When they flush the toilet, most people don’t think about what happens next. But for several hundred students at a private university in Washington state five years ago, what happened next was that scientists spied on some of their most intimate personal details. The researchers identified times of stress, probed the ethics of the students and calculated how many of them were bending the rules by taking drugs to help them with their degrees. The students had no knowledge of this at the time. And they probably still don’t.

Likewise, the citizens of dozens of European cities have no idea that their sewage is being sifted through right now, officially to protect them; or that the police are studying the results to track crime. The toilet bowl and its contents, once extremely private, are becoming very public indeed. It’s called wastewater-based epidemiology. Improved sensing techniques and analysis have made the contents of sewers and waste pipes a powerful source of data. And where there are data, there are researchers. Because although people may tell lies, the urine they send down the drain rarely does. Around for a decade or so, this analysis of waste water has mostly been used to obtain information that people would prefer others did not have — their use of illegal drugs, chiefly. Drugs broken down in the body leave telltale traces of metabolites, some of which can be found, quantified and back-calculated to work out how much of the original substance was present. Combined with a reliable estimate of the number of people who have, well, contributed a sample to the sample, the analysis can offer guidance on average consumption and how it changes.  

Some of the results are more worth noting than truly noteworthy. Cocaine use, unsurprisingly, peaks at the weekend. People in smaller towns and cities prefer amphetamines. And anyone watching the Netflix show Narcos — which chronicles the life and times of notorious drug lord Pablo Escobar — will be unsurprised to hear about the truly colossal amounts of cocaine that pass through the residents and into the waste water of the city of Medellín, Escobar’s one-time heartland. 

Even the study that involved the Washington students merely seemed to confirm what most people already accept: healthy university students take prescription-only medicines as ‘smart drugs’ to try to boost their cognitive abilities at exam time (D. A. Burgard et al. Sci. Tot. Environ. 450–451, 242–249; 2013).

A paper in the journal Forensic Science International this month offers an intriguing new possibility. Swiss researchers describe how they hooked up with drug-enforcement investigators to use waste­water analysis to shed light on the structure of drug markets, the criminals who controlled them, and how much influence police operations had on supply (F. Been et al. Forensic Sci. Int. 266, 215–221; 2016). The results are not foolproof — analysis of cannabis metabolites is chemically tricky, for example, and cannot distinguish between all sources — but the study did report some successes.

Heroin use in Lausanne was estimated by measuring morphine in the sewers and subtracting what was known to have been prescribed medically. Between October 2013 and December 2014, the scientists estimated that average daily consumption of pure heroin in the city was 13 grams. During the study, the police arrested two dealers, and analysis of phone records and interviews with users suggested that the dealers sold about 6 grams a day between them — about half the total market. This supported police intelligence that heroin, unlike other drugs such as methamphetamine, was supplied by a small number of local dealers who could be effectively targeted. You can flush, but you can’t hide.