DUI testing for marijuana has always been a hazy topic, with little reliable methodology as to how to actually determine impairment — since, “under the influence” of cannabis does not necessarily mean too high to drive. Now science has come one step closer to nailing down a reliable cannabis breathalyzer.
Several companies have been trying to develop marijuana breathalyzers, but by and large, they’ve had to confront the reality that testing a driver’s breath for cannabinoids is more complex than testing for alcohol. Now, however, scientists at the National Institute of Standards and Technology (NIST) have gotten one step closer to to measuring the vapor pressure of THC, the main psychoactive ingredient in cannabis.
Previously, researchers have been unable to measure THC’s vapor pressure because of its chemical structure — but finally being able to do this is a key step in building a reliable cannabis breathalyzer.
“Vapor pressure describes how a compound behaves when it transition from a liquid to a gas,” says Tara Lovestead, lead author of the study and a chemical engineer at NIST. “That’s what happens in your lungs when a molecule leaves the blood to be exhaled in your breath. So if you want to accurately measure blood levels based on breath, you need to know the vapor pressure.”
Vapor pressure is important because it indicates how far a molecule has gotten. Molecules with high vapor pressure, such as ethyl alcohol, will escape as a gas. That’s why you can smell alcohol like whiskey so easily. Unlike ethyl alcohol molecules, which are small and simple, THC molecules are bigger and more complex. They also often stick to one another, which causes the vapor pressure of THC to be lower. Often, it’s so low that it can’t be measured in full — to do so, the THC would need to be in a closed container while the pressure equalizes.
However, the NIST researchers found a way around that, using PLOT-cryo (porous layer open tubular cryogenic adsorption) technology. “PLOT-cryo is an extremely sensitive technique for capturing and analyzing things in the vapor phase,” says Tom Bruno, study co-author and NIST research chemist. “It was a natural candidate for this type of problem.”
The PLOT-cryo technology is so sensitive that it can identify and analyze even a few small THC molecules that escape as vapor. In the experiment, the NIST scientists measured the mass of pure THC molecules recovered from vapor, in order to calculate the vapor pressure.
“Fundamental measurements are the basis of standardization,” says Bruno. “We’re laying the foundation for the reliable systems of the future.”
However, despite reliability in measuring the amount of THC one has in their system, there’s still no standard for what quantity determines impairment. Everyone reacts to cannabis differently: the amount of weed that might cause one person to drive dangerously might hardly affect another user with a greater tolerance.
A breathalyzer still does not give a true reading on if someone is impaired. “Ultimately, if law enforcement’s priority is to better identify drivers who may be under the influence of cannabis, then the appropriate response is to identify and incorporate specific measures that accurately distinguish those cannabis-influenced drivers from those who are not, rather than relying on the detection of compounds that are not consistently associated with behavioral impairment,” says Paul Armentano of the National Organization for the Reform of Marijuana Laws. Other methods of determining impairment might included standardized field sobriety tests, and performance technology like My Canary, which tests cognitive impairment.
