A US research team has determined that developing a dependable cannabis breathalyser will require "more than a few years" of work, after a study found no measurable difference in THC levels in people's breath before and after they consumed cannabis.
The researchers collected breath aerosol samples — tiny particles produced deep within the lungs during normal breathing — through a device known as an "impaction filter".
Twelve participants provided samples prior to cannabis use and again one hour after consuming THC.
The results across both collection points were "somewhat similar", with post-consumption THC levels showing little variation from the baseline samples taken before use.
The study, carried out jointly by the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), was said by researchers to highlight just how difficult it is to create reliable breathalyser technology for cannabis.
NIST supervisory chemical engineer and study author Tara Lovestead said: "We were hoping to see if there was a difference in the amount of THC that we could collect in a participant's breath before and after they used a legal market cannabis product. And we expected to see higher THC concentrations in samples that we know were taken one hour after they used."
The results, however, showed no such variation, "adding to the complexity" of ongoing efforts to produce a test that works reliably.
"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," Lovestead said.
"It means that our hypothesis that it is really hard to develop a cannabis breathalyser is correct. This is hard, and it's going to take more than a few years of research."
She said the aim is to build the infrastructure capable of identifying recent cannabis use or potential impairment, and to "understand the physical and chemical properties of cannabis compounds so we can target them in breath collection devices".
The study's co-author and NIST materials research engineer Kavita Jeerage said: "A lot more research is needed to show that a cannabis breathalyser can produce useful results.
"A breathalyser test can have a huge impact on a person's life, so people should have confidence that the results are accurate."
NIST noted that the research remains "in its infancy", a situation it attributed to cannabis still being classified as a schedule one controlled drug in the US.
