A prototype device can tell if someone's high on marijuana and could soon be changing the way law enforcement officers deal with weed intoxication.

Researchers from the University of Pittsburgh in the US have unveiled a device that looks much like the standard breathalyser but can measure levels of THC, which is the dominant psychoactive compound in weed.

Tests for levels of marijuana are currently done using either blood, urine or hair samples.

The new device uses carbon nanotubes to bind the THC molecules and detect the compound through a person's breath.

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"The semiconductor carbon nanotubes that we are using weren't available even a few years ago," says lead author Sean Hwang.

"We used machine learning to 'teach' the breathalyser to recognise the presence of THB based on the electrical currents' recovery time, even when there are other substances, like alcohol, present in the breath.

"Creating a prototype that would work in the field was a crucial step in making this technology applicable," said Ervin Sejdic, PhD.

"It took a cross-disciplinary team to turn this idea into a usable device that's vital for keeping the roads safe."

As weed becomes legalised or decriminalised in more and more places across the globe, law enforcement will need a device like this to test if a driver is under the influence, much like they test for alcohol.

The device is said to be a promising first step in this direction.