“Sewage epidemiologist” doesn’t sound like the kind of job that will have applications flooding in. But it is a rapidly growing field. One of its primary goals is to figure out how levels of illegal drug use calculated from traditional approaches such as questionnaires and crime statistics tally with the more direct evidence from what ends up in the sewerage system.

For obvious reasons, illegal drug users aren’t always open and honest about their habits, so sewer data can help governments, researchers and police get a handle on the true scope of activity in a district.

Over the past six years or so, sewage epidemiology has appeared to agree rather well with other approaches to quantifying drug abuse. The technique isn’t about pinpointing individuals. Instead, the aim is to find out, for instance, that ecstasy use is particularly high in London, which could tell authorities whether they’re targeting their public health campaigns and police resources properly. It’s also important to measure drugs in wastewater because they are a pervasive contaminant. The quantities are big enough to raise concerns about effects on ecosystems.

Yet has sewage epidemiology to date really provided us with a true picture? A new study suggests that some previous research of illicit drugs in sewage might have underestimated the amounts being used.