The first flush of data has been released from a unique pilot project that aims to measure Canadians' marijuana consumption through their sewage, and the results are murky.

Statistics Canada sampled wastewater from treatment plants in Halifax, Montreal, Toronto, Edmonton and Vancouver between March and August and analyzed metabolites derived from THC, the main psychoactive compound in cannabis.

The goal of the study, which has been used in Europe for years, is to track pot usage before and after recreational marijuana was legalized this past October.

"This has been done elsewhere in the world, but we're the first jurisdiction to do it on an ongoing basis for so many cities," said Tony Peluso, assistant director with Statistics Canada.

Halifax higher than average

Preliminary results show the rate of cannabis use per capita was about 540 micrograms per person per week.

Of the five cities sampled, Halifax — by far the smallest site in the study — had a higher than average per capita consumption rate at 1,310 micrograms per person a week. However, the study also notes the margin of error for the city was quite large at 707 micrograms per person.

The East Coast city was followed by Montreal at 976 micrograms per person a week. Vancouver had the lowest average of the five cities sampled, at 288 micrograms per person a week.

Hard to measure

While this type of study, known as wastewater-based epidemiology, is cost-effective and can produce fast answers, Statistics Canada said results can be affected by a number of variables, including temperature, acidity, and the presence of industrial chemicals or bacteria in wastewater.

In its online summary of the results, the agency said "the disparity may not necessarily reflect a true difference in consumption, but may be the result of sampling or statistical methodology."

"For example, the wastewater treatment plant in Halifax was situated in the core area of the city and as a result, the consumption of cannabis measured there may not represent the city's entire metropolitan population," the summary said.

More sampling to come

In other cities, suburban treatment plants where cannabis usage might be lower were also sampled. As a result, Statistics Canada said intercity comparisons are not as important as tracking changes in the future.

"What we're more interested in is if Halifax is increasing consumption over time," said Peluso.

Sampling will continue until April 2019. The wastewater survey will be used in conjunction with individual surveys, but Peluso pointed out that people tend to underreport their cannabis use.

Statistics Canada is also using wastewater sampling to try to monitor usage of other drugs, including opiates, but Peluso said it's too early to determine if they can be accurately measured.