Hound Labs have invented a new breathalyser that tests for marijuana determining through extreme sensitivity how stoned one might be.

The new device is a lot more sensitive than breathalysers that check alcohol levels. They determine when someone has recently had THC - the ingredient that affects you when smoking marijuana.

As marijuana is starting to be legalised recreationally in many places, the new device has been labelled a "game-changer" by experts aiming to eliminate driving under the influence of the drug.

The new device will cost USD $5,000 and the single-use tests are USD $20 each. Photo / Hound Labs

Blood tests are able to show marijuana up to a month from smoking it but with this new test, if someone has smoked or eaten an edible in the last few hours it will show up just from their breath.

The company's director of business Warren Tolman told Tim Dower it works similar to breath testing for alcohol.

"The person breathes into the device and the device will give a reading about whether one passes or a warning will come yp saying there has been recent use."

"It's about creating a balance of public safety and fairness," said Mike Lynn, an experienced A and E physician who helped develop the test.

"I've seen the tragedies resulting from impaired driving up close. And I have a good idea how challenging it is at the roadside to know whether someone smoked pot recently. But I believe if someone is not stoned, they shouldn't be arrested."

The team behind the new device spent five years perfecting the test so it can tell the difference between recent and old consumption.

"Employers have the same fundamental problems as law enforcement. They need to maintain a safe workplace, but not have to worry about what their employees do in their free time. Someone can go home, smoke pot just like I'd enjoy a glass of wine, and not test positive," Lynn added.

"Employers are facing a workforce now that has close to full employment. They don't want to be firing valuable workers, especially for something that's legal in most states."

The new devices are expected to go into use throughout the US in 2020. The actual device is priced at US$5000 and the single-use tests are $20 each.