Proponents of fluoridation have been criticized for overstating the benefits, while opponents have been criticized for understating them and for overstating the risks. Systematic reviews have cited the lack of high quality research for the benefits and risks of water fluoridation and questions that are still unsettled. Researchers who oppose the practice state this as well. According to a 2013 Congressional Research Service report on fluoride in drinking water, these gaps in the fluoridation scientific literature fuel the controversy. Public water fluoridation was first practiced in 1945, in the US. As of 2015, about 25 countries have supplemental water fluoridation to varying degrees, and 11 of them have more than 50% of their population drinking fluoridated water. A further 28 countries have water that is naturally fluoridated, though in many of them there are areas where fluoride is above the optimum level. As of 2012, about 435 million people worldwide received water fluoridated at the recommended level, of whom 57 million (13%) received naturally fluoridated water and 377 million (87%) received artificially fluoridated water. In 2014, three-quarters of the US population on the public water supply received fluoridated water, which represented two-thirds of the total US population.