Hundreds of rivers across the world, including the River Thames in the U.K. and the Danube in Europe, are dangerously contaminated with antibiotics, according to the largest global study on river contamination ever conducted. The results were presented Monday in Helsinki and detail a worrying trend that experts at the University of York, which led the study, say could lead to complete antibiotic resistance that could “kill 10 million people by 2050.” The contamination is linked to human and animal waste and leaks from water-treatment facilities. Samples taken from the Danube in Vienna had seven different antibiotics at nearly four times the level considered safe, according to The Guardian. The study included samples from 711 sites across 72 countries and found antibiotics in 65 percent of the bodies of water. The concentration of household antibiotics exceeded safe levels in 111 samples. The worst had 300 times the safe limit.