Surface-water nitrate exposure to world populations has expanded and intensified during 1970-2

Excessive nitrate in surface waters deteriorates water quality and threatens human health. Human activities have caused increased nitrate concentrations in global surface waters over the past 50 years. An assessment of the long-term trajectory of surface-water nitrate exposure to world populations and associated potential health risks is imperative but lacking. Here, we used global spatially explicit data on surface-water nitrate concentrations and population density simulated by the consistent integrated assessment model, in combination with thresholds for various health risks compiled from epidemiological studies, to quantify the long-term changes in surface-water nitrate exposure to world populations at multiple spatial scales. During 1970-2010, global populations potentially affected by acute health risks associated with surface-water nitrate exposure increased from 6 to 60 million persons per year, while populations at potential chronic health risks increased from 169 to 1361 million persons per year. Potential acute risks increasingly affected Asian countries. Populations potentially affected by chronic risks shifted from dominance by high-income countries (in Europe and North America) to middle-income countries (in Asia and Africa). To mitigate adverse health effects associated with surface-water nitrate exposure, anthropogenic nitrogen inputs to natural environments should be drastically reduced. Moreover, international and national standards of maximum nitrate contamination may need to be lowered. By leveraging biogeochemical, socio-economic, and epidemiological knowledge from different disciplines, this novel assessment evaluates the role of humans in driving environmental changes and environmental effects on humans in a consistent manner.