Driving while high on marijuana can be as dangerous and illegal as driving drunk, but unlike alcohol, there's no way to detect dagga on your breath.
That could change, however, as University of Pittsburgh scientists are working hard to develop a breathalyser that can measure the psychoactive ingredient in marijuana. Although the technology may work, many questions must be answered before police start checking drivers.
Stoned behind the wheel
"We envision that this sensor can be produced as a breathalyser like an alcohol breathalyser," said lead researcher Alexander Star, a professor of chemistry and bioengineering.
As more US states legalise marijuana, it's likely more drivers will get behind the wheel stoned.
Since Washington state legalised marijuana in 2012, the number of drivers involved in fatal crashes who tested positive for marijuana rose from 8% in 2013 to 17% in 2014, according to the National Conference of State Legislatures.
Since current testing relies on blood, urine or hair samples, it can't be done in the field when a driver suspected of being high is pulled over.
Using carbon nanotubes, however, Star's team has found a way of detecting tetrahydrocannabinol (THC), the ingredient in marijuana that creates a high, in a user's breath.
These hollow nanotubes are 100 000 times thinner than a human hair. The electrical components of molecules in the breath bind to the tubes. Since different molecules bind at different speeds, the type of substance, including THC, can be detected.
Tests need to be done
Star said these sensors can detect THC with the same accuracy as mass spectrometry, the gold standard for detecting it.
So far, the device, which resembles a breathalyser used to test for alcohol, has only been tested in the lab, using breath samples that also contained carbon dioxide, water, ethanol, methanol and acetone.
Before the device can be made available for use by police, it will have to be tested on hundreds, if not thousands, of people, Star said.
Right now, unlike alcohol, no standard for THC intoxication exists or is written in any state driving laws. Setting a standard for how much THC is too much is a job for medical professionals, not chemical engineers, Star said.
So don't expect to have your breath tested for marijuana anytime soon.
Paul Armentano, deputy director of NORML, which advocates for the legalisation of marijuana, said establishing a standard for THC intoxication is no simple task.
Levels of impairment
"There is zero scientific data addressing the question of correlating the detection of THC or its metabolites in breath with psychomotor impairment, nor am I aware of any serious scientific investigations that have sought to do so," he said.
Experts, such as AAA and the National Highway Traffic Safety Administration (NHTSA), oppose imposition of such a per se limit, Armentano said.
"Some researchers and the NHTSA have observed that using a measure of THC as evidence of a driver's impairment is not supported by scientific evidence to date," he said.
Moreover, studies haven't been able to consistently correlate THC levels with levels of impairment, Armentano said.
"Ultimately, if law enforcement's priority is to better identify drivers who may be under the influence of cannabis, then the appropriate response is to identify and incorporate specific performance measures that accurately distinguish cannabis-influenced drivers from those who are not, and to prioritise greater officer training in the field of Drug Recognition Evaluators," he said.
The report was published in the journal ACS Sensors. Star is its editor-in-chief.
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