A new publication by the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) provides a rundown on how to scan through sewage to keep tabs of drug use trends in cities—and it contains a section explaining why public loos could be the next big thing when it comes to spotting newly-popular psychoactive substances.

Toxicologists and researchers attempting to put together a picture of which drugs are being used in a certain area have been analysing wastewater for about a decade. This involves collecting samples from wastewater treatment plants and scouring them for traces of substances: the higher the percentage of a substance, the more widespread its assumed consumption in the monitored community.

The procedure works well for drug epidemiologists on the lookout for the usual suspects: cocaine, heroin, amphetamine and so on. It is far less efficient, though, at gauging use patterns of new lab-made drugs such as so-called "legal highs"—chemicals with effects similar to illicit drugs, but not always explicitly forbidden.