A vapor-capturing technique called PLOT-cryo shows promise in measuring blood levels of THC, which is a large and complex molecule unlike ethanol, the main compound in alcohol.

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As more states have approved the use of recreational marijuana, researchers continue to consider what impacts being stoned will have on daily activities. A particular concern is people driving while high, which may lead to an increased risk of car crashes. In order to quickly and conveniently check driver impairment, a race has begun to see who can develop an effective and reliable marijuana breathalyzer.
 
New research, released by scientists at the National Institute of Standards and Technology (NIST) and published in Forensic Chemistry, may help companies in the effort. While it doesn’t lay out how an accurate breathalyzer might be built, it does lay the scientific foundation for one. The key lies in thermodynamics, specifically in vapor pressure.
 
Tetrahydrocannabinol, also called THC, is the main psychoactive compound in marijuana that causes the effect of being high. It works by attaching to cannabinoid brain receptors and releasing the chemical dopamine, which can produce feelings like euphoria, hallucinations, and a change in thinking.
 
Vapor pressure is a fundamental measurement of chemistry, defined as the pressure exerted by a vapor over liquid at equilibrium. It is the amount of pressure in a closed container when a liquid is evaporating and reabsorbing at the same rate.  
 
The tricky thing about THC, however, is that it is a large and complex molecule unlike ethanol, the main compound in alcohol, which is relatively small and simple. According to Tom Bruno, a NIST research chemist and co-author of the new paper, the vapor pressure of the THC compound is “very, very low,” which makes it difficult to measure.

Credit: Kelly Irvine/NIST

Bruno, along with fellow NIST researcher and lead author Tara Lovestead, found a way to skirt around this issue by measuring THC vapor pressure with a vapor-capturing technique called PLOT-cryo. It works by trapping vapors in a solid substance that coats the inside of a small tube. Because the tube is so small, Bruno said, the process is very efficient.
 
“You don’t have to collect a lot to get an analyzable result,” he remarked.
 
The PLOT-cryo device works like a very sensitive nose. It can sniff out different compounds from the air, and has been previously used to identify explosives as well as decaying flesh.
 
Knowing the vapor pressure of THC, Lovestead told Seeker, is going to help scientists understand “how much THC you would expect to leave the blood and go into the exhaled breath.”
 
But it is still too soon to know how these findings will apply to an actual breathalyzer product. Hound Labs, a company based in Oakland, California, recently made headlines by raising more than $8 million to create a marijuana breathalyzer.
 
There are many complications to developing such a device, especially one that tests specifically for THC. One main issue is that the levels of THC in the blood don’t necessarily correlate to someone’s levels of impairment.
 
“What matters is not the THC,” said Daniele Piomelli, a professor at the UC Irvine School of Medicine and an expert on cannabis. “What matters is if you are intoxicated.”
 
Piomelli pointed out that experienced marijuana users can build up a tolerance to THC, meaning that they may have high levels of THC in the blood but little actual functional impairment. On the other hand, novice users may have low levels of THC in the blood but still be extremely intoxicated. Piomelli suggested to Seeker that a better test for impairment is the classic behavioral field sobriety test.
 
Other experts disagree. Nick Morrow, a cannabis DUI expert and retired deputy sheriff for the Los Angeles County Sherriff Department, said that the only reliable test for marijuana right now is a blood test that can be time consuming and expensive, and therefore impractical at checkpoints in the field.
 
“The field sobriety testing process was built and designed around the alcohol model,” Morrow told Seeker. The problem with cannabis DUIs, he said, is that “you’re supposed to make an educated judgment.”
 
After 30 years working in law enforcement, Morrow said, “I don’t have confidence in the field sobriety tests.”
 
Marijuana sobriety tests are also complicated by the fact that, unlike alcohol, there is no set level of impairment. All 50 states have a set limit of 0.08 percent blood alcohol concentration —anything higher, and you’ll get a DUI. But in the case of marijuana, the laws vary state by state. In Colorado, for example, you can get a DUI if five nanograms of active THC are detected in the blood. In Nevada, the limit is two nanograms.
 
It also isn’t known how marijuana affects the rate of vehicle accidents. A study commissioned by AAA suggested that driving with a noisy child in the back of the car leads to the same amount of risk as driving while high. And it’s less risky than driving drunk or while talking on a hands-free cell phone.
 
Though they expect that their findings will one day help create an accurate breathalyzer, both Lovestead and Bruno are quick to acknowledge the limitations of their research. There are many things that still need to be understood: how THC causes impairment, how tolerance is created, and how a breathalyzer might work on people who use marijuana for medical reasons like chronic pain.
 
 “We’re at the very beginning of researching cannabis,” said Lovestead, “and there are so many questions that need to be answered before we can make informed decisions.”