The pee and poo of Chinese citizens have become veritable gold to police surveilling local drug use. Dozens of cities in China are now forensically analyzing sewage for drug consumption, according to Nature, which has led to the arrest of one drug manufacturer.

The city of Zhongshan, in Guangdong province, has been monitoring wastewater to evaluate drug-reduction programs, reports Nature. Police arrested a drug manufacturer based on the chemical analysis of human sewage, and “a handful of cities are planning to use [this kind of] data...to set targets for police arrests of drug users, some as early as next year.”

This process is called wastewater-based epidemiology (WBE), and works by testing for certain byproducts, or metabolites, that the body produces after metabolizing drugs. Some of these byproducts are present in human sewage, and can act like biomarkers for substances such as cocaine and weed, as previous WBE studies have shown. As it’s currently used, WBE is quantitative tool; combined with population or other data, it can potentially tell investigators how much of a drug is used, and how that changes over time.

Biomonitoring isn’t only used for drug research—it’s led to some benefits in disease studies, for instance—and poses different benefits and consequences depending on how it’s wielded.

Chinese President Xi Jinping recently doubled-down on anti-drug efforts, vowing to target trafficking networks and drug makers, according to state-run Xinhua News Agency. By the end of 2018, Nature reports that central and local governments will have poured 10 million yuan ($1.5 million) into WBE surveillance.

As Nature notes, experts largely agree that WBE is fairly reliable at estimating drug use, and might be a better gauge than law enforcement data, which “can be misleading because they are indirect measures.”

A study by Colombian researchers, for example, corroborated survey and police data about cocaine and cannabis use in the city of Medellín. And wastewater collected from 49 treatment plants shed light on heroin use in 24 Chinese cities.