Driving High: The Search for a Pot Breathalyzer

Before pot was legal in states across the U.S., determining how to handle stoned drivers was simple: possession or any levels of THC in the body was illegal, case closed. But as marijuana becomes legal in more jurisdictions, drug-impaired driving is becoming a worry to more and more legislators. And while developers and researchers are hoping that a THC breathalyzer may be the answer, the science is complicated. And as with most parts of the cannabis industry, getting people to agree on how it should be implemented is another struggle altogether. 

So far, the way that some states have tried to account for drug impaired driving is by creating what are known as “per se” laws – laws which designate a threshold of what level of THC is allowed in a driver’s system in order to legally drive, as measured through bodily fluids. But the trouble with per se laws is that the levels detected in blood or saliva may not necessarily detect impairment; they simply note THC’s presence in the body.

THC breathalyzers, on the other hand, could be the most reliable way to determine impairment and recent use. But while the technology for alcohol breathalyzers has been established for decades, it’s not as simple as replicating the technology for THC detection because of the differences in how THC and alcohol are metabolized.

Marilyn Huestis is the recently retired chief of chemistry and drug metabolism at National Institute on Drug Abuse’s Intramural Research Program. Her perspective on drug impairment is formed by 23 years of research on the effects of cannabis in human drug studies. In one of Huestis’ controlled studies, they administered cannabis to occasional users and daily users, and found that in daily marijuana users THC could be detected in breath for up to four hours, and in other, less frequent cannabis users for up to two hours. “We found that if you found it in breath it definitely meant recent use. That’s why it’s valuable,” says Huestis.

In breath, researchers found that they could measure THC in picograms (parts per trillion) – that’s 1,000 times smaller than the nanograms measured in fluids. “A picogram is a trillionth of a gram,” says Joe Heanue, CEO and co-founder of Triple Ring Technologies, a strategic partner of Hound Labs, a THC Breathalyzer developer out of California. “The way to think about that is a raisin weighs about a gram, so picture taking a raisin and cutting it in a trillion parts and trying to detect one of them… That gives you an idea of the kind of challenge we’re facing in these devices.”

Tara Lovestead, a research chemical engineer for the National Institute of Standards and Technology, has been working on vapor characterization and trace vapor detection for a decade. She says that while law enforcement is eager to have an analogous device to the alcohol breathalyzer for determining cannabis intoxication, THC has a much lower vapor pressure than ethanol. That means it’s much more challenging to detect in breath than alcohol.

“Picture cutting a raisin into a trillion parts and trying to detect one of them” says one scientist. 

“Not only is the vapor pressure of THC really low, it is predicted to be approximately 100 million times less than ethanol’s vapor pressure,” says Lovestead. “Thus, a breathalyzer designed to detect THC will have to be extremely sensitive, able to capture a needle in a haystack.”

Current THC detection methods include fluid samples, like blood and oral fluid. In addition to being more invasive than breath testing, they pose other challenges. “Alcohol distributes evenly throughout the body and in the water that’s present in our blood.” says Huestis. THC, on the other hand, has to be carried around the blood by transport proteins. That means that THC concentrations in blood, urine and oral fluids can remain for long periods, and aren’t necessarily a benchmark for saying whether someone is high. This is particularly true for daily users. In studies, even when chronic, frequent users had abstained for study purposes, THC remained in their blood for weeks.

But the risk isn’t just that someone who is not impaired could be convicted based on fluid detection levels; there’s also a risk that impaired drivers might be missed. That’s because even though THC remains in the blood for long periods, THC levels drop very rapidly after use. Huestis says that toxicologists have long known this, but her research team set about to determine exactly how quickly those levels dropped. Using human subjects and a driving simulator, researchers looked at not only how drivers performed while stoned, but also how the THC levels in their blood changed over time.

According to Huestis, 30 minutes following the driving test, the THC concentration in subjects was 74 percent lower than it was at the time of driving, and at 1.4 hours it was 90 percent lower. In the U.S., she says, the typical time to get a blood sample for suspected impairment after either a traffic stop or a crash is between 1.4 and four hours.

