Roadside breath tests are a staple of policing. Whenever officers suspect drivers of being drunk, they ask them to blow into a tube. This tube leads to a handheld device popularly known as a Breathalyzer that analyses the breath sample and outputs an estimate of the driver’s blood-alcohol level. Though not infallible, Breathalyzers are quick and accurate enough to help get drunks off the road before they harm themselves and others.
But what if the driver hasn’t been drinking? What if, instead, they’ve been smoking some fine, fine weed?
Like alcohol, cannabis is legal in many jurisdictions. Like alcohol, it can render users unfit to drive for several hours, long after their last dance with Mary Jane is but a hazy, munchie-filled memory. So, is there a Breathalyzer for cannabis?
The answer, so far, is no – but not for lack of trying. The latest effort comes from researchers at the US National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder. Led by Tara Lovestead and Kavita M Jeerage of NIST’s applied chemical and materials division, the team set out to measure the amount of tetrahydrocannabinolic acid (THC, the active ingredient in cannabis) in users’ breath, and to monitor how it changes over time.
Barriers to a cannabis breath test
Such studies are challenging for three reasons. One is that, unlike alcohol, relatively little THC shows up directly in a user’s breath. Instead, a cannabis Breathalyzer – let’s call it a Reefalyzer – would have to detect tiny amounts of THC in particles that form within the lungs and are then exhaled.
A further challenge is that THC can persist in the bodies of habitual users for weeks after any high has worn off. This means that a yes/no answer isn’t good enough: a practical Reefalyzer would have to distinguish between intoxicating and non-intoxicating levels of THC.
Finally, although Colorado is one of several US states to allow marijuana use for recreational as well as medical purposes, the drug remains illegal at a federal level. As a result, the federally-employed NIST researchers could not handle the drug they were trying to study.
A “federally compliant mobile laboratory”
In a paper published in the Journal of Breath Research, Lovestead and colleagues outline the ingenious way they overcame one of these challenges. To collect breath samples from cannabis users in a controlled way, the team developed a “federally compliant mobile laboratory” that met users at their place of residence. There, the researchers collected breath and blood samples before and after users returned to their homes to smoke high-THC cannabis from a local dispensary. Finally, the researchers used laboratory instruments to measure the amount of THC in the users’ breath.
So far, so good. From a Reefalyzer perspective, though, the results were disappointing. “We expected to see higher THC concentrations in the breath samples collected an hour after people used,” Lovestead told the NIST press office. In fact, the researchers found that pre-use and post-use THC levels spanned a similar range. “In many cases, we would not have been able to tell whether the person smoked within the last hour based on the concentration of THC in their breath,” she concluded.
Every breath we take
The team identified a few possible avenues for future experiments. One possibility would be to measure the flow rate of breath samples, to help identify outliers and investigate whether flow plays a role in aerosol capture. Another would be to perform tests on THC-spiked aerosols generated in a laboratory, rather than relying solely on human subjects.
The bottom line, though, is that the researchers say their results “do not support the idea that detecting THC in breath as a single measurement could reliably indicate recent cannabis use”. So if you’re waiting for Reefalyzers to appear alongside Breathalyzers in your favourite TV cop show – well, don’t hold your breath.
