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Statistics Canada is using a unique way of tracking the black market in cannabis since legalization: analyzing the sewage of five cities to determine how much pot residents are consuming.

The results suggest that between March and August Halifax had the highest rate of consumption per capita, at 1,310 micrograms per week, with Montreal coming in second at 976. Toronto came in at 451, Edmonton at 416 and Vancouver at 288. The areas surveyed represent almost 8.4 million Canadians.

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They are trace amounts – a microgram is a millionth of a gram. But Anthony Peluso, one of the researchers of the waste-water project and the assistant director at Statistics Canada, said the goal was to track consumption data consistently.

“There may be some flaws in it, but I think if the flaws are consistent over time, then the trend will emerge," he said.

When people smoke cannabis, the active ingredient THC is excreted from the body and flushed into the municipal waste water system. Statscan then analyzes this sewage water to determine the cannabis consumption levels in the city.

While this method has been used in Europe for 11 years, wastewater-based epidemiology, or WBE, is relatively new to Canada. It was designed as an alternative way of tracking cannabis use. Most users tend to under-report the amount of cannabis they consume, and the stigma associated with pot use – especially if it’s purchased from black market suppliers – makes tracking national cannabis use very difficult.

By calculating the total amount of cannabis use, Statscan is then able to subtract retail sales from legal vendors to estimate the size of the illicit market.

But M-J Milloy, an epidemiologist at the BC Centre on Substance Use and a professor of cannabis science at the University of British Columbia, said the data is completely unreliable.

“The problem is, the amount of THC that is detected in waste water is dependent upon how strong the cannabis [is], … The temperature of the water, the limpidity of the water, what kind of chemicals the city has put in their drinking water – all sorts of factors will affect how much THC is finally detected,” Dr. Milloy said.

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Everyone processes and excretes THC at different rates, he added. In addition, the way users choose to consume cannabis – whether they smoke, vape or ingest it – also affects how fast the body metabolizes and excretes the drug.

Even the amount of storm water in the sewage matters; the bacteria in storm water and the general pH levels in sewage could affect the results of a WBE survey considerably.

“In my view, these are fundamental flaws, and it renders any of their estimates unusable. Perhaps there will be developments in the future that will change my mind, but I don’t think there’s a lot of valuable evidence here at this point,” Dr. Milloy said.

With a file from The Canadian Press

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