A Story about Stoned Driving, Breathalyzers, and Vapor Pressures
The mushroom is growing. Cannabis is a household word. Millions of people are proudly holding up their med cards, perhaps, hopefully, with a revitalized existence from their use of Cannabis sativa. This isn’t just an Israeli, or Uruguayan, or Canadian, or American industry. Cannabis is globalizing; people are mobilizing. And with that spread of legality will likely come opposing technological advances looking to recapture lost revenue from the waning incarceration of citizens for possession and sales of cannabis.
Much has been made of “stoned” driving. The University of Toronto studied it in 1999. So did the United States Department of Transportation in 1993, concluding that “Of the many psychotropic drugs, licit and illicit, that are available and used by people who subsequently drive, [cannabis] may well be among the least harmful.”  A 2019 study at Kansas State University came to a similar conclusion. These studies provide very different conclusions than what mass media portrays, and of course, there will be studies that reach the polar opposite conclusion.
Not all that long ago, I was sent a press release that reported that “Americans Overwhelmingly Support Marijuana Roadside Sobriety Test.” Interestingly, the release was submitted by an insurance agency. It said that 77% of Americans want their policymakers to “work together to address the dangers of marijuana impairment to keep roads safe,” so like anyone reading any survey should do, I immediately looked for their “n,” or sample size. It was 835. So, 835 out of 327,200,000 people represent America? That’s 0.00026% of the population.
What about people driving under the influence of legal, prescribed pharmaceuticals? A 2017 study showed that out of 7,405 drivers, nearly 20% of them had ingested a medication that could impair their driving.  Are researchers working on rapid methods to determine if someone is potentially driving on Clonazepam, for example, a drug that can cause drowsiness, fatigue, and dizziness?
For law enforcement to be able to turn to breathalyzer technology, the molecules they want to measure need to be readily promoted into a vapor state like ethanol. Or, alternatively, non-volatiles can be measured in expelled breath with technology like SensAbues. With cannabis, delta-9-tetrahydrocannabinol (THC) is obviously the intended target of law enforcement. The National Institute of Standards and Technology (NIST) endeavored a research project to evaluate whether they could quantify the vapor pressures of THC (≥ 98 %) and ≥ 99 % cannabidiol (CBD) at different temperatures. 
The vapor pressure of a substance relates how likely it generates a vapor at specific environmental conditions. Substances with high vapor pressures at ambient conditions are labeled as volatile. Terpenes are volatile, which is why you can readily imbibe the pleasing bouquets of flowers, or spices, or hops, or cannabis. One study measured the vapor pressures of terpenes and terpenoids at 25°C/77°F.  (Remember – terpenes contain carbon & hydrogen; terpenoids contain another element like oxygen). The vapor pressures of the terpenes ranged from 100 to 550 Pa; terpenoids ranged from 1 to 130 Pa.  Clonezapam has a vapor pressure of 0.0000000000732 Pa at 25°C/77°F. Ethanol’s vapor pressure, however, is 12,400 Pa, which demonstrates the dichotomous difference between creating breathalyzer technology for the two substances.
The NIST researchers didn’t measure vapor pressures of THC or CBD at 25°C/77°F. They did calculate what the expected vapor pressures would be, however. The predicted vapor pressures of THC were calculated to be 0.0000257 Pa at 25°C (77°F) and 0.0000487 Pa at 30°C (86°F). Those calculated for CBD were even lower – 0.00000273 Pa at 25°C (77°F) and 0.00000616 Pa at 30°C (86°F), although a discrepancy in the author’s discussion suggests 0.0000320 Pa for CBD at 25°C.
Therefore, the lowest vapor pressure reported for the terpenoids, 1 Pa, is nearly 38,000 times higher than the predicted vapor pressure for the THC standard at 25°C. In fact, the researchers didn’t record an experimental vapor pressure for THC until the temperature was a balmy 61°C (142°F), and the value they recorded was 0.00193 Pa. 
So, regarding who flies higher at livable conditions? Why, terpenes of course!
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