Dowsing for danger: Arsenic is a metabolic poison that is present in minute quantities in most rock materials and, under certain natural conditions, can accumulate in aquifers and cause adverse health effects. Podgorski and Berg used measurements of arsenic in groundwater from ∼80 previous studies to train a machine-learning model with globally continuous predictor variables, including climate, soil, and topography (see the Perspective by Zheng). The output global map reveals the potential for hazard from arsenic contamination in groundwater, even in many places where there are sparse or no reported measurements. The highest-risk regions include areas of southern and central Asia and South America. Understanding arsenic hazard is especially essential in areas facing current or future water insecurity. Science , this issue p. 845 ; see also p. 818

Arsenic and Populace: The solubility of arsenic in groundwater aquifers is controlled by a number of hydrologic and geochemical factors. In rural communities that rely on groundwater for drinking water, the risk from exposure may pose a public health threat, especially when groundwater pumping can increase arsenic solubility. In an effort to provide a focused assessment of risk to arsenic exposure from groundwater, Rodríguez-Lado et al. (p. 866 ; see the Perspective by Michael ) constructed a geostatistical model that incorporates a number of factors that control arsenic solubility across China. Most of the risk centers in a few provinces—Xinjiang, Inner Mongolia, Henan, Shandong, and Jiangsu—but the total population exposed to arsenic levels above 10 micrograms per liter could be upwards of 19 million people.

The health of millions of people worldwide is negatively impacted by chronic exposure to elevated concentrations of geogenic fluoride in groundwater. Due to health effects including dental mottling and skeletal fluorosis, the World Health Organization maintains a maximum guideline of 1.5 mg/L in drinking water. As groundwater quality is not regularly tested in many areas, it is often unknown if the water in a given well or spring contains harmful levels of fluoride. Here we present a state-of-the-art global fluoride hazard map based on machine learning and over 400,000 fluoride measurements (10% of which >1.5 mg/L), which is then used to estimate the human population at risk. Hotspots indicated by the groundwater fluoride hazard map include parts of central Australia, western North America, eastern Brazil and many areas of Africa and Asia. Of the approximately 180 million people potentially affected worldwide, most reside in Asia (51–59% of total) and Africa (37–46% of total), with the latter representing 6.5% of the continent’s population. Africa also contains 14 of the top 20 affected countries in terms of population at risk. We also illuminate and discuss the key globally relevant hydrochemical and environmental factors related to fluoride accumulation.