Following up on a previous post about breath or blood tests for drivers suspected of marijuana DWI (also known as “drugged driving”), here is an NPR item from last year about difficulties in testing drivers for marijuana impairment. Unlike alcohol where breath and blood tests have long been available, and reasonably accurate, accurately testing either blood or breath for marijuana levels is not possible with current technology. In addition, marijuana blood levels are inherently different from alcohol because THC (the main psychoactive chemical in marijuana) is fat-soluble. THC behaves very differently from alcohol in the body. The above-linked NPR article quotes Professor Margaret Haney of Columbia University: “[T]hat changes everything. ‛It’s really difficult to document drugged driving in a relevant way . . . [because of] the simple fact that THC is fat soluble. That makes it absorbed in a very different way and much more difficult to relate behavior to, say, [blood] levels of THC or develop a breathalyzer.’ ”
The fat solubility of THC means that it can rapidly leave the blood stream and enter body fat, including the brain which is a very fatty organ. Because of this, an occasional marijuana user might see a rapid drop in their blood THC even though they are still high enough to be considered impaired. A test measuring THC levels in their blood would not accurately reflect their level impairment. On the other hand, regular users of marijuana have the opposite problem: their body fat may store THC for weeks after they have used marijuana and this THC will leach into their blood stream. A blood test given to a regular user of marijuana may thus show relatively high levels of THC even though they are not impaired by marijuana at all.
Marijuana edibles present another issue. According to Professor Haney: “With oral THC, it takes several hours for [blood THC] to peak, but it remains very low compared to the smoked route, even though they’re very high. It’s a hundredfold difference.”
These factors make testing people suspected of DWI for marijuana very difficult. This article notes that lawmakers want a single number – like the 0.08 BAC presumption for alcohol intoxication – to define marijuana intoxication for DWI purposes. But, for the reasons mentioned, a person’s level of impairment is not closely related to their level of blood THC. Occasional users may be impaired at a much lower THC level – and the THC level in their blood may fall below the legal cutoff very rapidly even though they still show signs of impairment. But a regular marijuana user may have blood levels above the legal limit when not impaired at all and even weeks after they have consumed marijuana. All of these issues make marijuana DWIs inherently hard to prove.