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When it comes to cannabis, the proof is in the peeing.

Canada’s national statistics agency has been collecting and testing municipal wastewater samples from five cities for traces of cannabis to gauge actual usage of the now legal drug. The results are surprising: In some cases they contradict other Statistics Canada data on pot use, while shedding light on the potential size of the black market.

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When cannabis is consumed, the body converts THC into THC-COOH, which can then be measured in wastewater — a nice word for sewage. Of the five cities measured between March 2018 and February 2019, Halifax, Nova Scotia had the highest average “load” of THC-COOH per capita, followed by Montreal, Toronto, Vancouver and Edmonton.