By Menaka Wilhelm

As a bill that would legalize recreational marijuana works its way through the Canadian Parliament, the government is gearing up to track cannabis consumption more closely than it has before. Statistics Canada has begun to do city-scale drug screening by monitoring what Canadians flush down the toilet.

Six cities have agreed to contribute samples from the place where all drains congregate — their wastewater treatment plants. Toronto, Montreal, Edmonton, Alberta; Vancouver and Surrey in British Columbia; and Halifax, Nova Scotia, will participate. All told, the network would capture data on drug use from about a quarter of Canada's total 36 million inhabitants.

Prime Minister Justin Trudeau had aimed to legalize marijuana by July, but the draft legislation still has a ways to go. After the Canadian Senate passed it on March 22, five committees are now considering changes.

Regardless of what happens with marijuana legislation in Ottawa, Statistics Canada has already begun testing sewage for signs of drugs. Canada joins several countries in Europe that sample wastewater for drugs annually. New Zealand has been collecting data from sewage since last year, and Australia tests nearly half of its population's wastewater for substance use.

Statistics Canada's main goal is to get an unbiased read of how legalization affects cannabis use. "There are things like surveys and whatnot where people report frequency of use, but the consumption numbers weren't quite as reliable as we would like them to be," says Anthony Peluso, an assistant director of Statistics Canada. Eventually the testing may be expanded to 25 cities, he says.

Ideally, Statistics Canada would like to estimate how much cannabis Canadians consume, in total, through the sewage measurements. It might be possible then to subtract legal sales and arrive at the amount of cannabis sold illegally, Peluso says.

But the route from a wastewater treatment plant to that kind of calculation gets really murky really fast. For starters, Peluso says, Statistics Canada has to consider some basic questions that get quite complex on a national scale: "The suburban users, are they peeing in the city but consuming in the suburbs?"

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