In states where marijuana is legal for adult use, the enforcement of things like driving while under the influence are more nuanced than the binary world of prohibition. There is no standard legal driving limit of THC across all 24 states that have reformed marijuana laws, but Hound Labs, an Oakland-based startup, has created a marijuana breathalyzer that has successfully differentiated between stoned drivers and sober drivers during roadside tests administered by police in California.

Hound Labs, co-founded in 2014 by Mike Lynn, an emergency room doctor, reserve deputy sheriff and venture capitalist, announced on Tuesday that California police conducted roadside tests with the company's handheld device and it was able to differentiate between drivers who have used cannabis recently and were impaired, and those who have used recently but were not under the influence.

"It's a balance of public safety and fairness-- impaired people should not be driving, but drivers should not be punished unfairly under arbitrary levels of THC that can be detected in the blood and saliva that don't correlate to impairment," says Lynn.

The device, which is called the Hound, measures levels of THC, the psychoactive chemical in marijuana, on the breath of drivers in parts per trillion through an extraction method. Like alcohol, THC can be detected in a person's breath for a short window of time, about two hours after ingesting. With one or two breaths, Hound Labs' marijuana breathalyzer can detect whether a person should be driving or not. (The device also measures blood-alcohol levels.)

The Hound was developed in partnership with scientists at the University of California, Berkeley and can detect THC and measure impairment regardless of the way a person ingested the drug, from smoking to edibles. THC is stored in body fat, which means a person can test positive for marijuana long after the drug's effects have worn off during blood, urine and saliva tests. Existing tests are not as easy and practical for police to conduct on the side of the road and cannot tell if a person just smoked and is intoxicated or is a frequent user. These facts have made it difficult for police in states where the drug is legal to differentiate between drivers who are not impaired and drivers who are impaired. Also, since blood and urine tests cannot measure if a person is high during the time of the test or smoked that day, more accurate testing will help prevent unimpaired drivers from getting arrested for THC being in their systems from a while ago.

The National Highway Traffic Safety Administration (NHTSA) says that smoking cannabis less than three hours prior to driving impairs and compromises driving performance. THC stays in blood, urine and saliva for a few weeks, which can lead to a false positive. But THC only stays in a person's breath for two hours, which makes testing for THC through the breath a more accurate approach to measure impairment. Although a legal limit for THC levels doesn't exist yet, Lynn is hoping his tool can help establish a standard.

To collect data and help create a baseline level of THC, Lynn is doing police ride-alongs, stopping drivers who are driving erratically. If a person appears to be high on marijuana, or admits to it, the police officer will do field tests. If the person fails the sobriety test, the officer will ask if they want to take a THC breathalyzer. If the person says yes, it gives Lynn a chance to collect data in the field.

"We have pulled over guys who smoked 20 minutes ago and blew very high levels, while others were not clearly impaired and blew low levels," says Lynn. "Right now, there is no standard, but through field tests we are collecting data and hopefully researchers will be able to create a standard like they did with alcohol and blood-alcohol levels."

Hound Labs is not the only company making devices to test levels of THC. Cannabix, which is working with Richard A. Yost, who helped develop the alcohol breathalyzer, is also making a THC breathalyzer. Colorado-based Lifeloc Technologies is also developing one. Colorado's state police are testing devices that detect THC in a person's saliva, The Cannabist reports.

Lynn hopes the Hound can eventually become law enforcement's standard and replace expensive, impractical and inaccurate blood and urine tests.

Lynn says the standards regarding detecting THC, compared with detecting a person's blood-alcohol levels, are "completely arbitrary." Lynn says that blood and urine tests cannot determine whether a person ingested THC thirty minutes, three hours ago, or three days ago, which is a pivotal time frame for police to be able to differentiate.

"It's hard to tell me someone who smoked three days ago is still stoned, but blood and urine and saliva tests cannot tell the difference between three hours and three days," says Lynn.

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Hound Labs hopes its THC breathalyzer will hit the commercial market in 2017 and has raised $4 million since founding in 2014. The company hopes to raise more capital before going to market. Lynn expects to set the price of the breathalyzer at around $1,000, which is similar to alcohol breathalyzers.

Lynn would not say which police department conducted the tests, but Hound Labs will launch another round of tests with the Lompoc Police Department in California. The first roadside tests were trials and drivers caught stoned were not arrested but safely brought home.

"What is the threshold, how many picograms of THC in a breath correlates to impairment? We don't know yet, no one knows what level in breath correlates to impairment," says Lynn. "We hope to help find that window and set the standard."