Vapor pressure is how a compound behaves when it turns from a liquid into a gas which is what the lung organ does when someone exhales. It is this science that has allowed technology for alcohol breathalyzers to be developed as well as the potential for marijuana, specifically THC, breathalyzers to be developed.
Marijuana is now legal for recreational or medicinal use in at least 28 states and the District of Columbia. But driving under the influence of marijuana is illegal no matter which state you’re in. To enforce the law, authorities need a simple, rigorous roadside test for marijuana intoxication.
Although several companies are working to develop marijuana breathalyzers, testing a person’s breath for marijuana-derived compounds is far more complicated than testing for alcohol.
But scientists at the National Institute of Standards and Technology (NIST) have taken an important step toward that goal by measuring a fundamental physical property of the main psychoactive compound in marijuana, delta-9 tetrahydrocannabinol (THC). Specifically, they measured the vapor pressure of this compound—a measurement that, due to the compound’s chemical structure, is very difficult and has not been accomplished before. The results were published in Forensic Chemistry .
“Vapor pressure describes how a compound behaves when it transitions from a liquid to a gas,” said Tara Lovestead, a NIST chemical engineer and the lead author of the study. “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.”
Law enforcement agencies are interested in a breathalyzer because roadside collection of blood or urine would be impractical and invasive. Lovestead is not designing a breathalyzer herself. Rather, by measuring this fundamental physical property, she and her colleagues are laying the technical groundwork for manufacturers to develop accurate devices.
While this research is an important step forward, more research will still be needed to understand how breath levels of THC correlate with blood levels, and what blood levels of THC indicate that a person is too impaired to drive.
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