The search for a reliable method of determining levels of cannabis impairment continues as another pathway is closed off. Researchers say that saliva tests are poor indicators of impairment—meaning THC breathalyzers might be as ineffective as traditional THC tests at indicating if a driver is impaired.

Present Methods Ineffective

Cannabis prohibitionists often list drugged driving among their top concerns when debating against adult-use cannabis legalization. The National Institute on Drug Abuse (NIDA) says that THC is the illicit compound most frequently found in the blood of drivers who have been involved in vehicle crashes. But THC tests can return positive weeks or even months after cannabis use—long after the psychoactive effects of the drug have worn off.

This has lead experts to determine that conventional tests for cannabis use like blood or urine analysis are poor methods for detecting cannabis impairment. Looking to capitalize on the need for accurate THC impairment tests as more states legalized adult-use cannabis, a number of entrepreneurs began developing different THC breathalyzer devices and are touting the new technology as the answer to the drugged driving problem.

How Breathalyzers (Are Supposed to) Work

Saliva analysis for drug testing has been around for many years, but the results can take time to come back. Breathalyzers analyze saliva and evaluate THC levels in minutes rather than hours or days.

“The fundamental challenge is that THC exists in breath in concentrations that are something like a billion times less than alcohol,” Hound Labs CEO Mike Lynn told reporters. Hound Labs worked to produce one of the first THC breathalyzers available on the market. “That means you need a breathalyzer that’s literally a billion times more sensitive if you’re going to use it for marijuana. It’s like identifying 25 or 30 specific grains of sand on a beach that’s well over a mile long.”

According to an October 2021 report in the journal Science Translational Medicine, the express probe for on-site cannabis inhalation (EPOCH) method of analyzing saliva is able to detect THC with high sensitivity in minutes. According to researchers, the half-life of THC in oral fluid was reported to be 1.4 hours, so this method should be more effective at determining current THC levels than the more typical blood or urine tests.

Does It Hold Up?

But some experts have questioned whether THC levels are even a good metric to judge impairment. Unlike alcohol, cannabis appears to have drastically different effects for different users, and researchers are only beginning to broach the topic of THC tolerance.

Last summer University of New Mexico researcher Sarah Stith told The Paper. that her team was beginning to study marijuana tolerance in medical cannabis patients. “There’s a central story here,” said Stith. “It seems that THC makes everything more intense, so at baseline, you’re going to get more symptom relief with higher THC, but you’re also looking at more negative side effects. What seems to be happening with tolerance—although it’s certainly not peer reviewed or published yet—is that it lessens both of those effects.”

If true, the influence of tolerance needs to be addressed when considering policies to detect impairment. Breathalizer technology, like fluid testing, can only determine the level of cannabinoids concentrated in a person’s system. None of these testing methods can determine how high a person actually is and how the drug is affecting their ability to drive.

A new study out of the University of Sydney in Australia that was recently published in the journal Neuroscience & Behavioral Reviews put this idea to the test by comparing data from studies in 28 publications involving the consumption of cannabis. “Higher blood THC concentrations were only weakly associated with increased impairment in occasional cannabis users while no significant relationship was detected in regular cannabis users,” said lead researcher Danielle McCartney.

According to the data, no significant relationship was found between blood THC concentration and driving performance for drivers who used cannabis on at least a weekly basis.

“Of course, this does not suggest there is no relationship between THC intoxication and driving impairment,” McCartney said. “It is showing us that using THC concentration in blood and saliva are inconsistent markers for such intoxication.”

The Future: Brain Scans

Just as one group of scientists seems to be shutting down the dream of answering the drugged driving problem with breathalyzers, another group says the real answer lies in noninvasive brain imaging technology.

According to The Harvard Gazette, researchers at Massachusetts General Hospital say they have been able to determine individual levels of cannabis intoxication through the use of functional near-infrared spectroscopy (fNIRS). The results were published in the journal of Neuropsychopharmacology.

The 169 participants of the study were given either edible cannabis or a placebo and received fNIRS brain scans before and after the dose. Participants who received the edible and reported feeling intoxicated were found to have more neural activity markers in the prefrontal cortex region of the brain than participants who reported low or no intoxication.

“Our research represents a novel direction for impairment testing in the field,” said lead author Jodi Gilman. “This is a critical issue because a ‘breathalyzer’ type of approach will not work for detecting cannabis impairment, which makes it very difficult to objectively assess impairment from THC during a traffic stop.”

Even if scanning driver brains proves to be the best method for cannabis impairment detection, there is no indication that the anyone is currently working to make a roadside version of the technology. For the time being, law enforcement agencies will have to rely on officers’ abilities to recognize the signs of impairment without the aid of any fancy gear. To this end, a number of agencies across the country are currently developing training programs to educate officers.