Difficulties in testing for THC mean that curbing cannabis use amongst drivers hasn't been all that straightforward. Though marijuana use can be detected in the saliva for up to 24 hours after use, it can show up in blood and urine samples for anywhere up to a month. Existing methods like blood and urine samples therefore make it hard to determine whether a driver is actually impaired at the time that they jump behind the wheel. But companies like Canada's Cannabix are working on portable breathalyzers designed to test exclusively for recent use of the drug, a solution that could be of great assistance to law enforcement personnel in keeping impaired drivers off the road.

The spark may have been lit for a marijuana breath test in 2007, when a roadside survey carried out by the US National Highway Traffic Safety Administration found 8.7 percent of people driving at night time on the weekends were doing so under the influence of THC. In the time since, four US states have legalized recreational marijuana use, with others expected to follow suit.

But developing a more precise means of testing has been slow going. Cannabix's solution has been in the works for around two years, with the company only recently announcing an alpha prototype. It is not divulging a whole lot about how the patent-pending system will actually work, only to say it will detect THC use within a two hour period and provide instant positive or negative results. The company says it is now developing a beta version for third-party testing.

While Cannabix may be making the most noise about achieving a marketable product, there are a number of parties breathing down its neck. In April, a pair of University of Akron students announced they were developing The Cannibuster, a smartphone-sized device designed to test levels of THC in minutes using saliva. Meanwhile, researchers at Washington State University are developing a THC breath testing device that relies on ion mobility spectrometry.

A study undertaken by the National Institute on Drug Abuse (NIDA) last year suggested that breath testing for marijuana was feasible in principle using a SensAbues collection device. This Swedish-developed system captures metabolites in the breath with a polymetric filter and then screens for THC in the lab using mass spectrometry.

It tested the breath of 13 frequent users and 11 occasional users who had each smoked a single marijuana cigarette. The system returned positive results in all but one of the occasional users, with overall positive tests declining with the passing of time: 77 percent of breath samples taken at 1.4 hours tested positive, 54 percent at 2.4 hours and zero at three hours.

So there's no shortage of potential solutions on the horizon, but a haze still surrounds whether these efforts can actually translate to reliable, roadside breath testing systems. This largely stems from the fact that unlike alcohol, which remains in the bloodstream until you become sober, THC tends to linger long after its effects have worn off. So a real device that only returns a positive test if the subject is acutely affected by marijuana is yet to be comprehensively detailed.

This has unsurprisingly drawn criticism from users and non-users alike, with the prospect of people falling foul of the law despite using the drug days or even weeks before testing a potential consequence of marijuana breathalyzers. And that's not to mention the complexities in defining exactly what level of THC concentration constitutes an impairment for drivers.

Some states, such as Washington and Oregon, impose a five nanograms per milliliter limit, while others enforce a zero tolerance approach. Following their studies, researchers at the NIDA proposed experiments where subjects would take simulated driving tests while under the influence of marijuana to determine the actual THC concentrations that align with impairment.

So despite all the huffing and puffing, there appears to be some work to do before the science is truly settled and a patrol officer strolls up to your car with a marijuana breath test in tow.

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