That’s why Huestis thinks per-se laws are inadequate. “It’s a terrible number because, if you’re an occasional smoker who doesn’t have any tolerance, 5 nanograms [a common legal threshold for THC] is going to be below that level in only two and a half hours. And the occasional user is going to still be very impaired at two and a half hours.”

Policy makers constantly press Huestis for a number – a content level, like a blood alcohol level – that can definitively say that someone is impaired. “Now that we have done so much research on chronic, frequent cannabis users, I tell them, there is no one number that will clearly distinguish between impaired and non-impaired people,” she says.

Mostly, campaigns to educate the public about drug impaired driving speak to the recreational pot user. Posters hang in dispensaries that warn users about edibles’ delayed reaction, and urging people to plan a ride before they’re high. The Colorado Department of Public Health and Environment offers guidelines around marijuana use, including suggestions for when it might be safe to drive after consuming cannabis. “As a rule of thumb, if you’re smoking, wait at least six hours before driving. If you’re ingesting, wait at least eight,” their website advises.

But, for daily medicinal users it can be a bit more tricky. The Colorado Department of Transportation says that medical marijuana needs to be treated like any other prescription drug that can impair driving – if a drug causes impairment, it’s illegal to drive. Huestis agrees, but notes that guidelines around suggestions for driving for those who use daily is a difficult question to answer. Ultimately, she says, it’s something that society is going to have to make a decision on.

Huestis points to the .08 limit in the U.S. for alcohol as an example of a socially acceptable limit, and compares it to most other countries, whose driving thresholds for alcohol are much lower. “With alcohol we’re more liberal and we allow for more possibility, but we have more people killed on the roads than almost any other western country.”

And yet, companies like Triple Ring Technologies and Hound Labs are still eager to get a product to market. Heanue, of Triple Ring, says that though they’re still in the testing phase, his company is hoping to be working with commercial and law enforcement partners in the third quarter of this year. Their technology will be able to detect both inhaled and ingested forms of cannabis in the breath. In an effort to make the detection process less invasive, Hound Labs’ is developing a breathalyzer that will be able to detect both THC and alcohol in one instrument. 

“As a rule of thumb, if you’re smoking [pot], wait at least six hours before driving,” warns the Colorado Department of Health. “If you’re ingesting, wait at least eight.”

Mass spectrometry has been the gold standard for making laboratory measurements on breath samples because of the tool’s extreme sensitivity. But, because mass spectrometry devices are large, expensive instruments – table-top or even larger – they’re not practical to take into the field. That’s why, Heanue says, Hound Labs developed a technique that is portable and that has the same sensitivity as mass spectrometry. “In fact,” he says, “the way we validate our device is to do measurements with both our technology and mass spec and show that they yield comparable results. Mass spec has been a critical tool for us.”

Though there are other THC breathalyzers in development – notably one from Cannabix Technologies, in Vancouver, Canada – nothing is ready to go on the market right now. “We’re all waiting,” Says Huestis. “When they have something, we scientists are going to say, prove to me that you don’t have false positives. Prove to me that you don’t have false negatives. Tell me when you can first start detecting it in breath, tell me when you last detect it in breath. What’s the detection window? How are you confirming? We have a lot of questions [but] we’re ready and we’re excited. And if it works I think it’s going to be a great tool.”

Others, though, are feeling a bit more cautious with their optimism. Heather Thompson holds a PhD in molecular and chemical biology and is a cannabis educator and advocate based in Colorado. She agrees that breathalyzers would be a better tool than checking serum levels via blood, urine, or oral fluids because of the likelihood that they have to implicate chronic, frequent users even when they’re not actually high. But, her concerns around any tests for impairment mostly center around bias – and who gets flagged for the testing of impairment to begin with.

“I just worry about the impact that that has on marginalized communities because it does impact them more than dominant communities,” says Thompson. The rules about impairment, she says, are predominantly determined by white people. “And marginalized folks get screwed by that over and over.”

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