A small-scale, preliminary study by researchers at the University of Colorado-Boulder suggests developing a breathalyzer test for cannabis users will be a challenge, if possible at all.

Published earlier this year in the Journal of Breath Research, the “proof of concept” study was the first step in a project between the university and the National Institute of Standards and Technology to develop a standardized practice for detecting cannabis impairment.

Police have been able to detect the blood alcohol concentration of drivers with breathalyzers for decades.

Something similar for cannabis has proved not to be 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,” Cinnamon Bidwell, a CU Boulder professor of Psychology and Neuroscience, said in a news release. “That suggests that this isn’t going to be easy, and a lot more careful research needs to be done to get it right.”

CU Boulder received a $600,000 grant from National Institute of Standards and Technology to continue its research.

Medical and recreational cannabis spending combined in the U.S. could hit $33.6 billion by the end of year, according to MJBizDaily — which tracks national cannabis sales.

In Colorado, cannabis sales — medicinal and recreational — peaked in 2021 at $2.2 billion. Sales have been dropping ever since. In the state’s July report, the latest available, cannabis sales in Colorado had reached $920 million. At the same point last year, sales had already topped $1 billion.

In 2014, Denver became the first city in the United States to permit regulated retail marijuana sales.

In the years since, recreational use has become legal in 23 states and Washington D.C., and in 38 states for medical use, according to the National Conference of State Legislatures.

Driving under the influence, though, remains illegal.

Impaired driving in the state accounts for one-in-three traffic deaths each year, according to the Colorado Department of Transportation. For the last five years, Colorado has had, on average, 47 cannabis-related fatalities in crashes across the state.

Drug testing methods to detect cannabis rely on blood, urine and hair samples, which cannot be conducted in the field.

The CU Boulder project is the latest to enter a burgeoning market of cannabis  research, including the University of Pittsburgh, and emerging breathalyzer prototypes.

Unlike alcohol, in which there is a correlation between blood concentration and impairment, more Delta-9-tetrahydrocannabinol, or THC, consumed from cannabis does not equate to greater intoxication.

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

“There is a large commercial enterprise saying that this is not only doable, but already exists,” Bidwell said. “But the science is just not there yet.”