Professor Wang told Digital Trends:
"Detection of marijuana from blood or urine in a reference lab is not difficult, but it is difficult or impractical at the roadside. Most law enforcement officers are not authorized to take blood samples, while taking a urine sample on spot is extremely inconvenient. Detection of marijuana from saliva would bypass the trappings with either blood or urine samples."
The professor's technology works by using nanoparticles that fit THC (or reagent molecules) perfectly. When the results come back, the tester can see how many nanograms of THC there are for every milliliter of saliva. At the moment, Wang's technique is merely a proof of concept, though his lab plans to begin making the actual handheld device law enforcement can use.
Lawmakers are still trying to iron out marijuana-related driving rules in the US, since it's pretty tough to say if one is too high to drive. A recent study commissioned by the AAA Foundation for Traffic Safety, for instance, determined that THC in the blood might not be a useful indicator for how intoxicated a person is. Our body absorbs the chemical in a different way than it does alcohol. And since THC dissolves in fat rather than water, it can remain in fatty tissues such as the brain for a very long time. An NHTSA expert told New Scientist that devices like this still need to be tested more thoroughly in order to establish what THC measurements typically lead to bad driving.