City leaders in Spokane, Washington, want to know just how much pot residents are smoking, now that it's legal there. Sewage might hold the answer.

The primary author of Washington state's recreational marijuana law, attorney Alison Holcomb, made this suggestion to the city's marijuana policy subcommittee at a meeting on Tuesday. About 50 city leaders and residents make up the group, which attempts to grapple with what legalization means for the city of about 210,000.

"We don't have really good data on usage and perceptions of harm," said Jon Snyder, a Spokane city council member. "It's funny how the sewage thing has really captured people's imagination."

Testing sewage could help to determine the extent of marijuana usage in Spokane, where some traditional survey methods have been upended by legalization and others are seen as less than completely reliable.

Arrest and incarceration rates were always subject to an enforcement bias, but are now essentially non-existent in Washington state. Hospital emergency rooms sometimes test for marijuana for people who come in as a result of an overdose, but those patients are typically in the hospital due to a different drug. And many officials are dubious that people honestly self-report in phone surveys, even after legalization.

Holcomb's suggestion was inspired by research from a small group of American scientists in the Pacific north-west. In two separate studies conducted in Washington and nearby Oregon, environmental chemists and a public health researcher looked for traces of drugs in wastewater.

"In some ways, I think my most surprising finding is that it works," said Caleb Banta-Green, an epidemiologist and professor at the University of Washington's Alcohol and Drug Abuse Institute. "When I first heard about it I was skeptical. I thought, 'How is this going to be sensitive enough?'."

He and two chemists teamed up to study tiny quantities of drug metabolites in sewage, measuring parts per billion with a state-of-the-art mass spectrometer.

One chemist, Daniel Burgard, a researcher at the University of Puget Sound in Tacoma, Washington, tested sewage downstream a of college campus for metabolites of amphetamines commonly used as "study drugs", Ritalin and Adderall.

"The amphetamine levels go through the roof during finals," Burgard told Environmental Health News in January. Data collected from sewage was used in conjunction with results of self-reported surveys.

In another study, Banta-Green and Jennifer Field, an environmental chemist, tested sewage across 96 cities in Oregon, around 2.4 million people and 65% of the state's population, for methamphetamine and cocaine. Though higher concentrations were found in urban areas, methamphetamine metabolites were found in every city.

As Banta-Green and Field point out in their research, sampling sewage tests everyone. As a result, when paired with data collected in other ways, sewage sampling could better illustrate the extent of the marijuana using population in the area, as well as helping researchers understand whether people accurately self-report and if prevention efforts work.

Scientists in the north-west United States are not the only ones doing this; researchers are testing sewage around the world.

In Europe, 19 cities across the continent are having their sewage monitored, with help from Banta-Green and a €5m grant from the European Monitoring Centre for Drugs and Drug Addiction.

Monitoring sewage made international headlines in 2005, when Italian researchers discovered as much as 4 kilograms of cocaine residues per day traveling the Po River near Milan.

Nevertheless, sewage data in Washington won't replace information compiled from surveys, interviews and hospitals. But it could offset weaknesses of those sources, making estimates more accurate and multi-dimensional. It's also cheaper.

"I do lots of different types of research," said Banta-Green. Data is usually "the most expensive thing". "In this case, the data are close to free because you're saying can we have some of your free sewer water, 100 milliliters," he said.

Some studies have tested for cannabinoids, and the metabolites of the psychoactive ingredient in marijuana, THC. Nevertheless, Banta-Green said his group is not quite ready.

"If we store those samples now, we might be in a better place in two years to calibrate those samples," said Banta-Green. The sewage can be frozen and tested later. "This area of research was in its infancy in '08, and I like to say we're in our adolescence," said Banta-Green.