Windows of impairment and detection

There is currently no reliable roadside test for cannabis impairment—while some experts believe we’re still a long way off, Hound Labs says the time is now

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Driving high is illegal, no matter what state you’re in. But proving someone is or isn’t under the influence of cannabis at the time of a traffic incident (or accident) is still a complicated proposition—for a number of reasons. Chief among them is the simple fact that THC affects almost every single user differently. 

“Unlike with alcohol, there is not a linear relationship between cannabis in someone’s system and impairment,” Jenny Lynn, the CMO and co-founder of Hound Labs, a cannabis breathalyzer company, says. 

Different people have different tolerance levels based on their frequency of use, she explains, and cannabis is metabolized either very rapidly or very slowly depending on how it’s consumed. Traces of THC can stay stored in fat and hair for weeks, and it can show up in blood samples days after consumption—even though the actual high only lasts a few hours. 

“In many states, employees have the legal right to use recreational cannabis,” Lynn says. “Someone could smoke at a barbecue on a Saturday, and risk testing positive [for THC in their blood] on Monday morning.” And their urine and hair would test positive for weeks. 

Lynn, along with her partner Dr. Mike Lynn, believe they may have developed a solution to that problem, though. The new Hound Labs cannabis breathalyzer, which is commercially launching next year, is as sensitive as a dog’s nose, she says; and it only detects cannabis on a person’s breath, within the impairment window of three to four hours. 

“The technology we created is one billion times more sensitive than an alcohol breathalyzer,”  Lynn says. 

However, Marilyn Huestis, a toxicologist and adjunct professor in the Department of Epidemiology and Preventive Medicine at University of Maryland, doesn’t buy it. She hasn’t seen Hound Labs’ research, but she asserts that chronic users can actually retain THC in their system after a month of total abstinence, and can still exhibit cognitive impairment for up to three weeks.

“We’ve proven that,” Huestis claims.

She describes one study in which chronic frequent cannabis users were kept in her “closed research unit” and deprived of weed for a full 30 days. Some of the study subjects left, she says, because they started “going into cannabis withdrawal.”

Those who stayed had their hair, blood, urine, sweat, cognitive function, psychomotor performance, and more tested regularly for the month. Over which time, Huestis says, cannabis never left their system. 

“When we did this work on chronic frequent cannabis users, I could still measure THC in low levels in the blood . . . 30 days after last use,” she says. “I want to be frank, I was surprised.” 

In the same study, they tested “critical tracking” and “divided attention”—two areas “that are very important for driving performance,” Huestis says. “We showed significant impairment in the chronic frequent cannabis users . . . at one week, two weeks, and three weeks after last access to the drug.”

Does that mean cannabis users should avoid driving for three weeks after each use? 

Not according to a comprehensive meta-analysis of 80 scientific studies published earlier this year in the journal Neuroscience & Biobehavioral Reviews. The analysis, titled “Determining the magnitude and duration of acute Δ 9-tetrahydrocannabinol (Δ 9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review,” concluded that the window of impairment for THC is somewhere between three and 10 hours. 

“Overall, our results confirm that Δ9-THC impairs aspects of driving performance and demonstrate that the magnitude and duration of this impairment depends on the dose provided, route of administration and frequency with which cannabis is used.” the study determined. 

Huestis agrees with that, at least. She just doesn’t believe that we have any “window of detection that matches closely with the window of impairment.”

But the Lynns, of Hound Labs, say that we do. They claim to have developed a cannabis breathalyzer that can detect picograms (measured in the parts per trillion) of THC on a person’s breath. And while it won’t definitively tell you how high a person is, it will tell you if they’ve used cannabis in the last few hours, and how many picograms of THC are still on their breath. This development could have big implications for states like Colorado, where driving high is a serious crime but there is no reliable method for testing. 

“One of the big tricks was figuring out how to capture enough breath to measure THC and doing it in a predictable and consistent way,” Lynn says. 

She explains that in order to determine if a person is under the influence of cannabis, they had to develop technology to determine when the person might have used it. Because THC degrades so quickly on the breath, and because it appears in such tiny amounts, Hound Labs had to develop an ultra-sensitive method for capturing breath. 

“We have done over 8,000 tests with humans who are smoking THC and we have refined our technology based on the results,” she says. When someone breathes into one of their breathalyzers, the breath is directed into two separate chambers: one that produces an immediate result, and another that can be saved for evidentiary purposes. 

“What a typical alcohol breathalyzer is doing is providing a reading. And then that reading is compared to research that others have done on the impairment level for alcohol,” Lynn says. “We function the same way: we are providing a reading, noting the presence of THC in the breath, and then that will be compared to the research that exists [on THC impairment].”

Huestis isn’t satisfied by that. 

“[Hound Labs] has been saying for more than a year that they have a product ready to go,” Huestis says. “What we scientists say is, ‘Great. Show us the data. Where are the data that show that you can both sensitively as well as specifically identify THC and what’s your window of detection?’”

Lynn simply points to a study published in Clinical Chemistry, titled “Correlation of Breath and Blood Δ9-Tetrahydrocannabinol Concentrations and Release Kinetics Following Controlled Administration of Smoked Cannabis.”

That study—which used the Hound Labs breathalyzer device in their experiments—concludes: 

“THC can be reliably detected throughout the presumed [three hour] impairment window following controlled administration of smoked cannabis. The findings support breath THC concentrations as representing a physiological process and are correlated to blood concentrations, albeit with a shorter window of detection.”

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