A decade after Colorado and Washington became the first U.S. states to legalize recreational cannabis, law enforcement officials still face a perplexing challenge: How can they tell if someone recently smoked a joint or ate a gummy and is too impaired to drive?

With alcohol, the answer is relatively straight forward: Since 1931, with the advent of the “drunkometer” and 1954 with the invention of the more portable and refined breathalyzer, police have measured ethanol (a.k.a. ethyl alcohol) in breath, as a pretty reliable roadside measure of intoxication.

With cannabis, new research shows, it’s not so simple.

“In our pilot research, we found that in regular cannabis users, their breath around an hour after use is not looking a whole lot different than their baseline measure on days that they haven’t used at all,” said Cinnamon Bidwell, assistant professor in the Department of Psychology and Neuroscience at CU Boulder. “That suggests this isn’t going to be easy, and a lot more careful research needs to be done to get it right.”

The small “proof of concept” study, published in the Journal of Breath Research in May, marked the first step in a unique collaboration between scientists and engineers at the National Institute of Standards and Technology (NIST) and researchers at CU Boulder.

Their goal: to develop reliable, standardized industry protocols for detecting cannabis impairment via breath—or at least determine whether it’s possible at all.

NIST chemical engineers Tara Lovestead and Kavita Jeerage pioneered the research with $2 million in grants from the U.S. Department of Justice’s National Institute of Justice. They partnered with Bidwell, a leading cannabis researcher. In September, NIST granted CU Boulder $600,000 dollars to help continue the research.

The project comes as a handful of U.S. startups have already begun marketing cannabis breathalyzer prototypes to law enforcement, employers and cannabis users. 

“There is a large commercial enterprise saying this is not only doable, but already exists. But the science is just not there yet,” said Bidwell, noting that only about half-dozen studies exist, some of them industry sponsored.

‘Needle in a haystack’

While still imperfect, blood alcohol breathalyzers have nearly a century of science behind them, including universal standards for collecting and analyzing breath and calibrating devices. None of that infrastructure exists for the cannabis breathalyzer industry.

“Alcohol breathalyzers are built from studies in tens of thousands of individuals. Cannabis breathalyzer research is in the 1930–50s stage of development,” said Jeerage, who did postdoctorate work in chemical engineering at CU Boulder.

The chemistry of alcohol and cannabis is also different: Unlike ethanol, which is exhaled in copious amounts in a gaseous vapor, the main psychoactive ingredient in cannabis (tetrahydrocannabinol or THC) is carried in aerosol particles, tiny molecules surrounded by fluid from the deep lungs. 

NIST research has shown that individuals exhale 1 million times more ethanol with a single breath than cannabis users exhale of THC in 12 breaths.

“With THC, it’s like looking for a needle in a haystack,” said Lovestead, who earned her PhD in chemical engineering from CU Boulder.

THC also lingers in fatty tissues, showing up in blood and fluids for weeks and making it hard to tell whether a person used an hour ago or days ago. (While some breathalyzer marketers report that THC clears breath more quickly, making this method an ideal way to detect recent use, there isn’t enough data to state this conclusively, said Bidwell.)

With alcohol, there is a clear correlation between blood concentration and impairment. But more THC in blood does not necessarily equate to more intoxication, research has shown.

To complicate matters further, some people use cannabis regularly to treat medical conditions.

“I get emails all the time from lawyers trying to defend people whose blood draws indicated they have THC in their system but who insist they did not use recently. It’s a huge problem and a matter of social justice,” said Bidwell, director of the Center for Health and Neuroscience Genes and Environment (CUChange). 

Not ready for prime time

For the study, the researchers used a simple, filter-based device to collect breath samples from 18 volunteers. Participants were asked not to use cannabis the day before giving their baseline samples or prior to the experimental session. All were asked to purchase a specific strain of cannabis flower containing 25% THC. 

One day, CUChange’s mobile laboratory, a white Sprinter van, pulled up in the participant’s driveway. (Because cannabis is illegal at the federal level, researchers are not allowed to handle or administer it, so Bidwell’s team developed the mobile lab system to evaluate the effects of legal market cannabis use.)

After giving blood and breath samples in the van, participants were instructed to go into their house, use cannabis and return to the van for cognitive and psychological tests of intoxication and to breathe into the collection device. 

The samples were shipped to NIST for processing and analysis via a technology called liquid chromatography with tandem mass spectrometry.

Surprisingly, only eight participants showed the anticipated increase in THC after cannabis use. In three post-use breath samples THC was not detected at all. In several other samples, THC levels in breath post-use were similar to or lower than baseline levels.

“Our results do not support the idea that detecting THC in breath as a single measurement could reliably indicate recent cannabis use,” the authors concluded.

That is not to say that it’s not possible, they added.

In future research, the NIST team hopes to get a better understanding of the chemical properties of THC, experiment with other methods of detecting it in breath, and develop standard reference materials that labs could use to calibrate their equipment.

The NIST-CU Boulder team plans to launch their next study soon, involving at least 40 participants and more than a thousand breath samples from various time points to determine how quickly THC in breath peaks and dissipates. They also hope to learn whether comparing breath THC measurements over time could be useful to identify or rule out recent use.

“How can we take what we know from science and develop a reliable tool that can be used both to protect people who are using safely and draw a line for unsafe use?” said Bidwell. “That’s the ultimate goal.”