Here are the challenges police could face in laying charges for pot-impaired driving

A wave of new case law could be coming down the pipe as law enforcement prepares to put new drug-impaired driving powers to the test.

Speaking during a technical briefing on Tuesday, government officials stressed they are ready for cannabis legalization but expect to see the courts set clearer interpretations of the laws around impaired driving as they begin to lay charges and move them through the legal system.

At issue will be the tricky nature of how the body metabolizes pot.

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THC, or tetrahydrocannabinol — the active component in cannabis that produces the feeling of being high — can show up in the blood days after an individual consumes marijuana.

That raises questions about how police can prove the amount of THC in the blood was making the person actually high while driving, versus lingering in their blood from consumption days prior.

While officials said someone can be charged for exceeding the legal limit, they can also be charged with impaired driving even if they have less THC in their blood than meets the criteria for a legal limit charge.

For example, the federal rules say it is a summary offence — similar to a misdemeanour south of the border — to drive with between 0.2 and 0.5 nanograms of THC per millilitre of blood.

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A person can be indicted if they test positive for more than 0.5 nanograms, or if they have a prior summary conviction.

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The charge for exceeding those limits, however, is separate from the offence of impaired driving, the officials said.

And someone with 0.1 nanograms of THC in their blood involved in an accident could still be charged with impaired driving even without the second charge of exceeding legal limits.

READ MORE: Young Canadian drivers at increased risk of car crash, even hours after getting high

So how do police figure out if that amount of THC is from days ago?

Officials said deciding whether to lay a charge will come down to a judgement call on the part of law enforcement.

Their comments offer clarification on remarks made by Justice Minister Jody Wilson-Raybould on the West Block last month.

READ MORE: When will you be charged for driving after smoking pot? ‘It depends on a case-by-case basis:’ Minister

Wilson-Raybould was asked to explain at what point someone might face a charge for impaired driving after consuming pot.

She said officers would make those decisions on a “case-by-case” basis, but did not explain why that was the case.

Bill Blair, the minister for border security and a former chief of the Toronto Police Service, also spoke with reporters Tuesday afternoon.

He said police across the different provinces are ready for legalization to take effect at midnight.

But he would not say when the government might consider pardoning those with past convictions for things like pot possession.

“There’s an appropriate time to deal with that,” he said. “That date is fast approaching, as you know, but we have not yet reached it.”

Officials say law enforcement will continue open investigations into existing cannabis charges that began prior to legalization.

The decision whether to ultimately push those cases through to prosecution will be up to the Crown.