC is rapidly absorbed into the blood stream after smoking and impairment levels are at their highest during the acute phase, which last up to an hour, after which the level of impairment begins to subside rapidly. Recent scientific and medical studies have indicated that driver’s with active THC concentrations in the whole blood of less than 5 nanograms per milliliter (5ng/mL) have no greater crash risk than that of drug free drivers. The risk of a crash related accident begins to exceed that of a sober driver when active THC concentration levels reach 5 – 10 ng/mL in the whole blood. See, Grotenhermen et al. Developing Science-Based Per Se Limits for Driving Under the Influence of Cannabis: Findings and Recommendations by an Expert Panel. Sept. 2005.
There have been several studies that have compared the degree of impairment caused by marijuana with the impairment cause by alcohol. Comparison of the level of impairment of both the physical and mental driving skills need to operate a vehicle indicate that a THC concentration of about 4ng/mL in blood serum, from either smoking or ingesting marijuana, is associated with the same overall level of impairment as a blood alcohol content (BAC) of 0.04. Thus, the level of impairment produced by a BAC of 0.08 corresponds similarly to a THC concentration of 9-10 ng/mL in blood serum. See, Grotenhermen et al. Developing Science-Based Per Se Limits for Driving Under the Influence of Cannabis: Findings and Recommendations by an Expert Panel. Sept. 2005.