Once recreational cannabis is legal, Statistics Canada wants to track the consumption of the drug more closely than it has in the past.

That's why, as reported by NPR, the agency has begun drug screenings in six different Canadian cities by taking a closer look at what we're flushing down the toilet.

Vancouver, Surrey, Edmonton, Toronto, Montreal, and Halifax have agreed to share samples from their wastewater treatment plants with Statistics Canada. In total, they account for approximately one quarter of Canada's population of 36 million. 

It's a practice common in Europe, where wastewater samples are tested for drugs on an annual basis. Australia and New Zealand also collect drug use data based on sewage samples. 

For now, the testing will remain in the six cities listed above, but Anthony Peluso, assistant director of Statistics Canada, told NPR that the testing could eventually be expanded to as many as 25 cities across the country.

While researchers say detecting tetrahydrocannabinol (THC) in diluted wastewater is "relatively straightforward", using that number to determine exactly how much cannabis is being consumed is a whole different ballgame.

It comes down to something scientists call the consumption-excretion relationship, which depends on a number of different factors, including whether is ingested, smoked, or vaped.

Tolerance and other biological and metabolic differences among individuals also make determining the exact amount of cannabis being consumed in a given population a challenging number to land on.

While the main goal of Statistics Canada is to determine how much cannabis Canadians consume by measuring the THC present in sewage, Peluso also suggested that, by subtracting legal sales from the number it may or may not arrive upon, the agency could attempt to determine the amount of cannabis being sold illegally.