In less than three months marijuana will officially be legalized across Canada and Oct. 17 will be a big day for law enforcement, as research is still underway to determine how the practical implications of the new legislation will be rolled out.

Earlier this month the federal Ministry of Public Safety announced it is launching a three-year study into the effects of cannabis on drivers. The nearly million dollar study will place drivers aged 19 to 45 in simulated driving scenarios to see how different levels of THC – the psychoactive ingredient in cannabis – affects their driving ability.

Why the government decided to look into this now and not earlier remains a mystery (the Ministry has not replied to multiple requests for comment). What is even more unclear is how drivers’ sobriety will be determined in the fall.

Scott MacDonald is a health sciences professor at the University of Victoria who recently retired as assistant director at the Canadian Institute of Substance Use Research (CISUR).

He explained that law enforcement officers will use an oral test to check the THC level in a driver’s saliva. A blood test would later be administered to find the exact levels of THC in the individual’s system.

Bill C-46 is aiming to change the criminal code regarding impaired driving, and sets two threshold levels for THC in the bloodstream, with the first at two nanograms, the next at five ng. Theoretically, the higher the levels, the higher the impairment – and potentially the higher the fine.

The problem, MacDonald said, is how THC is metabolized by the body.

“THC binds to the fat cells of the body, so they’re kind of released slowly over time,” he said. “You’re not really high, but it can go back into your blood and remain in your system for quite some time.”

He noted that this is very different from alcohol, which is eliminated from the body at a very constant rate.

Heavy marijuana users – those who smoke every day – can build up quite a tolerance. They can hold THC in their systems up to five days after they smoked, reaching levels of at least two nanograms in an oral test even though, at that point, they would not present any symptoms of intoxication.

“What happens when people have cannabis is their THC levels rise very rapidly, and could [increase] to over 100 nanograms in the first 15 minutes,” MacDonald said. “Then, they decline very rapidly by 90 per cent in the first hour, then kind of level off. It’s when the THC has levelled off that it’s very difficult to pick a cut off point that would be accurate.”

MacDonald believes the numbers to be used to test if someone is high, are too low.

“This is a step above zero-tolerance; they’re not impaired-based laws like we have with alcohol.”

Marijuana edibles bring an even greater challenge, he said, since ingested cannabis not only takes longer to kick in – averaging about an hour – but also creates a completely different chemical reaction.

“It goes directly to stomach and liver and when it’s metabolized, it creates a new compound called hydroxy-THC,” MacDonald said. “It’s more potent than THC itself, but no one’s been able to accurately measure hydroxy-THC for an impairment level.”

Law enforcement officers will received an approved screening device for oral tests soon, though at this point they don’t know which brand it will be, or when they will receive them.

Sgt. Shannon Perkins works with the traffic section of the Victoria Police Department. She said any such device will only be a tool to go along with their standard field sobriety test (SFST), an impairment check that requires the driver to walk heel-to-toe in a straight line then turn, during which the officer observes their eyes.

In response to MacDonald’s concerns over potential problems with low THC thresholds, Perkins said the specific numbers aren’t the point.

“We need to not get so hung up on levels or the presence of drugs,” Perkins said. “The device leads us to levels, which goes into a larger part of an entire investigation.”

She warned, however, that regardless of how low an individual’s levels may be, different people will react differently to drugs and should simply not drive if they’ve had cannabis.

“If it’s still in their system, how do we know it won’t affect their driving?” she said. “We’d love to tell the public how much they can have, but because we can’t provide that, they need to take responsibility … if the user doesn’t bear responsibility, should society?”

The B.C. Association of Chiefs of Police have instructed police departments to have 30 per cent of their front line officers trained in SFST by October in anticipation of higher instances impaired driving.

“It’s a state of change,” Perkins said. “Police agencies don’t have all the answers yet. There will be case law arguments and changes, and that’s how it goes.”

Driving high on marijuana will still be illegal when pot becomes legalized, but the impairment enforcement rules still need fine tuning.