With more states legalizing recreational marijuana, enforcing DUI laws becomes more complicated. Alcohol intoxication can be easily measured during a traffic stop—the driver just blows into a tube—but testing for pot requires blood, urine, or hair samples, and results only show whether someone has THC in their system, not whether they’re currently under the influence.

That’s got scientists—and venture-backed entrepreneurs—racing for a better way: a breathalyzer for weed. California startup Hound Labs raised $65 million to develop such a product, and released a small study showing how it could work. A Canadian firm called SannTek Labs has Silicon Valley backing for a similar device. Most recently, researchers at the University of Pittsburgh debuted their take on the weed breathalyzer. Like a typical alcohol-based breathalyzer, it has a mouthpiece you blow into and a digital display to show results. Breathing into it sends air flowing over carbon nanotubes thousands of times smaller than a human hair that help identify THC molecules, even among other substances in the sample.

It’s much easier to label someone a drunk driver than a dangerously stoned driver. Since California legalized, pot-smoking drivers have managed to pass field sobriety tests. A Michigan medical marijuana user charged with DUI took his case to a week-long trial, where it was decided that the THC in his system wasn’t enough to impair him.

The University of Pittsburgh prototype was tested under laboratory conditions—we don’t yet know how it could work (or not) in the real world. (Defense attorneys love to point out flaws in today’s alcohol-only breathalyzers, which have a high margin of error even if used correctly.) Another loophole is weed breathalyzers may only pick up THC that’s been smoked; edibles would create a pretty expansive loophole when it came to detecting THC use through breath. (Saliva is another easily obtainable fluid that researchers have considered testing.)

The unanswered question is still what our standard is for THC "intoxication,” since it affects people in radically different ways. Research has yet to establish something as clear-cut as blood alcohol concentration (BAC) as a legal standard, and it’s not clear it’s even possible. The lawyer for that Michigan man called on lawmakers to set standards, as did California weed entrepreneurs. So far, states are reluctant: There are still a lot of unknowns, not least because federal prohibition continues to make it difficult to study marijuana.

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