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How bad is marijuana second-hand smoke?

Just 10 minutes of moderate exposure to marijuana smoke is enough for blood levels to exceed the legal driving limit, according to research out of the University of Calgary.

Dr. Fiona Clement, the lead author of the Canadian Medical Association Journal Open study, said she was surprised by the results.

“People who are passively exposed, so not smoking themselves, can test positive in their blood or urine for THC levels above 5 nanograms per mililitre, which is being put out as the legal limit, with relatively modest exposure conditions,” she said.

“So think about something, an enclosed space maybe the size of a regular kitchen, all of the windows closed, relatively poor ventilation, and a joint being passed around while you’re having a conversation.”

Clement said biologically, there’s no way to prove it was secondhand exposure versus active consumption. On top of that, no roadside tests are available yet, and police are relying on subjective tests.

“It will be contentious I think, because different people will show different kinds of impairment at different levels,” said Clement. “Not all bodies store or process THC the same. So not everybody would present with the same kinds of impairment at that same level.”

On top of legal consequences, the Ontario Lung Association is sounding the alarm on the health effects of secondhand pot.

“We know they share a lot of the same chemicals, metals, particulate matter, as tobacco smoke,” said Sarah Butson with the Ontario Lung Association. “33 of those are known carcinogens, which means they’re cancer-causing agents.”

Researchers point to the hard lessons already learned as a good indicator of the potential effects of pot.

“The devastating health effects of tobacco are well-documented and generally well-accepted by the public,” said Butson. “So if you’re going to use cannabis, don’t smoke it, look for those alternative modes of consumption.”

“We’ve learned some hard lessons from tobacco,” said Clement. “Where if we reflect back to – not too long ago actually  – you could smoke inside, you could smoke on airplanes, and all those areas which are very poorly ventilated. We do know exposure of that kind is bad and can lead to serious health outcomes for people.”

Researchers also note: there are lots of unknowns about the longer term effects of passive exposure to marijuana.

Public consumption laws are set at the municipal level and at this point different cities are adopting different rules to mediate that exposure.