Up to 220 million people worldwide, with approximately 94% of them in Asia, could be at risk of drinking well water containing harmful levels of arsenic, a tasteless, odorless and naturally occurring poison. The global scope of this persistent public health issue is revealed in a new study, in which researchers present the most accurate and detailed global prediction map of groundwater arsenic concentrations to date. It reveals previously unidentified areas of potential arsenic contamination, including parts of Central Asia and broad areas of the Arctic and sub-Arctic. Trace amounts of arsenic occur in virtually all rocks and sediments but rarely at concentrations high enough to cause adverse health effects. Nevertheless, arsenic is toxic; at high levels, it causes a wide range of maladies, including neurological disorders and cancer. Because dissolved arsenic can accumulate in aquafers, drinking contaminated groundwater is a major source of exposure. Consequently, the World Health Organization's (WHO) guideline concentration for arsenic in drinking water is 10 micrograms per liter. While the severe public health risks from arsenic contamination are well-recognized, arsenic is generally not included in the standard suite of tested water quality parameters. And due to incomplete and unreliable records and spotty testing, risk assessments are often fraught with uncertainty. Joel Podgorski and Michael Berg compiled data from 80 groundwater arsenic studies worldwide and used machine learning to model global arsenic risk. The resulting map revealed global groundwater contamination hazards, including in regions with few or no reported measurements. According to the results, the highest-risk regions include areas of Asia and South America. "Disparities in coverage of regulatory requirements in the U.S. have left over a million rural Americans unknowingly exposed to arsenic with a high proportion of socio-economically and behaviorally vulnerable groups," writes Yan Zheng in a related Perspective.